Every figure on this site should be traceable. This page explains where our numbers come from and how we calculate them.
Installation costs
UK. We use installation prices recorded by certified installers through the MCS scheme and published government cost data, cross-checked against published price guides. We show a range and, where available, the recorded average.
US. We show two figures: the median price quoted on comparison marketplaces, and the industry-wide average. The gap between them is real and we explain it on our cost pages.
Savings and payback
We calculate savings as:
- the solar electricity you use yourself, valued at the price you would have paid for it, plus
- the electricity you export, valued at your export rate.
Payback is the installed cost divided by the yearly benefit. We show a range, because the result depends on how much power you use at home and which export tariff you choose.
Our examples do not assume electricity prices will rise. If they do, payback will be shorter.
The assumptions we use
| Assumption | UK | US |
|---|---|---|
| Electricity price | Ofgem price cap unit rate | Average residential rate for the state |
| Output | Regional generation per kW, adjusted for roof direction | State generation per kW, adjusted for roof direction |
| Export value | A stated export rate, which you can change in the calculator | The state's or utility's export rules |
How often we update
- Prices and savings: reviewed regularly, including when the UK price cap changes.
- Grants, loans and incentives: reviewed when the rules change.
- Export rates: reviewed regularly, because they change often.
Every guide shows the date it was last updated and the month its prices were checked.
Sources
Each guide lists its sources at the bottom. We prefer primary sources: government departments, regulators and official scheme data.
Corrections
If you spot an error, tell us. We correct mistakes as soon as we can.