The Powerwall was installed at the end of February 2022 so there’s now data from a full year of it working alongside the solar panels and heat pump to analyse. Here we go…
Headline stats
| House total energy consumption | 8637 kWh |
| Import from Grid | 4770 kWh |
| Total Solar Generation | 4405 kWh |
| Export from Solar | 531 kWh |
| House power from solar % | 45% |
| Solar self consumption % | 88% |
| Total Electricity bill (excluding standing charge) inc VAT | £732 |
| Average cost per kWh – inc solar/exc solar | 8.5p / 15p |
| All data Mar 2022 to Feb 2023 |
The above figures are close but not exact – the bill cost comes from our actual bills which don’t quite line up with month ends – over a year a few days one way or another won’t make much difference. Next year will be able to do better as Home Assistant is tracking the data every day.
One number stands out – only £732 for electricity for the whole year – that’s heating, hot water, cooking, lights, etc. and 5000 miles plus of EV driving. We were amazed. How come?
At the current (Feb 23) price cap rate of 34p per unit 8637 kWhs would cost over £2900! There’s three factors that combine to get the bill right down.
- EV Tariff
- Charging battery overnight
- Using more of the solar energy we generate
We have an EV – so can take advantage of EV tariffs which offer really low rates for a few hours each night. For all last year we were on Octopus Go Faster. Five hours overnight at 8.3p/kWh, the rest of the time it was 30.8p per unit. Just changing from price cap to Octopus go would get the cost down to just over £2000. So simply having an EV saves us £900 on our electricity bill!
The battery can charge overnight at these cheap rates so we can use the cheap electricity during the day cutting down our use of peak rate energy.
| Energy Source | Grid Import | House Consumption |
| Cheap rate | 3505 kWh | 2080 kWh |
| Peak rate | 1199 kWh | 2172 kWh |
| From Solar | -531 kWh (export) | 3874 kWh |
Charging the battery overnight shifted 973 kWh of consumption from peak to cheap rate – saving around £220 at last years rates, next year will save around £300 on current pricing. By maximising our use of cheap rate – laundry, dishwasher, EV charging – we can really get the average unit cost down – excluding solar our average cost per kWh over the whole year was about 15p.
Our solar panels provide almost half of our electricity for free – the battery more than doubles the amount of solar we can use.
Here’s some stats from the solar PV system showing how solar use has changed since the battery was installed – these compare 2021 to 2022 so not exactly the same as the headlines table but close enough.
Before the battery we were using 40% of the solar we generated, with the battery that’s up to 88% – around 2000 kWh of peak rate electricity that we don’t have to import – that’s about £600 in 22/23 and will be more like £800 in 23/24 if prices don’t change. Including Solar in the average rate calculations brings our cost per kWh consumed down to only 8.5p.
Don’t get too excited about all that export we lost – export units only earn 4.1p – so £100 in 2021, £20 in 2023. It’s far better to self use solar generation if possible.
Yes, we got lucky – fixing on Octopus Go faster before prices soared. Our Go Faster tariff ended at the end of February, using the new prices our electricity bill for last year would have been around £920, a 25% increase – actually a bit higher as the cheap rate period has dropped from 5 to 4 hours.
In short the EV, solar panels and battery combine to drastically reduce our electricity bill – genuinely didn’t believe the numbers until checking everything twice for this post!