Sorry guys, but size does matter
Get your minds out of the gutter, I’m talking about solar generation and battery size. When we first bought our system, we were recommended to get 27kWh of battery. We didn’t have the spare cash when we bought our initial system, so we bought half of that.
Having just shelled out for the heat pump, we were looking to spread the cost and see what we actually need. We might have been left without the option to do so, but thankfully, that was not the case, though getting any more capacity might be tricky.
Generally, over the winter, the 13.5kWh battery we bought did pretty well. It’s the first day of meteorological spring in the UK, as we are getting better 3.0 and 3.5kW off our roof and we only need 1.4kW to run the house (heating, water heating, and the washing machine), the rest is being stowed in the battery and when a cloud passes, the battery is filling in the dip. So far today, 22% of our power has come from the sun, 3% from the powerwall (charged by the sun) and the rest from the grid – 19.4kWh of which have warmed the house and heated the water – I have not only washed my dark delicates but had a lovely bath earlier.
Now, I do know some hardy souls who turn off the heating as soon as spring starts, they just heat the water. We don’t do that – I am not saying the house is heated to 22°C, or that we’re wearing summer clothing, I love my jumpers and warm socks, so a barmy 16-19°C suits us, the lounge is currently 18.1°C which is where I am writing this piece, largely achieved by the infrared power of the sun heating the room. Solar gain is a beautiful thing, this time of year.
Indeed, I am making use of it to warm up the veggies I am using in a butternut squash soup later, which if I’m lucky, can be cooked from power from the battery.
Yawn, so what?
This, oh so boring shit, is so worth doing. I am enjoying all the benefits of modern life, without having to pay for the power. At £0.2603 per kWh, this matters.
For February, we were charged for 1,044.60 kWh of electricity, but our usage was 1,179.4 kWh. Our heat “usage” over February was 2,720kWh, while our “spend” was 685 kWh. That gives us a seasonal coefficient of just under 4 (from 2720 divided by 685). Not bad.
Semi-interesting, go on…
Whatever! We’re not turning our central heating off – it’s between -2 and 0°C in the UK around 5am and I like to be warm. So, I am enjoying the sunshine today.
Hope you are too! But going forwards, we have now got that 27kWh of battery. Which means, during the summer, we are off grid – a good solar day for us is 20kWh coming off the roof – if we are in the office (i.e. not heating the house, boiling the kettle, or cooking food), that power goes out to the grid.
In the UK, that is not a disaster, we get paid for that “sale of power”, though in reality, it’s our neighbours who potentially get to enjoy the use of that greener energy.
The battery changes that: now it’s summer, electricity prices are now £0.2602 per kWh, so everything we can use gives takes money off our bill. But thanks to the longest days being upon us, our usage is down too – barely 11kWh, so a full battery or good day of solar means we have enough electricity stowed for a really dark couple of days.
On average, during June, we fill up our battery easily by midday. We barely empty it either.
Timing our dishwasher to go on at 2pm (our peak generating time) means that we get less money for our exported energy but are paying significantly less for our electricity.
We’re doing the same with the EV. Nothing gets charged while the sun is not giving power. Not the house battery, nor the car. Phones, watches, laptops, all wait until the sun is providing.
Our Samsung Smart things does some of the watching for us. Making handy hints of when to put on the washing or dishwasher. Or make a cup of really strong tea…
Post script: it’s not what you do, but the way that you do it.
One of the things to watch with this connected to the grid but drawing as little as possible life style, is the rate things are powered at…
What does that mean? Well, a fast kettle in the UK draws about 3kW for a very short time, but it can mean you need more than your solar panels are giving just at that moment. Switching to a 1kW takes longer to boil (same amount of energy but over a longer time frame) but probably keep within the levels being produced by the solar cells. The difference is, with the 3kW kettle, you will be paying the energy company even though you have solar cells.
The electric car is a great example of this. We have 10kW, 5kW, and 2.5kW chargers: if we’re using the grid, it makes sense to go for the 10kW one for as short a time as possible, where-as for the solar cells, the 2.5kW makes much more sense. Limiting the charging to the suniest part of the day, also makes a great deal of sense. But the downside is, it’s going to take much longer to charge the car.
My parents had bought some solar cells some years prior to retirement. Retirement meant a huge reduction in earnings, so they did this to the ulitmate extreme and, please bear in mind, they didn’t have a battery.
So, they recorded things on their sky box and watched them during daylight hours. All cooking was done at midday and either thigs were reheated or eaten cold in the twilight. They went to bed when the sunset and time shifted, to only be awake during the daylight hours. They got their electricity bill down to less than £30 a month on average, and vast majority of that was the standing charge.
A small part of me really admires this, but with a full time job, it’s not something we have to do.
Posted: July 26th, 2025 under Driving off the grid.