How this calculator works (our assumptions, in the open)
Most solar "calculators" either hide behind a lead form or bury their assumptions. We put ours on the page, because the numbers only mean something if you can see where they come from. Here's exactly what drives your estimate:
- Your usage — we estimate your annual kWh from your bill, netting off a standing charge of roughly £208/year and dividing by the 26.11p unit rate (the Ofgem price cap for 1 July–30 September 2026).
- Generation — about 950 kWh per kWp per year for a well-sited UK roof, adjusted for your roof direction (south is best; east/west loses ~15%; north far more).
- Self-consumption — the share you use at home rather than export: ~45–50% without a battery, rising with daytime use, and up to ~80–90% with a battery.
- The value of each unit — a self-consumed unit avoids ~26p of import; an exported unit earns ~12p (Octopus Outgoing, as at July 2026). That's why using your own power is worth roughly double selling it.
- Cost — about £1,500–£2,000 per kWp installed with 0% VAT (residential solar is 0% VAT until 31 March 2027, then 5%); a battery adds roughly £4,000–£5,000.
- Payback — indicative cost divided by the annual saving, shown as a band. UK domestic solar typically pays back in 7–12 years on panels warrantied for 25 years or more.
What the calculator can't see: shading from a chimney or a neighbour's tree, your roof's condition, your exact half-hourly usage, or the specific quote you're offered. Those can move the result meaningfully, which is why the honest figure only comes from a survey. Use this to decide whether it's worth getting quotes — then let an MCS-certified installer confirm the detail. You can also cross-check against the independent Energy Saving Trust calculator.
What the numbers usually look like
For a typical 3–4 bed home, a 4 kWp system costs around £6,000–£8,000, generates ~3,400–3,800 kWh a year, and — with moderate daytime use — saves roughly £400–£700 a year, for a payback around 9–12 years. High daytime users, EV or heat-pump homes and larger 5–6 kWp systems can do better; small bills, north-facing roofs, shading, or being out all day do worse. If your result looks marginal, that's useful information — see when solar is not worth it before spending anything. For the full breakdown, read how much solar panels save and the payback period explained.