Solar panel calculator
Start from your roof, get a system size, and see what it would generate, save and export, and how long it takes to pay for itself.
How this is calculated
Usable roof area gives a system size in kWp, using the output per square metre you enter. Generation is that size multiplied by the annual yield per kWp for your part of the country.
Generation splits two ways. What you use in the house saves you the full retail rate you pay. What you export earns the export rate, which is usually a good deal lower. The split between those two is the single biggest lever on whether solar pays.
Payback is the installed cost divided by the annual benefit. The projection then applies your degradation figure year by year, which is why the later years contribute slightly less than the early ones.
Not included: any maintenance, an inverter replacement at around year twelve to fifteen, or changes in electricity prices. A rising electricity price shortens payback and a falling one lengthens it.
Rates used · 2026/27
| Typical yield | 750 to 950 kWh per kWp a year |
| Panel density | about 0.20 to 0.22 kWp per m² |
| Self-use without a battery | 30% to 50% |
| Self-use with a battery | 60% to 80% |
| Export rate | set by your supplier, not the government |
| Inverter replacement | commonly year 12 to 15, not included |
Last checked July 2026. Source: HMRC and gov.uk.
Worked example
40 square metres of roof at 75% usable gives 30 square metres of panels, or a 6.30 kWp system. At 850 kWh per kWp that generates 5,355 kWh a year. Using 40% yourself saves £525 at 24.5p, and exporting the rest earns £482 at 15p, so £1,007 a year against an £8,820 install. It pays for itself in 8.8 years and returns £23,715 over 25.
Common questions
How much roof do I need?
At about 0.21 kWp per square metre, a 4 kWp system needs roughly 19 square metres of usable panel area, so around 25 square metres of roof once clearances are allowed for.
Does a battery make it worth it?
It raises the share you use yourself, which is worth the gap between your electricity rate and the export rate. Whether that repays the battery cost depends on the size of that gap, so try it both ways.
What if my roof faces east or west?
Expect roughly 15% to 20% less than due south, though an east-west split can suit a household that uses power morning and evening. Lower the generation figure to reflect it.
Is the export rate guaranteed?
No. The Smart Export Guarantee obliges larger suppliers to offer something, but the rate is theirs to set and varies enormously. Check what your supplier actually pays before assuming.