Solar Payback Calculator
How long a solar system takes to pay for itself, given what you self-consume rather than export.
Solar economics turn almost entirely on self-consumption.
How the solar payback calculator works
Solar economics turn almost entirely on self-consumption. A unit you use yourself saves the full retail rate; a unit you export earns the feed-in rate, which is typically a quarter of it or less. A system with 30% self-consumption and one with 70% can have paybacks years apart on identical hardware.
Output depends on array size, local sun hours and a performance ratio covering inverter losses, heat, dust and orientation. Around 0.75 to 0.85 is realistic; anything above 0.9 is optimistic.
This ignores panel degradation, which is roughly half a percent a year, and any rebate, which is per-market and dated. Both push the real answer in opposite directions.
Formula: annual saving = self-consumed × retail rate + exported × feed-in rate
Worked examples
| Inputs | Payback period | Note |
|---|---|---|
| 6.6 kW at 40% self-consumption | 5 years 1 month | about 5 years |
| The same system at 70% | 3 years 4 months | markedly faster |
| Exporting almost everything | 10 years 5 months | far slower |
FAQFrequently asked questions
How long does solar take to pay for itself?
Commonly four to seven years, but it depends far more on self-consumption than on hardware.
Why does self-consumption matter so much?
A unit you use saves the full retail rate; a unit you export earns the feed-in rate, often a quarter of it or less.
What is a performance ratio?
The share of theoretical output actually delivered after inverter losses, heat, dust and orientation. 0.75 to 0.85 is realistic.
Does this include rebates?
No. Rebates are per-market and change, so enter the cost you would actually pay after any you receive.
Do panels degrade?
Yes, roughly half a percent a year, so output at year 20 is around 90% of year one. That is not modelled here.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- Ofgem — the UK energy regulator
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.