Four steps and two assumptions. You can check every one of them, and change the ones that are about your household.
This is the only figure we do not make up any part of. It comes from PVGIS, the Photovoltaic Geographical Information System published by the European Commission's Joint Research Centre. It is free, open, needs no registration, and it is the standard reference for this.
We queried it for a 0.8 kWp system with 14 per cent system losses, at one representative point per region, in four mounting arrangements: a garden frame at 30 degrees facing south, an ideal pitched roof at 35 degrees facing south, a vertical south wall, and a vertical east or west wall.
These are ideal conditions. Unshaded, correctly angled, at one city in your region. If a tree, a chimney or a neighbour's extension shades the panel for part of the day, you will get less. Treat the number as a ceiling.
A plug-in panel generates in the middle of the day. Electricity you do not use at the moment it is generated is exported, and you get nothing for it unless you have an export arrangement.
This is where a battery would normally come in, and it is exactly what the rules exclude. A battery on its own, or a kit with one built in, falls outside the approved class. So the share you use is set by your household, not by your kit.
| Someone home during the day | We assume you use |
|---|---|
| Rarely, house empty on weekdays | 35 per cent |
| Some days | 50 per cent |
| Most days | 70 per cent |
These are our assumptions, not anyone's published figures, and the calculator prints whichever one it is using on screen. If you know your own consumption pattern better, adjust the setting until it matches.
Whatever you pay per kWh. The slider starts at 25p, which is a round number rather than a claim. Put your own in, it is on your bill.
Generation, multiplied by the share you use, multiplied by your rate. That is the whole sum. There is no fifth step and no adjustment factor we have not told you about.
Payback would need the all-in cost of an installation, including a socket or circuit work where those are needed. That is not currently sourceable in the UK, so we do not print one.
If you want this done for your actual address rather than a regional point, ask.
Tell us the postcode and where you are thinking of putting it. We will run the numbers for your exact spot rather than a regional average, and tell you if the permitted development rules rule it out before you spend anything.