Solar Panel Calculator: Savings and Payback for Your Roof
Work out how much electricity solar panels on your roof would make, how much of it you’d use, what they’d save and how many years they take to pay for themselves, with or without a battery. For the UK, Ireland and mainland Europe, in € or £.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Work out your solar
Solar Panel Calculator
See what solar panels on your roof would make, how much of it you'd use, what they'd save and how many years they take to pay for themselves, with or without a battery.
1 Your roof
2 Your electricity
On your bill or your supplier's app. A typical home uses about 2.700 kWh a year in the UK and 3.500 kWh in Spain or Ireland.
When you're home Battery3 What it costs
Pays for itself in
–
Yearly output
0 kWh
Your use it covers
0%
Gain over 25 years
0
Where the power goes
Breakdown
Rules and grants where you live
Rules and payments change: check the current position with your installer.
What to ask installers to quote for
Get at least three quotes for the same system so you can compare them.
What's next?
How it's worked out: yearly output = panels × watts × sunshine for your area (PVGIS, EU Joint Research Centre, best angle, 14% system losses) × a factor for the roof's direction × shade. Used at home directly: the smaller of the output and your yearly use × 30%, 45% or 60% by when you're home. A battery adds up to its size × 0,9 × about 150–300 useful cycles a year (more in sunnier areas). Saving = power used at home × electricity price + power exported × export payment. Payback = cost ÷ yearly saving. The 25-year figure assumes output drops 0,5% a year and prices stay as entered.
Estimates only. Real output depends on the exact angle, shade and weather, and savings on your tariff and how you use power. Quotes vary a lot, and the inverter usually needs replacing once in 25 years.
How it works
Output, use and payback
Solar panels save money in two ways: the power you use at home instead of buying it, and the payment for the power you export. The calculator works out both and compares them with the cost. The sources and assumptions behind it are on our How we calculate page.
What the panels make
Each kW of panels makes a set amount a year where you live: about 900–1,000 kWh in the UK and Ireland, and 1,500–1,600 kWh in southern Spain (PVGIS, the EU’s solar data). A south-facing roof gets it all; east or west about 18% less, and shade cuts it further.
How much you use
Panels make most power around midday, so you only use part of it as it’s made: more if someone is home in the day. A battery stores some of the rest for the evening. Power you don’t use goes to the grid.
What it saves
Power you use saves your full electricity price; exported power earns the export payment, which is usually much less. Add the two for the yearly saving, then divide the cost by it to see how many years the panels take to pay back.
Examples
How long do solar panels take to pay back?
The same calculator for a few typical homes. Sunnier places pay back faster, and a good export payment helps as much as more sunshine.
Illustrative only, from the calculator above: 4.3 kWp unless stated, some daytime use, no shade. UK: 2,700 kWh a year at £0.26, export 12p, £1,650 per kWp and £600 per kWh of battery. Ireland: 3,500 kWh at €0.30, export 19c, €1,900 per kWp less the €1,800 SEAI grant. Spain: 3,500 kWh at €0.30, export 7c, €1,300 per kWp. Your own quote and tariff will differ.
Tips
Before you get quotes
A few checks make it easier to compare quotes and get the most out of the panels once they’re on.
Size it to your use
Panels that make far more than you use mostly earn the export rate, which pays back slowly. Size the system to your yearly use, unless you plan an electric car or a heat pump, which use a lot of daytime power.
Compare like with like
Get at least three quotes for the same number and size of panels, and check each includes scaffolding, the inverter, any battery and the paperwork for export payments or a grant. Compare the cost per kW.
Use the power in the day
Once the panels are on, run the washing machine, dishwasher and tumble dryer when the sun is out, and charge an electric car at midday. Each kWh you use instead of exporting saves the difference between the two prices.
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Common questions
Frequently asked questions about solar panels
Common questions about the size, output, cost and payback of solar panels on your roof.
How many solar panels do I need?
Enough to make roughly what you use in a year. A typical UK home uses about 2,700 kWh, and each kW of panels makes about 1,000 kWh a year in southern England, so about 3–4 kW: 8 to 10 panels of 400–450 W. In southern Spain the same panels make about 60% more.
How much do solar panels save a year?
It depends on how much of the power you use at home and what you’re paid for the rest. A 4.3 kW system in southern England saves about £700 a year with a 12p export payment; the calculator works it out for your own home, tariff and area.
How long do solar panels take to pay for themselves?
Usually 6 to 12 years. In the UK about 10 years for a typical system; in Ireland about 6–7 years with the SEAI grant and the Clean Export Guarantee; in central and southern Spain about 6–7 years. Panels usually last 25 years or more.
Is a solar battery worth it?
A battery lets you use more of your own power in the evening, but it adds a lot to the cost, so it often makes the payback longer. It’s more worth it when the export payment is low, or with a time-of-use tariff that lets you charge it cheaply at night.
Do solar panels work on an east or west facing roof?
Yes. East or west roofs make about 18% less than a south-facing one over a year, but spread the power over the morning and evening, when you’re more likely to use it. North-facing roofs make about a third less and are rarely worth it.
Do I need planning permission for solar panels?
Usually not for a house: in the UK and Ireland most roof panels count as permitted development. Flats, listed buildings and some conservation areas need permission. In Spain, check your town hall’s rules; many towns also give IBI or ICIO tax discounts.
How much do solar panels cost?
In 2026, about £1,500–1,800 per kW fitted in the UK (around £7,000 for a 4.3 kW system), €1,800–2,000 per kW in Ireland before the €1,800 SEAI grant, and €1,000–1,500 per kW in Spain. A battery adds roughly £/€500–600 per kWh.
Are the results exact?
No. They are estimates: output depends on the exact angle, shade and weather, and savings on your tariff and how you use power. Quotes vary a lot, so compare a few, and expect to replace the inverter once in 25 years.