About SunSum and how the calculator works
SunSum is a free UK solar panel cost calculator built on published UK irradiance, pricing and Smart Export Guarantee data. This page explains who runs it, exactly how the estimates are calculated and how the service stays free.
What SunSum is
SunSum is a free, independent solar panel cost calculator for UK homes. Answer eight quick questions and it returns an indicative system size, install price range, annual saving and payback period for your property. It is not an installer and it does not sell panels: it exists to give homeowners a realistic, pressure-free starting point before they speak to anyone.
SunSum launched in the North East of England and now covers the North of England, with national figures available for any UK postcode.
How the calculator works
Every estimate is modelled rather than guessed, using published UK data:
- Generation starts from the UK average of about 850 kWh per kWp per year, adjusted by a regional irradiance factor for your postcode area (the North of England runs at roughly 93 to 95% of the UK average).
- Roof direction and shading scale that generation: a south-facing unshaded roof keeps close to 100%, with east and west-facing roofs typically 80 to 85%.
- System size is recommended from your electricity usage, inferred from your monthly bill, and your property type.
- Install prices use 2026 UK market bands: most homes fall between £5,000 and £12,000 fully installed, with 0% VAT applied to residential installations.
- Savings combine the grid electricity you stop buying at around 27p per kWh with Smart Export Guarantee income at roughly 15p per exported kWh.
- Payback is the installed cost divided by the total annual benefit, which puts most UK homes at 6 to 10 years.
Why we show ranges, not quotes
Anyone giving you an exact price without seeing your roof is guessing. SunSum deliberately shows ranges built from real install data, and the binding figure comes from an MCS-certified installer after a roof survey. We would rather be honestly approximate than precisely wrong.
How the free service is funded
SunSum is free for homeowners and there is no charge at any point. If you choose to be contacted after your estimate, we pass your details, with your consent, to a single trusted MCS-certified installer covering your area, and that referral is how the service is funded. One local company, not a list of firms competing for your phone number. If you are only researching, you can take the estimate by email and nobody calls you.
Data and privacy
We store only what the estimate and any consented installer follow-up require, and we are ICO registered. You can request deletion of your data at any time. The full detail is in the privacy policy linked in the footer.
Contact
Questions, corrections or feedback on the methodology are welcome at [email protected]. If a figure on this site looks out of date, tell us and we will check it against current market data.