Payback is install cost divided by annual saving, but the annual saving is the part that needs care. Electricity you generate and use yourself avoids the import rate, which defaults here to 24.5 pence per kWh. Electricity you export earns only the export tariff, commonly around 15 pence and sometimes far less, so the split between the two changes the answer substantially.
That split is the self-consumption rate. A house that is empty on weekdays might use only 30 per cent of what the roof produces; one with a battery, a hot water diverter or someone at home can reach 60 to 80 per cent. It is an editable input because it depends entirely on the household rather than on the equipment.
On a £7,500 installation generating 3917 kWh a year with 45 per cent self-consumption, you avoid buying 1763 kWh at 24.5 pence and export 2155 kWh at 15 pence. That is £755.09 a year, giving a payback of about 9.9 years. Panels are normally warranted for 25 years, so the years after payback are the actual return.
The formula
cost- Total installed cost including inverter and scaffolding
generation- Annual output in kWh
self-use- Share of generation consumed on site rather than exported
import- The unit rate you avoid paying, in pence per kWh
export- What your supplier pays for exported units, in pence per kWh
How it works, step by step
- Enter the total installed cost, not just the hardware.
- Enter the annual generation figure, from a quote or the output calculator.
- Set your realistic self-consumption share.
- Enter both the import and export rates; the gauge shows payback in years.
Worked examples
£7,500 system, 45 per cent self-consumption
Of 3917 kWh you use 1763 kWh yourself, worth £431.90 at 24.5 p. The other 2155 kWh exports at 15 p for £323.19. Total £755.09 a year, so payback is 9.9 years.
What a battery changes
Lifting self-consumption to 75 per cent moves 2938 kWh to the import rate, worth £866.74 a year. Payback on the same £7,500 falls to 8.7 years — before adding the battery cost, which must go into the cost field.
How to read your score
Frequently asked questions
Should the battery cost go in the install cost?
Yes, if you are buying one. A battery raises both the cost and the self-consumption rate, so putting it in one field and not the other flatters the result badly. Add its full installed price and then raise the self-consumption figure.
Why does the export rate matter so much?
Because exported units are worth whatever the tariff pays, which is often a third to a half of the import rate. A system that exports most of its output is earning at the lower rate on most of its generation, which lengthens payback considerably.
What about panel degradation?
Modern panels typically lose around 0.5 per cent of output a year and are warranted to retain roughly 85 to 90 per cent at 25 years. It has almost no effect on the payback year itself, but it does reduce the lifetime total, which is why the 25 year figure here applies it.
Does this account for electricity prices rising?
No, deliberately. It holds both rates flat so the result reflects arithmetic rather than a forecast. If import prices rise faster than export rates, real payback will be shorter than shown, and the higher your self-consumption the more that helps.
Is the inverter replacement included?
Not automatically. String inverters commonly need replacing somewhere between years 10 and 15, at perhaps £800 to £1,500. If you want to be thorough, add that to the install cost before reading the lifetime figure.
Solar payback reference
| Install cost | 30% self-use | 45% self-use | 60% self-use | 80% self-use |
|---|---|---|---|---|
| £4,000 | 5.7 yr | 5.3 yr | 4.9 yr | 4.5 yr |
| £5,500 | 7.9 yr | 7.3 yr | 6.8 yr | 6.2 yr |
| £7,000 | 10.0 yr | 9.3 yr | 8.6 yr | 7.9 yr |
| £7,500 | 10.7 yr | 9.9 yr | 9.2 yr | 8.5 yr |
| £9,000 | 12.9 yr | 11.9 yr | 11.1 yr | 10.2 yr |
| £11,000 | 15.7 yr | 14.6 yr | 13.6 yr | 12.4 yr |
| £14,000 | 20.0 yr | 18.5 yr | 17.3 yr | 15.8 yr |
Self-consumption is the single biggest lever here — it is worth more than a cheaper quote.