Work out what an 800W kit actually pays back
Most payback figures you will read are a single national average. Yours depends on what you paid, what your electricity costs, and how much of the generation you are home to use. Change the six numbers below and the ledger on the right recalculates as you type.
| Line | Amount |
|---|---|
| Annual generationSystem size multiplied by generation hours | — |
| Used at homeDisplaces units you would have bought | — |
| ExportedSold back to your supplier | — |
| Annual bill savingHome-used kWh at your import tariff | — |
| Annual export incomeExported kWh at your SEG rate | — |
| Total annual saving | — |
| Ten-year net positionTen years of saving, less what the kit cost | — |
Read this before you quote the number. gov.uk estimates £70–£110 a year for an 800W system; your result depends on orientation, shading and how much you are home in daylight. This is arithmetic on the figures you entered, not a forecast.
How this is calculated
No hidden fudge factor. Four lines of arithmetic, run in your browser:
annual generation (kWh) = (system watts / 1000) x generation hours
used at home (kWh) = annual generation x used-at-home %
exported (kWh) = annual generation - used at home
bill saving (£) = used at home x import tariff / 100
export income (£) = exported x export tariff / 100
total annual saving (£) = bill saving + export income
payback (years) = kit price / total annual saving
ten-year net (£) = (total annual saving x 10) - kit price What this model does not do
- No panel degradation. Real modules lose roughly 0.4–0.5% of output a year. Over ten years that is a few percent off the later figures.
- No tariff inflation. Electricity prices rising would shorten your payback; falling prices would lengthen it. Both are guesses, so we model neither.
- No discounting. A pound saved in year eight is treated as a pound, not its present value.
- No inverter clipping. If you enter a panel rating well above 800W, a compliant plug-in inverter caps AC output at 800W and the model will flatter you. The calculator warns you above 1,000W.
- No standing charge, VAT or maintenance. Domestic solar is currently zero-rated for VAT in the UK; a kit price you type in is taken at face value.
Where the defaults come from
- 950 generation hours — the middle of the band MCS and the Energy Saving Trust use for a well-sited south-facing UK array.
- 26.5p import — approximately the Ofgem price cap electricity unit rate. Your bill is the authority, not this figure.
- 15p export — a mid-range Smart Export Guarantee rate. Rates vary by supplier from roughly 1p to 25p.
- 50% used at home — a deliberately unheroic assumption for a household that is out for part of the day.
If you think one of these is wrong, change it. That is the point of the page. The guides set out the underlying rules, and the ledger lists what certified kit currently costs.
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Track the real number, not the estimate
This page models what your kit should do. The Solar Savings Ledger is the spreadsheet and printable logbook for recording what it actually did — daily generation, monthly savings, export income and payback-to-date against what you paid. After a year you stop estimating.
See the Solar Savings Ledger — £4.99One-off payment, instant download. No subscription.
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