Most solar calculators give you a big number because of one hidden assumption. Here is how to do it properly, with your own figures.
The four numbers you need
1. Your electricity price On your bill. Currently around 26.32p per unit under the price cap.
2. Estimated annual generation, in kWh, from a quote. As a rough guide, 1kW of panels produces around 850 to 1,000 kWh a year in the UK.
3. Your self-consumption, as a percentage. The share of generation you use at home rather than export. This is the number that decides everything.
4. Your export rate Currently around 12p to 13p per kWh on a flat tariff.
Prices last reviewed 4 Nov 2026.
The calculation
Units used at home = generation x self-consumption %
Units exported = generation - units used at home
Annual saving = (units used x your electricity price)
+ (units exported x your export rate)That is it. There is no complexity being hidden from you.
Why self-consumption decides everything
A unit you use at home saves you the full import price. A unit you export earns you roughly half of it.
So the same system, on the same roof, generating exactly the same amount, is worth substantially more to a household that uses most of it than to one that exports most of it.
Without a battery, a typical UK household uses well under half of what it generates. With a battery it rises substantially. An electric car or a heat pump raises it further.
If a calculator or a quote assumes you will use most of your generation and your house is empty from eight until six, the number it gives you is wrong.
Do it twice
Run the calculation with the installer’s self-consumption figure, then again with a figure ten percentage points lower.
If the case only works on the optimistic version, treat the quote with caution. If it works on both, you have a robust answer.
What to add to the picture
Not in the calculation but real:
- Electricity prices have generally risen over time, so a saving against future prices is worth more
- If you later add a heat pump or an electric car, your self-consumption rises and the system improves without further spending
- Panels last well beyond any payback period
Working against you:
- The inverter will need replacing at some point
- Output falls slightly each year
- There is currently no VAT on electricity from 1 Oct 2026 to 31 Mar 2027, so the electricity you are avoiding buying is temporarily cheaper and your saving temporarily smaller
The figure with a source behind it
Government’s own estimate is around £500 a year for rooftop solar, from the Solar Roadmap. It is an average and it will be wrong for any particular house, but it is the one savings figure on this site that has a primary source behind it.
Source: Solar Roadmap. Checked 4 Nov 2026.