Finding the chase and cutting to it

This is another look at how our heat pump is working for us.

Our home energy set-up is reasonably straight-forward.

We have power coming in from the grid and our roof-mounted solar panels. The power from the roof is either used directly, or stored in our batteries (27kWh of storage).

We also have an EV, a 2014 Nissan Leaf (24kWh battery) and a vehicle to grid charger, so theoretically, we can draw from that source too. (We haven’t done that for a while).

We then have our air sourced heat pump, providing all our hot water and any heating we need.

Like everyone else, we have fridges, ovens, toasters, hobs, microwave ovens, computers, TVs, LED lighting, exhaust fans in bathrooms and the kitchen, vacuums, steam cleaners, an iron, dishwasher, washing machine, a condensing tumble dryer, hair dryers, etc. The modern tools of life.

Cut to the chase: how much power did you use over the summer, please?

Oh, yes, that’s why we’re here.

Firstly, summer is everything from the 1st June 2025 to the 31st August 2025.

Weather-wise, it has been a warm, dry season in the East of England. A little cloudy but being near the coast, we have that cloud on occassion. It makes our temperatures a little cooler, but does mean we don’t get quite as much power off our roof. So, let us look at those figures first.

Solar generation.

Month	Power generated (kWh)
June	497.6
July	393.3
August	422.1

Which is a reasonable year, not great, but not terrible.

Our usage is broken into two categories: heat and hot water, and general use.

Month	Heating (kWh)	Other (kWh)	Total (kWh)
June	38.1	409.5	447.6
July	43	361.6	404.6
August	41.4	390.2	431.6

Heavy users!

Yes, but some of that is the car. A good 100kWh a month, at that point we’re looking at much more usual usage.

So, what energy have we bought from the grid?

Month	Generated (kWH)	Used (kWH)	Net (kWH)	Bought from the grid (kWH)
June	497.6	447.6	-50	68
July	393.3	404.6	11.3	53.70
August	422.1	431.6	10.5	73.4

Hold on, how come you’re generating more energy than you’re using in June and still buying from the grid?

The batteries are not perfect, we get between 10% and 11% loss from the power stored in the battery – so for every 1kWh stored, we get 900Wh or 890Wh out. This is called the round trip efficiency, and is given in terms of 100%-loss %, so ours is 89% to 90% efficient.

We buy some power from the grid – the battery software does its best to match our demand with our usuage but without going into “island mode” and delibrately going “off-grid”, that is going to happen.

We can’t programme the system to only use the battery during the summer. It has to be done manually, which needs a great deal of discipline, and working full time, that is a big ask.

It’s also likely to hurt the batteries if we drain them consistently – there are definitely days we could have been off-grid without dropping below 20% of the battery, but it is hard to spot them ahead of time…

We have managed to donate some back to the grid too – the Powerwall does try to even out what it takes from the grid with what it donates. That is captured by a separate meter and for us is about 170 kWh which earns us our SEG (solar export guarantee) is just under £0.12, so circa £20.

What about the moolah?

It’s so dirty to talk about money – only kidding.

So, we have 92 days of standing charge, and 195.10 kWh of usage, giving us a total of £49.20 for energy charges and £40.25 of standing charge.

Summer we paid £89.45 for using that energy off the grid, or £29.81 a month. (We could say that we take the SEG money off this bill, which would be £69.45 or £23.15 a month).

I know some are doing better than this by using their batteries to store energy from off peak times, but I don’t think £29.81 is too bad, when 46% of that is from the standing charge and VAT.

Our standing charge rate is relatively low – in our circumstances, that makes much more sense as there are some days we only pay £0.04 for the electricity we use off the grid…