Finding the chase and cutting to it

This is another piece on how air sourced heat pumps (ASHP) work…

Oh no, can’t we talk about anything else?

Not yet, and bear with me. This is a fundamental thing to consider when trying to get the most out of your pump. But it helps to understand how a heat pump works to get understand why this makes such a big difference.

Fundamentally, a heat pump, like an air conditioning unit or fridge, uses a compressor to pull heat out of the air they draw in through the fan. ASHP use the principle of heat dynamics and some chemicals that have a low boiling point, that allows it to transfer as much heat out of the air as is possible, into the compressor. This squeezes the gas, generating high levels of heat which is conducted to the heating coils, which heat the water going to the radiators or the hot water tank.

This is why ASHP are so efficient compared to a fan heater or immersion heater, that squeezing effect can be thought of as a multiplier for the heat “pulled” out of the air. The chemical in our pump is propane, and it’s boiling point is -42°C, so basically, our heat pump works well even if the outside temperatures would kill a mammal like us.

The heat pump does have a back up plan, for when the “flow temperature” being generated is lower than the one needed to heat your water or rooms, there is a back up electrical heater.

Which is the point of this post.

Finally! Which is what, please?

When the ASHP was installed, it was given some settings to describe how it should behave.

For the summer, the maximum flow rate (or the maximum temperature coming off the compressor) was set to 50°C, which would work well for us, if we weren’t trying to use our house as a heat store.

We bake the house at midday and, fundamentally, only heat our hot water once a day. As we have solar cells, our time of day is midday’ish – I shift the time based on our house’s solar elevation (I talk about solar elevation on 14th February 2026).

What?

We get free solar engergy off our roof (well not free to install, but we bought the panels many moons ago, so it is free now). Plus, our solar energy is definitely carbon free (their carbon debt being paid off back in 2016, please see Number Crunching Part 1). Making the most of that energy is really important to us for those very reasons – keep our costs down and our CO₂ emissions to a minimum.

OK, I’ll bite, is this your only option?

We could sell that back to the grid and then buy back energy later in the day. But our rates don’t change: Ovo’s Simpler Energy Plan has a low standing charge, so while the units are a little higher than other plans in Ovo’s range, we benefit during the summer when we’re effectively off-grid.

Which all means that our heat pump is not working for us as well as it could for these reasons.

Why is that?

The coefficient of performance (COP) for our heat pump is around 4.1 per annum, that means for every kilowatt hour of energy it consumes, it produces four point one kilowatts of heating. Basically 1kWh = 4.1kWh. That’s made of the electrical heater and the compressor.

The electrical heater has a lower COP, about 1. Our best strategy is not to use the heater unless necessary.

Yet, when I looked at the figures that was exactly what was happening! So far (today is the 24^th January), we’ve used 734kWh in the compressor and 50.4kWh from the heater. Why?

Well, the heat pump has a setting called “maximum flow temperature”, which has a little leeway, but it states how much of the flow temperature comes from the compressor. Ours, like many set in the UK, was set to 50°C. Everytime we try to heat our hot water up to 55°C once a day, the electrical heater was being activated. For at least £13.10 worth of electricity, that we shouldn’t need!

So, I’ve set the maximum flow rate to 57°C. This should allow us to get to our 55°C easily without needing to turn on the heater. We could set the max flow temperature all the way up to 65°C, but that might get the compressor over exerting itself at times, and I don’t mind a fiver or so spent to keep everything working well.

Is it really going to make that much of a difference?

Let’s look at last year, if you remember, we can have a look at the way our heat pump used the energy it consummed.

	Total in kWh	Compressor in kWh	Electrical Heater in kWh	Heat generated in kWh
Nov 2024	544	509	34.5	2,330
Dec 2024	603	585	18.5	2,640
Jan 2025	926	863	62.5	3,390
Feb 2025	686	653	32.7	2,720
Mar 2025	486	463	22.9	2,010
Apr 2025	187	183	4.5	1,010
Jun 2025	113	111	02.0	632
Jul 2025	38.1	37.8	0.3	220
Aug 2025	43	42	1	222
Sep 2025	90.7	88.9	1.8	537
Oct 2025	231	227	3.7	1,240
Nov 2025	510	493	16.7	2,130
Dec 2025	617	597	18.4	2,600
Jan 2026	1,000	940	62.5	3,440
Feb 2026	?	?	?	?
Mar 2026	?	?	?	?
Apr 2026	?	?	?	?
Total	6,074.8	5792.7	282	25,121

This is something we’re going to try until April, I will let you know how it goes, probably with a graph.

If we can reduce the power used, because the compressor is more efficient than the heater, it’s probably something we’re going to keep doing!

Is it worth it to save 122.6kWh between January 2026 and April 2026?

From a money point of view that’s 122.6*£0.26 ~ £31.88. Is it worth the squeeze? But if we can reduce our CO2 levels further, it’s got to be worth it, hasn’t it?

Breaking the 80% Ceiling of current diabetes technology.

For decades, the clinical approach to insulin replacement has treated the human body as a predictable, linear machine. Standard “Total Daily Dose” (TDD) and “Basal/Bolus” ratios operate on the assumption that if a specific amount of insulin is delivered, a predictable result will follow. While early insulin pumps introduced distinct profiles for weekdays and weekends, and modern Hybrid Closed Loops (HCL) now react to real-time sensor data, these systems remain fundamentally limited, based on reactive responses to basals that rarely change.

Most current HCL users hit a “plateau” of 80–85% Time in Range (TIR). This is because these systems are reactive; by the time a sensor detects a glucose excursion, the physiological event—whether stress, hormonal shifts, or delayed digestion—is already in progress. They are chasing a moving target using a linear lens to view a chaotic biological system.

Embracing Chaos: The Attractor Model.

Drawing on James Gleick’s Chaos Theory, we must recognize that diabetes is a dynamic system where small variations in initial conditions lead to vastly different outcomes. A “standard” Tuesday basal rate may fail on a Tuesday where the user is stressed, at a different altitude, or at a specific point in a menstrual cycle, or a storm is brewing changing the environment in which the pump is operating.

In 1997, I began conceptualizing a model that moved away from “averages” toward a framework of attractors. In this framework, an attractor is any quantifiable influence, such as altitude, barometric pressure, or site location, that pulls the metabolic system toward a specific state. Instead of viewing these variables as “noise” to be filtered out, my framework treats them as deterministic pivots that shift the system’s equilibrium.

Quantifying the “42 Factors” with 2026 Technology.

The 2018 publication of the “42 Factors That Affect Blood Glucose” by Adam Brown provided a taxonomy for these influencers. However, the real breakthrough lies in our ability to quantify them in 2026 through the ubiquity of wearables and the Internet of Things (IoT).

By utilizing a smartphone as a central processing server and wearables (watches and sensors) as data donors, we can establish individual “biological signatures” tied to the person with the pump. We no longer need to guess the impact of laying in bed, a commute, or an intense exercise session; the hardware provides the high-resolution data streams (accelerometers, temperature sensors, GPS) to establish these values in real-time.

The Core Logic: Predictive Tailoring.

The framework operates by taking a personalized “base” basal rate and modifying it through a cumulative total of active attractor coefficients.

• Environmental Attractors: data such as temperature, altitude, and barometric pressure are pulled from the internet and localized to the user’s environment. These may impact the user or the equipment.
• Physiological Attractors: menstrual cycles, amount of sleep, or activity types, stress levels, and illness are mapped via calendars and biometrics.
• Decay Factors: the model accounts for the “lag” or “tail” of certain events, such as the lingering metabolic effects of long-distance cycling or the degradation of an insulin infusion site over several days. Even the degradation of the insulin in the pump.

By multiplying the base rate by the sum of these active attractors (which may be positive, negative, or zero), the system suggests a predictive basal rate for the next 24 hours. This moves the HCL from a reactive “safety loop” to a proactive, contextual model.

Safety, Autonomy, and the “Human in the Loop”.

A critical component of this non-linear framework is user autonomy. Unlike current “black box” systems that make decisions in secret, this model suggests a “sane” value to the user, who retains the power to accept, modify, or ignore the suggestion.

Furthermore, to prevent the risks of cross-contamination, especially in households where multiple people may require insulin replacement from a primary carer, the framework utilizes a physical cable connection for uploading basal rates to the pump. This ensures that the specific attractor model and pump ID are securely encoded to the correct individual, eliminating the risks associated with wireless interference or signal crossover.

Conclusion: A Unified Path Forward.

By unifying the “42 Factors” into a dynamic, nonlinear framework, we provide the Hybrid Closed Loop with the context it currently lacks. We can finally account for the pre-menopausal woman’s cycle, the athlete’s recovery, and the traveler’s changing environment. This is not just about better mathematics; it is about reclaiming autonomy and achieving results that finally break through the linear ceiling and coping with the mistakes non-contextual hybrid loops make across the world.

Transparency Note: This post contains affiliate links to energy partners like OVO (marked with [Ad]). If you click them, I may earn a commission. I am sharing my personal data and journey as a homeowner; please remember that energy performance is highly specific to each property. Individual results will vary [AD].

I’ve obviously had some time recently to get into the nitty gritty of our heating system, tuning the heat pump settings to match the weather conditions. Not the sort of thing that people usually do with their heating.

Up to a point, I have been interested in how it all works, but my driver is to reduce our carbon footprint: which has largely been achieved just by letting things run.

Isn’t that enough? Can we talk about something more interesting, please?

Bear with me, this is interesting to some.

From our figures, the heat pump did save us money without anything more. But, what if it were more marginal than that? What if we had to tighten our belts? How do we do the best that we can? How can we keep the heat pump running as long as it should do (with proper servicing)?

I have to say, the window figures and their payback were a bit of a shock. I am sitting here with it 4°C outside, feeling very cozy and having spent only 10.6 kWh to feel that since midnight (overnight it dipped to 2°C). The heat pump buzzed along happily, ramping up for when we got out of bed.

Air sourced heat pumps (ASHP) are very different to gas boilers, as heat pumps like “humming along”, having periods where they can flex up and gently ramp back down. Gas boilers are a bit happier to just switch on and off as required. To save money, switch the gas boiler off for a bit, let things cool down then heat them back up when you want more heat.

ASHP efficiency is all about the ambient air temperature. We’ve positioned ours to take advantage of the morning sun, which helps take the edge off the frost in the air around the pump. More importantly, we’ve tucked it into a sheltered spot to protect it from the prevailing south-westerly winds and the infamous East Anglian ‘lazy wind.’ By blocking these winds, we reduce ‘wind-chill’ on the unit, which prevents the pump from having to work harder or entering its defrost cycle more often than necessary.

Heat pumps work by extracting heat from the air they draw in, so when the wind is blowing directly into it, it struggles to pull out as much heat. Think of blowing on the skin on the back of your hand, that air is colder than the surrounding air because it is being forced directly on to your skin – this is a similar principle.

Tuning the heating curve.

The other thing I’ve started doing with the heat pump is changing its “heating curve”. This is the temperature it tries to get the “flow temperature” coming out of the pump for any given input temperature.

If we had underfloor heating everywhere, a heating curve of 0.7 would likely to be a perfect heat curve when teamed with the Tado™ smart room thermostats and smart TRVs on the radiators. Unfortunately, our upstairs circuits are all radiators, and some aren’t really big enough for the rooms they are in (we have plans to swap them out in the summer). So, on a warmer day, like today, I find that setting the curve to be 0.8 works for us. During colder days, we find a heating curve of 1.1 is more suitable to keep us comfortable.

 

The heating curves we’ve used over the past seven days.

The heating curve sets the water going into the underfloor heating pipes and the radiators. At warmer temperatures, the heat pump sends cooler water to these pipes as it doesn’t need to bring the rooms up to temperature in the same way, it is not fighting the temperature against the thermostats but working with the environment.

So, why not just set the curve to 1.1 and leave it there? Well, if we did that, we’d use much more energy and it wouldn’t allow the heat pump to coast when everything is a bit warmer. At the moment, having switched it from 0.9 last night to cope with a drop to 2°C, we had the pump coming on earler and harder over night, but is now gently working, keeping everything warm without much effort.

We do not allow the house to get cold, even when it is resting, the pump is running very gently. As I’m typing this at noon, the pump is pre-heating the house for when the sunsets. We’ll run this way until April, when the batteries from our solar generation will start to earn their keep again. Last year, we spent very little on electricity for our heating and hot water between March and September. Over the summer itself, between June and August, I will turn off the heating circuit completely as I did last year.

What about when you leave the house?

Well, unless we’re away for a number of days, it is not worth switching off for a few hours. Allowing the house to get cold does not save us money. Indeed, I am using the house as a thermal store for our energy.

So it’s a different way of looking at the seasonal problem, in the UK?

Pretty much. And so much cheaper, in terms of power used, than the gas boiler. Remember, the best we ever did was 12.9 MWh for the gas boiler compared to 3.9 MWh for the year with the heat pump. And that was before we started actively tuning what we were using.

When we had the pump installed, we were told to “set it to 32°C and enjoy”. We’ve running it between 21°C and 25°C (which mirrors our rooms thermostat settings) against variable heat curves. We’re more than enjoying it.

Can I be honest, here? I love data: I am a bachelor of science even though I did maths and computer science, and some of my natural scientist friends do look down on that “type of degree”. Can I be honest, I do think maths is more of an arts subject, as we don’t tend to do experiments.

Computer science is arguable. Yes, it is language based, but the vast majority of coders do test their code to ensure it is doing what it should be doing…

That’s not why I’m writing this piece. This is all about getting an infra-red camera.

Why, oh why, would you bother writing about that?

Well, because it’s kind of fascinating. We all know what we learnt at school and from the various adverts and articles discussing how to save energy at home by “keeping the heat in” – that’s insulation to you and me.

But as a mere human, it’s very hard to know if the measures you are taking are making any difference.

Without measuring, of course. I could have bought a dedicated unit: instead I bought a dedicated charge-coupled device that doesn’t have a filter to cut out infra-red rays. It plugs into the USB C port on my phone which launches the app which can display the data (image) being captured by the camera.

It came first thing this morning and I had a busy day at work, so I wasn’t able to play with it until my afternoon tea break. Oh my, it is amazing.

Downstairs, we don’t switch off the heating during the day, turning down the thermostat to 16°C. Internally, where we get thermal gain from our windows, the floors and furniture are warmer than their surroundings.

Externally, that heating effect is heating bricks that take much longer to cool down – so taking pictures at 7pm, 2 hours after the sun has set is probably showing heat coming off the bricks that is completely expected! If I’m keen I will get up early tomorrow when that effect will be minimal. Then I should be able to see if our insulation is being effective.

Some things are really interesting – if we don’t wear slippers, the camera picks up where we have just walked. Our footprints glow white then fad to yellow, orange, red, then fade completely.

We can see the heat conducting and convecting off our radiators or pouring through the underfloor pipes. We now have a means to see where our heating pipes are, so long as we’re pointing the camera at them when they first come on.

Outside, we can see the heat pour from the air vents in our walls and windows. The garage is particularly lossy, not least because it is very difficult to make it air tight. It also has the pipe work for the heating and hot water. We, and the previous owners, had lagged much of the pipe work in there as a result, but of course we can see if that is being impactful. The gaps are obvious.

But mostly, it looks like it is all working reasonably well.

So, how do you make use of the camera, and understand the results?

Good question. The camera calibrates itself to the surroundings, which means the scale is shown each and every picture.

Take this one of the lounge floor just starting its heating cycle just after tea.

The dark areas are cooler than the white areas but scaled for the image in question. Another picture will be at a completely range.

You know that advice to not dry items on radiators. This is why:

The radiator is nice and warm, but the towels are just soaking up all that heat and not distributing it around the room.

The tiles are colder, 21.2°C – the dark colour is not a good sign. It means we’re probably losing heat through this interior wall.

This picture shows that little heat is being lost through this external wall. While the window vents and wall vents are leaching heat through them. My beloved is glowing bright, as his body temperature is about 36°C – it was a little cold out in the dark that night and his clothing is shielding it a little.

OK, so now what? What are you going to do with this “knowledge”?

Well, nothing today, per se. But, it has spurred us into getting a blind for the last uncovered window in the house – a thermal black out one, of course.

I am very tempted to get something for our internal doors – it’s frightening how much of a rooms heat pours into the hallway.

We also seem to have some gaps in our roof insulation. We need to ensure this is breathable insulation but it’s tempting to pull our finger out and close those gaps.

Of course, that will all help to reduce our carbon footprint and save some money. It’s been a cold autumn. Even so, we are using dramatically less electricity to heat our house than we used gas – how can I say that? Well, a low usage year for our gas consumption was 13 MWh, where-as we have only used 3.97 MWh last year.

Of course, now the windows have been installed, I am looking forward to seeing the new results.

The results with the new windows.

What else can we do with this kind of knowledge?

Of course, this is not the only thing we can do with the camera.

The temperature measurement means I have been able to tune our heat pump to run really efficiently.

Heat pumps are basically mecahnical compressors taking heat from their inputs and use an exchanger to heat the water for your hot water supply and radiators and/or underfloor heating circuits. Compressors work efficiently (both economically and in terms of longevity) if they run gently, running at a constant rate and temperature.

So, ideally your input flow to your radiators is hitting about 35°C-50°C. By the time it hits the last radiator in your run, you shouldn’t be losing more than 5°C – you can only know that by measuring it or painstakingly working out the gradient from the radiator thermostat graphs and deriving the differences between each of your radiators…

Anyway, I used the camera to see that we were dropping about 10°C across our upstairs run, meaning our last radiator is struggling to get up to temperature with a input flow temperature of 42°C. Looking up how this works (Google or Gemini are your friends here), our strategy has changed.

First, change the temperature levels.

Our pump has three settings. Reduced will be set to 22°C, Normal to 23°C, and comfort to 25°C.

Next, look at the heat curve.

Our pump was originally set to “0.7”. That works really well for our underfloor heating circuits, but the radiators did not get up to temperature unless our temperature levels were set to 23°C, 25°C, and 27°C!

I have reset the curve to “1.1”, this changes the flow temperature that is set once the outside temperate (the ultimate input for the heat pump, the air temperature is where the heat is being extracted from). With a curve of “1.1”, the flow temperature going in to the radiators is set at 50°C when the outside temperature is below 1°C. At 10°C, the flow temperature is 40°C, and 20°C when the outside temperature is 20°C.

Finally, timing which radiators come on when.

Our ensuite tado radiator needs to be on most of the time, as that means the radiators in the 2nd ensuite and the bathroom are on. So, during our heating window (when we’re making the most of our solar generation), it comes on for an hour longer than the other rooms.

So far, it seems to be making a difference. Our electricity usage curve has smoothed out. Of course, I am mucking about with this on the coldest day of the year!

Now, what you are looking to do is keep your rooms at a reasonably constant temperature during the peak times and let them gently cool down when electricity is at its peak charge rate.

During the summer, we will be using our solar panels to heat our water completely, but spring and autumn will see us use the house as a thermal store, so we can move off using the grid as much as possible.

The winter is all about being comfortable. Last year, January saw us using 1,327.00 kWh, with 926 kWh being used by our heating.

Now, there are some things we need to consider. Our radiators worked well for a high output boiler, but we have to tune our heat pump to suit them. Ideally, three of our radiators would be swapped out. Looking at the camera and the electricity usage, it would be worth doing that for the bathrooms. Next week I will be looking at some tuning we can do for our heat pump, given these insights!

Hope this gives you some thought on your planned setup for your heat pumps or boilers!

Last week, we looked at the cash return on replacement windows, which, on the face of it looked marginal at best. But of course, we have made other investments over the past twelve years.

Easy steps, let’s look at the LED lighting.

About a year, just in saved electricity. Initial cost was about £350 for the whole house, and the kitchen lights alone (£250) paid for themselves in three months.

What about the solar panels?

Our panels cost circa £6,000. With the SEG and FIT, and the money they have saved us in terms of not having to buy electricity, they paid off in about 5 years.

The tado heat controls.

Like the lights, a chunky cost, incurred back in 2014 and 2015, over six months, but around £750 for our eight smart radiator valves and five thermostats, and one hub. The radiator valves were the initial step and proved themselves, so expanded the system to the whole house.

These allowed us to go from using c. 17MWh in gas a year to c. 14MWh in gas a year, just when gas prices trebled. So, two-three years to pay off.

Can the same be said for the heat pump?

From Number crunching part 2 we can see the power used to heat our house has gone from 13,270.888 kWh to 3,966.20 kWh – we’re going to pretend we have bought this electricity, just to keep the numbers simple.

At today’s prices, gas is £0.0603, so the gas would have cost £800 and a standing charge of £0.35 per day, so annually that’s £928.

The electricity has already had its standing charge paid for because we need the fridge power, lights to come on when it is dark and to be able to cook our food – we only pay that charge once. We’re not using gas for anything but heat and hot water.

So, at £0.2527 per unit, our heating has cost £1,002.26 – if we’d paid for it all. But we didn’t, during the summer, a good 750 kWh was used straight off our solar cells or batteries. So let’s run that again, giving £812.73. Which is cheaper than the gas per annum by £115.27, so in hundred years, or so it pays off. Our boiler was dying anyway – a new boiler would have cost £7,000 (it’s a big house, it needs a big powerful heat pump or boiler, many houses do not need as much capacity) – so should we say the difference is what we’re paying off? In which case, 43 years!

I suspect we’ve spent more than we needed to, as we punched big holes in to our house to do the renovations back in the tail end of November 2025. I’ve also learnt a great deal about how to balance what we’re generating against what we’re using.

Of course, without the smart radiator valves and thermostats, it would have been a different story. Without the fabric of the house being well insulated, it would not have come in as quickly either.

I’m cheating a little, because we already had the solar cells – we were about 35% self-powered last year for everything, including our car. We don’t have to pay twice for the standing charge.

Buying our new windows should help accelerate that period.

But I think that is a good way to look at the total cost and pay back period. This wasn’t a big bang but a series of smaller steps.

In the East of England, we have an “artic airmass” causing freezing weather. It’s 2°C out there at the moment, and has been as low as -6.5°C in our bit of the world over the past couple of days.

So, what is it like living with an air sourced heat pump in this climate?

It’s fine. Obviously, outside is chilly. But the house is maintaining its heat and the heat pump is working well. At 1°C, we’re getting a coefficient of performance of 11.5kW / 4.05kW = 2.84. In other words, nearly three times as much heat out of the system for each kilowatt consumed.

So, it is still out-performing a boiler.

We’re still getting some energy out of our solar panels – the pitch of our roof and the orientation mean that the snow melts well. That meant that yesterday, we got 5.1 kWh, and today we generated 3kWh.

Our daily consumption is quite high for heating. Yesterday, we used 58.8kWh for heating. Today we used 60.5kWh! During June 2025, our entire consumption for the month was 43kWh and that all came off our roof!

But, we’re sitting quite pretty. Our battery is carrying a little extra power, so if there is a power cut, we can keep warm for a little while. We also have a wood burner, if needed.

But so far, it all looks good. Hope you are keeping safe and warm out there.

A question was asked after publishing Is there such a thing as breaking even? “Does the carbon cost for windows repay sooner or later than the financial cost?”

I am going to find that a difficult question to answer, as we had some substantial building work done to put in a French door and take out a window and a French door on opposite ends of a wall and have a single big patio door put in.

So, let’s having a look at the energy saving compared to the old windows against what we can expect given the u-values for the new windows.

A quick reminder of what a u-value is: u-values are the rate at which heat is lost from a surface and is dependent on a few things such as materials, air gaps, and is measured in Watts per square meter a degree Kelvin or W/m²K. (Kelvin’s are used here as they are the scientific unit (also known as the SI unit) for temperature, unlike the Celsius unit and you then don’t need a pesky ° symbol when typing this out). The lower the u-value, the better.

From the previous article, we know that the temperate loss for the new windows will be, so we’re looking to compare that to the old windows. Some people who’ve studied this as part of their day job work this out to be: Annual kWh Savings = (u-value of old window – u-value of new window) * area of the window in question in m² * “Degree Days” * 24 / 1000.

The first three terms of this equation are explained in full in the equation! The “24/1000” converts u-value into kWh. The “Degree Days” is a new term here and refers to the average temperature for the location in question. A good source is the Enman degree day data, which show how much a building needs warming up and cooling down based on the external temperatures. In the south of England, that’s approx. 15.5°C and can be sourced from the https://www.degreedays.net/#generate site or https://www.gov.uk/government/collections/digest-of-uk-energy-statistics-dukes. Of course, from the last post, you can see the UK is a properly seasonal place…

I hate to say this, but a spreadsheet is our friend here.

Window	Area m2	Old u-value W/m2K	New u-value W/m2K	Degree days °C	Annual saving kWh
Spare room north	1.019	5.8	1	15.5	2.199
Spare room south	1.019	5.8	1.1	15.5	2.419
Master bedroom	3.500	2.8	1.4	15.5	5.104
Ensuite (estimated area)	1.000	5.8	1	15.5	2.158
Bathroom	1.019	4.8	1.1	15.5	2.002
Sam’s office	1.019	2.8	1.1	15.5	1.168
Jon’s office	1.019	2.8	1.1	15.5	1.168
Landing 1	1.019	2.8	1	15.5	1.062
Landing 2	1.019	2.8	1	15.5	1.062
Kitchen doors	3.471	2.8	1	15.5	3.615
Kitchen by doors windows	0.819	2.8	1	15.5	0.853
Kitchen south not sink	1.650	2.8	1	15.5	1.719
Kitchen south by sink	0.984	2.8	1.4	15.5	1.435
Lounge French doors	3.374	2.8	1	15.5	3.514
Lounge windows by French doors	1.649	2.8	1	15.5	1.718
Lounge patio doors	3.488	2.8	1.4	15.5	5.086
Gym	1.586	4.8	1	15.5	2.831
Cloakroom	0.643	4.8	1	15.5	1.148
Grand total					40.261

This is just plugging in the values in the table and getting the annual saving, then adding up the total.

The old u-values are estimated – a 2009 window is between 2.6 and 2.8 W/m²K, where the windows were failing, I have made them 4.8 or 5.8 depending on the level of failure. The ensuite windows are an estimate – these are shaped windows positioned in an opened area of roof. We know they were leaking.

Basically, the savings per annum in terms of power are going to be 40.261 kWh. Which is £10.47 per annum at £0.26 a unit. I could have used a different number for the old u-value, but mostly the windows were just about still working.

So, does it cost in quicker in terms of financial cost depends on how much the windows have cost and the cost of electricity or gas. The lower the u-value, generally, the more expensive the window. As you can see here, we had many windows being done! And that doesn’t include the building work.

What we are seeing is much better heat retention in the rooms where things were really bad and in every room, the energy needed to lift the temperature in the room is much less – in our bedroom, we’ve turned down the thermostat to make it more comfortable. In the spare room, where the windows were completely blown, the room holds its temperature beautifully. For us, with our EPC B rated house, this is more impactful than for someone in a house with badly insulated walls, floors, ceilings, and rooves. It should mean we retain our thermal envelop.

It’s also much quicker to vent the rooms, due to changing how the windows open, and significantly more light comes into the rooms.

But basically, it is going to take us a while, based on these values. And it is only worth it if your walls have a good u-value too, otherwise the heat leakage will come through the lower insulated path…

I’m tidying this up as I am sitting with the outside temperature hitting -2°C with the heat pump barely ticking over, so I think it’s been worthwhile.

This is a look at the second half of 2025, and a follow up to Number crunching part 1.

The summer saw us pretty much “off grid” apart from backup. Of course, we still have to pay the standing charge, currently a whopping £0.4595 a day, though I know that is better than many companies charge, and for us, during the summer, that suits us fine – a higher unit cost gives us a lower connectivity charge: yes please.

Our set up is this. We have an EPC rated B house, with good insulation, an air sourced heat pump supplying hot water and space heating, solar cells, 27 kWh of batteries, an electric oven, hob, microwave, washing machine, tumble dryer, electric car, etc. A pretty typical set up for the most part – central heating etc.

I am now going to fill in that table from Number crunching part 1, with the figures from July to December.

Month	2023 electricity usage (kWh)	2025 electricity usage (kWh)	% difference
Jan	697.10	1,327.00	190.4%
Feb	537.60	1,044.60	194.3%
Mar	615.65	560.70	91.1%
Apr	547.15	205.80	37.6%
May	492.45	186.75	37.9%
Jun	493.51	68.00	13.2%
Jul	787.54	53.70	6.8%
Aug	678.75	73.30	10.8%
Sep	609.60	204.80	33.6%
Oct	471.45	585.150	124.1%
Nov	577.10	895.90	155.2%
Dec	625.30	1,087.70	173.9%
Total	7,133.200	6,293.40	88.2%

So, our billing usage has varied a great deal. During the summer, we were almost off-grid, though not quite. Next year, I may gear things like when we heat water and use the battery in a completely different way. I certainly started to do that in November and I am keen to see the numbers for January and February to see the difference that makes. Moving the hot water to be heated during the day means we use far less electricity from the grid – using the insulating properties of the house fabric and new windows meant that December’s usage was slightly better than November. (Partly expected as we had big holes in the lounge and master bedroom while building work was going on).

Making the most of the air sourced heat pump has a completely different mindset in play. Heat is more abundant during midday. So it makes sense to use the heat pump to heat hot water, a thermal store if you like.

Likewise, for heating the house. If you have good insulation and fewer loses through your windows, heat the house during the peak of day. Closing thermal bridges has become my favourite hobby over the past few weekends.

Yes, but what is your actual usage, not just what is coming off the grid?

Let’s fill in that table too.

Month	Electricity usage (kWh)
Jan	1,400
Feb	1,200
Mar	1,000
Apr	670
May	590
Jun	447
Jul	405
Aug	432
Sep	546
Oct	749
Nov	985
Dec	1,170
Total	9,594

OK, so basically that is a lot of electricity being used. But you’re generating about 35% of your needs over the year.

Pretty much. The batteries mean we’re actually using that – we have almost no earnings from exporting electricity. Interestingly, Tesla gave us a brake down of our power stowed over the year, and we’re in the top 7% of users who’ve gone off grid!

The huge difference is in the heat pump. It means, we have no other fuel expenditure in the house.

I can pull out our electricity usage for heating and hot water.

One of the important figures here is the SPF. This tells us about the efficiency of our heat pump or for how much power we put in, how much thermal energy (heat) do we get out? We calculate it from (thermal energy) / (heating usage). The estimate from the manufacturer was 3.51, so we’re doing a little better in our little bit of the world.

While ours is doing relatively well, it is worth considering that it is an average, based on the heat available from the air being used as a source. The air temperature gets hotter, the heat pump does better.

So, of our 9,594 kWh being used, 3,966.20 kWh was used to heat our hot water and house. Our car used another 2,000 kWh or so. Our cooking, cleaning (vacuum, washing machine, tumble dryer, hair dryer, steam cleaner, iron), and gardening (lawn mower, hedge trimmer, leaf blower/vacuum), computers, lighting, venting fans in bathrooms, mobile phones, refridgerator, freezer and insulin pump charging used 3,627.80 kWh.

Of course, we’re buying 6,293.40 kWh of that from the grid. Which makes us relatively heavy users.

Is the heat pump better than using gas?

Shall we look at the figures. As before, I am comparing to 2023 as it’s the last complete year’s figures I have.

The table below is a comparison of the gas we used to heat the house in 2023 and the electricity we’ve used to heat the house using the heat pump.

Month	Gas Usage (kWh) 2023	Electricity usage (kWh) 2025	Percentage difference
Jan	2,021.181	925.00	45.77%
Feb	1,931.237	686.00	35.52%
Mar	1,408.429	486.00	34.51%
Apr	735.847	187.00	25.41%
May	353.446	113.00	31.97%
Jun	17.835	38.10	213.62%
Jul	0.000	43.00	100.00%
Aug	62.561	41.40	66.18%
Sep	569.986	90.70	15.91%
Oct	1,657.321	231.00	13.94%
Nov	2,134.510	510.00	23.89%
Dec	2,378.535	615.00	23.92%
Total	13,270.888	3,966.20	29.89%

Basically, the answer to the question, is the heat pump better than using gas? is a no brainer: yes. The month by month comparison shows the ASHP outperforming the gas boiler every time. 13,270.888/3,966.20 = 29.89%. Basically we’ve used a third of the power. And that’s with big holes in our external walls during late November and early December! NB: I may do this again next year, just to show the difference it made in late November and early December.

Oh wait, what about the summer months?

Ah, what you can’t see in the “Gas Usage” figures is the solar diverter we were using from May to August. Rather than use CO₂ generating gas to provide hot water back in 2023, we were using free electricity from our solar cells. It meant our gas totals were artificially lower, but they were still blown out of the water by the energy usage of the heat pump for heating the water.

The solar diverter has a coefficient of performance (COP) of 1, or 1 kWh of energy is converted into 1 kWh of heat. The gas boiler has a COP of 0.8, or 1 kWh of gas generates 0.8 kWh of heat. Our heat pump varies between 3.7 and 5.7, so for every 1 kWh of energy we put into the system, we get 3.7 to 5.7 kWh of thermal energy out – a yearly average of 4.1 or so.

Yes, the heat pump is not as good over the winter, but it is still miles better than a gas boiler, achieving 410% the heat output compared to a unit of electricity bought from the grid.

Basically, you are saying it has all been worthwhile then?

The aim has been to future proof our house and reduce our carbon footprint. By reducing our energy needs, we’ve minimised our running costs against changing fuel and energy prices, to protect against inflation, and give us a predictable base on which to live our lives and plan for our retirement.

I could have spent this money in the stock market. My father was a stock broker, in 2004 he sold stocks in Scottish Power and used the proceeds to buy solar cells. The solar cells gave him a return of 5.7%, way better than Scottish Power shares could have given him at the time. It protect my parents and us from the shocks in the energy supply market following the war in Ukraine.

Plus, like us, it allowed him to reduce his carbon footprint. When we moved in to our house in 2013, our energy use generated 2.4 tonnes CO₂ per annum to give us heat and power. Now it generates 0.3 tonnes CO₂ per annum. I call that a return on investment.

I was already committed to doing what was in my power to help reduce my carbon footprint. My parents showed me a way in which I could effectively do that, without giving up anything but a little thought and money.

I am hoping I have done the same for you. Shown you what is possible, within your budget. Even if it is only switching a light off when you leave a room, or not using a vehicle for a fare stage, or taking a stroll rather than park in the car park closest to the shops, everything helps. You make a difference.

What about the mullah?

As we’ve used 6,293.40 kWh from the grid, at £0.26, that equals £1633.14 in usage or £149.78 a month, including the standing charge of £0.4595 a day. Our house is 244m², compared to an average house size of 85m², so we’re doing OK: a house nearly three times as big as the average house is not paying any thing like three times the average direct debit for energy (which is between £143 and £154 per month including usage and standing charge). We’re not cold, we’re not sitting in the dark, we are running an electric car off that energy too and cooking with it.

Yes, electricity is more expensive than gas, but the efficiencies are costing in for us.

Last year, we managed approx. 35% off grid. By using power shifting with the heat pump settings, my hope is closer to 42%, though that depends on the solar generation figures: we might do better or worse.

Hope this has helped you form you plans for making the shift away from gas central and water heating, whether it’s to a heat pump or infrared panel heaters. Happy 2026.

The UK, that is. We have, as a nation, declared a set of tough targets:

• Net Zero greenhouse gas emissions by 2050.
• Key interim targets include cutting emissions by 81% by 2035 (vs. 1990 levels)
  and
• achieving a net zero power system by 2030.

We’ve got some politically sticky situations about transportation of said power, especially sourced from off-shore wind farms or tidal power plants. It is far cheaper to install and maintain overhead power cables, but these have an impact on residents of the proposed paths. We should put a stake in the ground and see how the UK is fairing now there’s only 4 years left on those original targets for our power system – net zero power by 2030.

How is UK measuring its carbon intensity?

Most of my figures come from NESO – National Energy System Operator – as the UK privatised its energy sector, NESO was put in place to gather such statistics and ensure, along with OfGem, that industry regulations were being met, or better yet, exceeded.

The one we’re interested in today is NESO’s tracking of the UK’s commitment to Net Zero, explained by them if you follow the link above… One of the arguments I often hear against electric vehicles and heat pumps is that coal or gas is being burned to generate electricity – that really isn’t the case any more.

Indeed, it has not been the case for a while now. The last UK coal power plant shut in September 2024. But let’s look at where we are today, or rather November 2025.

We can see from November 2025’s figures that much of our electricity came from wind power. Gas was the 2nd biggest source still, but nuclear came next in the ranking. Hydro power generates 2.2% of the electricity generated in November 2025. The numbers vary month by month, as you’d expect, solar power comes much higher in the ranking in June, making up nearly 11.9% of the mix rather than 2.2% in November!

Importantly, the Zero Carbon figures demonstrate that the majority of the power in the UK is coming from zero carbon sources (wind, solar, nuclear, and hydro) during November 2025. When it is windy, 91% of the electricity generated comes from renewable sources (at 5am on the 30th November 2025)!

In November, our energy demand was 26TWh with peak demand being at 4:30pm on the 20th November of 43,666 MWh at 16:30, when people get home from school and turn on lights and the kettle.

NESO publish an instaneous carbon insensity map, that can help you work out the source of the generated energy and whether you want to move to using greener power. (NB – to see the map on the web page you need to scroll quite far down the page!)

The image above shows how much of the natural gas used in the UK generates electricity or goes into the “Distribution networks” – this is the gas burnt by boilers for hot water and heating purposes, and the sources of that gas. Russia was a major supplier of most of Europe before the Ukraine invasion, that has changed dramatically – those figures would appear under LNG or liquified natural gas imports, America is now the biggest importer of gas to the UK. Stored gas is where cannisters or tanks are used in individual homes.

But let’s get back to our look at where we are in the UK in terms of milestones and records…

Some records broken up to now.

2025 saw Max Solar generation being broken, with 14,023MWh (14GWh) on the 8^th July 2025.

The Max Wind power record was broken on the 5^th December 2025 and saw the power needs in the UK being met when 23,835 MWh (nearly 24GWh) were generated for a period of 30 minutes.

Carbon Intensity, or how much CO₂ is produced for each single kilo-Watt-hour of energy generated, was approx. 177 gCO₂/kWh in 2025 compared to approx. 207 gCO₂/kWh in 2024. Indeed, on the 15^th April 2025 the UK was running with a Carbon Intensity of 19 gCO₂/kWh and on the 1^st April 2025 we ran for 97.7% with Zero Carbon.

As we move to wind and solar power, we need some background generation to fill the gaps when it is not sunny or windy: cloudy days are often the ones where little wind is available to use to generate power.

Two such sources are nuclear and hydro-power. Nuclear makes up 10-20% of the UK generation mix at any point.

Hydro-power is even less. But water could be a great way to “store” excess solar or wind power. Systems which pump water up to a high reservoir when there is a glut of greener power sources and then release it when the sun has set or the wind has died down is a great way to do this – if you have the time check out this youTube entry talking about pumped storage hydropower (PSH).

Now, one thing you’ve seen me talk about is the efficiency of batteries in terms of round trip – pumped storage hydropower is about 70-80% efficient, so not quite as good as a lithium battery, but the real beauty of the system is in the net zero costs. Water is one thing we have in the UK, it doesn’t need to be mined, and more importantly, PSH can store power for hours of use – 10 hours is not a common duration for such a system. Which is difficult to do with a lithium battery.

Now, where I live in the UK is not ideal for a water dam generating hydro-electricity. But systems could and have been built e.g. Dinorwig Power Station in Snowdonia, Ffestiniog Power Station in Gwynedd, and Cruachan Dam in Scotland. Most of the other hydro-power plants in the UK are river and sea systems.

River sourced hydro-power is the most common type of plant in the UK, like Clachan, utilising a dam across a fast flowing systems of streams into a loch and reservoir in Argyll, Scotland.

Personally, it’s one reason I would love to live by a flowing river. An archimedes screw can be installed providing efficient generation of electricity with little harm to wildlife, like the one on the River Stour, Suffolk rated at 11kWh, it provides a third of the energy used by the National Trust’s Field Studies Centre located at Flatford Mill.

Tidal power is of course open to a little island country like the UK. We have a couple of tidal stream systems (one in Orkney and one in Anglesea) and a dam on the Severn Estuary harnessing tidal range power. Less suspectable to draught than river systems, tidal power gives a predictable fall back position but is much more invasive to the environment than archimedes screws. And expensive – expensive and invasive, not ideal. But the UK is breaking ground in this area, leading the technology across the globe.

Of course, even if we do harness that power, we need to transport it back to homes where people can use it, that hold second paragraph in this article. On Farming Today, 14^th December 2025, a home in Northumbria using a diesel generator for its electrical generation was discussing the issues of achieving Net Zero.

Several listeners wrote in asking why not move to using solar power with batteries or at least wind power. The owner of the farm walked jerry cans to the generator because a lorry could not park close enough to the abode to make a delivery! Installing solar cells was not going to be trivial for this family. Also, the chances of supplying enough solar power during the winter to power the house were small, given the get 7 hours 45 minutes of daylight at a h_max of 90°-55°-23.44° ≈ 11.56°. And that’s not mentioning the difficulties of installing said batteries…

Remote locations in England, Northern Ireland, Wales, and Scotland mean that many people do not have access to mains electricity even in the 21^st Century. Please consider that is the case for some in your country when you are sitting in your home in a town or city.

Back to the chase, please? How is the UK doing, please?

OK, but I think I’ve demonstrated the UK has some way to go and needs to ensure no-one is left behind. But we’re making in-roads and every step on that journey will help the UK reach its goal in 2030.

As you know, I’ve been passionate about making the most of the excess power generated in the UK during our peak solar periods.

I am sure I haven’t influenced Australia, but I love this idea and wish it were copied across the world: free-energy-while-the-sun-shines.

Power shifting is something I have spoken about and makes so much sense. Batteries can be up to 93% efficient, but making use of the electricity fresh off the grid makes a great deal of sense. Giving free electricity to all users is a no brainer, as per the example Australia is setting the world. Even if people use it to charge their own batteries.

Dishwashers, water heaters (something we do), washing machines, and tumble dryers are all prime candidates for doing the chores while we work. Slow cookers and timed cooking can do the same for food – making bread while the sun shines, so to speak. We set our heating timers to heat our water during peak solar time to use our water heater as a solar battery, we do a few of the rooms round the house too, dramatically reducing our costs on sunny days.

Rather than running these remotely, a simple timer delaying when the machine starts is a boon for achieving this while you are in the office. It was common in such machines between 2003 and 2015. Slowly the industry moved to wifi control, which is not as good because you are meant to start this at the right time! I don’t know about you, but I am a little busy while I’m working during the day…

What we have been doing that made a bit of a difference is ensure we’re not using power between 15:30 and 19:30 where at all possible – when the UK experiences its peak draw. Heating happens during the day, car charging, etc. Our hot water disinfectant cycle is done between 12:00 and 14:30 every Friday during the winter, and 13:00 to 15:30 during the summer – using Gemini, I work out the next week’s max solar generation times.

Of course, it’s not fool proof. Storm Claudia has been raging today, though the East of England. So I moved our disinfectant cycle to yesterday. Hence, our usage has been lower on the darkest day we’ve had in 2025.

We could hook up our solar diverter to our car – with the heat pump, even on a cold day it makes no sense to use an immersion heater. Why? Well, the immersion heater is 100% efficient (every 1kWh we put in we get 1kWh of heat) but the heat pump is at least 350% efficient, for every 1kWh we put in we get 3.5kWh of heat out. We do not use the solar diverter to heat water, as a result.

We’re at the point where it makes sense to do the right things for the right reasons otherwise you are not helping anything, or indeed making things worse! We’re getting close that being the case.

Optimising from here on in is not going to be trivial. We are going to have to make conscious decisions for each and every step.

Surely, you are doing enough?

Can anyone ever do enough? I ask as it always seems there is some area in which we personally can give a bit more. If enough people do that, we can achieve net zero as a nation, and wider, as a planet.

Call it our little gift over Christmas, and beyond. Merry Christmas, or hope you had a happy Hanukkah.

Please note: I wrote this before the massacre in Australia. I do not agree with what is happening in Gaza or what is happening in retaliation, like these events. I hope we can all find better ways to settle our differences in 2026.