I’m aware this post is at risk of coming across as very entitled, given how many people are struggling with energy bills and the general cost of living crisis at the moment – and almost certainly facing a further large increase in energy bills in January 2027. The intention is simply to provide a fact-based real-world illustration of what can be achieved with a high-performance, all-electric home and a significant up-front investment in on-site electricity generation and storage – which I realise most people are not privileged to be able to afford.
The Tesla Powerwall 3 solar PV inverter and battery system was commissioned on 31 October 2025 and so has just completed a full year of operation, alongside the original (much smaller) SolarEdge PV inverter installation. Both the Generation and Consumption statistics are highly seasonal (Generation being highest in Summer and Consumption highest in Winter, due to the electric heating) so it doesn’t make much sense to analyse anything less than a full year of data.
The ‘official’ kWh readings from the supplier’s SMETS2 meter are summarised below:
| Import (kWh) | Export (kWh) | |
| 2025-11-01 | 34,038 | 10,390 |
| 2026-11-01 | 47,228 | 27,005 |
| Difference | 13,190 | 16,615 |
In total, that’s a Net Export of 3,425 kWh – i.e. there was more than 3.4 MWh of ‘excess’ solar PV generation after taking account of all of the on-site usage (including on-site EV charging). Note that the Import figure is not the same as Usage; there was significant off-peak Import used to charge the Battery overnight and then Exported the next day – although some of the Usage was also satisfied by solar PV generation. The data from the Powerwall says that the total ‘Home – Used’ figure was 13,676 kWh – although that seems like a lot, based on the sub-metering for individual devices; for example:1
- The Ground Source Heat Pump in the House used about 1,500 kWh
- This covers Central Heating and Hot Water
- The Air-to-Air Heat Pump in the Outbuildings used about 1,200 kWh
- Including some usage for Air Conditioning in the Summer
- The Electric Car Charge Points used about 750 kWh
- The two MVHR ventilation systems used about 525 kWh
The translation of the Net Export figure into pounds and pence is not a simple calculation since it depends on the tariff rates for Export and Import, which vary by time of day. However, the 40 kWh of battery storage means that very nearly all of the Import was at the off-peak rate (currently £0.066 per kWh courtesy of Intelligent Octopus Go) and all of the Export was at its flat rate (currently £0.12 per kWh). Using those figures and ignoring the fact that the rates were slightly different historically and there was some VAT involved:
- The Imported electricity cost £870.52
- The Exported electricity earned £1,993.71
Adding in the standing charges (£185.24 for the year) gets to an overall annual total of -£937.96 (the negative sign indicating that Octopus Energy paid out over £900).
The actual monthly credits and debits on the energy bill over the course of the whole year confirm this figure: almost exactly -£900.
The original intent was not really to make money but more to have the property generate at least as much zero-carbon energy as it consumes. Achieving that with a 3.4 MWh ‘buffer’ (for additional consumption – e.g. additional EV mileage – or reduced generation) feels about right. Whether or not this net export results in a profit depends heavily on the available tariff rates. In future years, the trend is surely for summer-daytime solar generation to exceed demand, making the value of middle-of-the-day exports effectively zero and so reducing the income available from exporting excess solar generation.
- It’s not completely clear where the losses resulting from charging and discharging the battery get accounted for. I’ve noticed that the ‘Home’ consumption figure seems to increase by a few hundred Watts when the battery is either charging or discharging heavily, which would be tending to artificially inflate the ‘Home – Used’ figure and might explain why this seems high. ↩︎
Solar PV Generation & Storage – First Full Year Summary by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.