Our sources and solar calculations
Here’s where our information comes from and how we work out the estimates you see on the site.
Our main sources
- Energy Saving Trust: practical advice about panels, batteries, costs and savings.
- Government, Ofgem and councils: funding, tax, planning and export-payment rules.
- Electricity suppliers: tariff rates, application requirements and contract terms.
- MCS and consumer protection organisations: installer checks, certification and payment protection.
Each guide links to the sources used. Rules can differ across the UK, and suppliers can change their offers, so check the linked guidance before applying or agreeing to work.
A few useful terms
- kWh
- A unit of electricity, as shown on your bill. It measures how much electricity you use, generate or send to the grid.
- kWp
- The combined rated power of your panels under standard test conditions. It is a way to describe system size.
- Solar electricity used at home
- The share of your panels’ output used by your household instead of being sent to the grid.
- Export rate
- The payment for each unit of spare electricity you send to the grid.
Estimating electricity generation
The amount your panels can produce depends on their size, your location, roof direction, roof slope and shade. Our optional postcode lookup uses PVGIS, a solar resource from the European Commission’s Joint Research Centre, to estimate output for a location.
A postcode estimate cannot see a nearby tree or check your roof. Ask your installer for an estimate based on a survey, with the effects of shading explained.
Estimating savings
The calculator adds the money saved by using solar electricity at home to the payments for spare electricity sent to the grid. It subtracts your yearly upkeep budget.
- Bill savings: electricity used at home × the price you pay per unit.
- Export payments: spare electricity sent to the grid × the export rate.
- Years to cover the cost: installation price ÷ the first year’s net savings and payments.
The 20-year result also allows for the yearly fall in panel output you enter. It keeps electricity prices and the annual upkeep budget unchanged and subtracts the initial installation cost.
Loan interest, separate roof work and future replacement bills are not calculated individually. Allow for these when judging affordability. A battery needs its own cost and savings assessment; the calculator does not model battery charging schedules or storage losses.
Understanding the example figures
The calculator starts with example figures so you can try it straight away. They are not a recommendation for your home. Replace them with your bill, quote and tariff details wherever possible.
Try the solar savings calculator.
About the export comparison data
Our July 2026 comparison data records selected suppliers’ published rates and conditions on the date shown. “Not stated” means the supplier did not clearly answer that point in the information we checked. It does not mean the condition cannot apply.
The downloads include source links and reuse information. For a current offer, follow the supplier’s link and read its latest terms.
Tell us about a correction
If a source has changed or a calculation needs a clearer explanation, contact us with the page link.