1. What size system are you considering?
Each kW needs roughly two to three panels' worth of roof. Larger systems need more roof area.
2. Where in Great Britain is your home?
Southern England and South Wales get the most sun; the North of England and Scotland get less. Each option is one city's figure, so pick the nearest. Prices are for Great Britain, so it doesn't cover Northern Ireland.
3. Which way does your roof face?
Pick the closest match. North-facing roofs lose nearly half the output, so payback is much longer.
4. Adding a battery?
A battery stores spare daytime solar to use in the evening. How much that adds depends on how much you use: less with Ofgem's typical 2,500 kWh a year than in a home that uses more. We assume a 10 kWh battery, 8 kWh of it usable, and that extra use (an electric car, say) is spread like the rest; charged overnight, it uses less of the solar.
Check a recent bill, or pick the closest match below.
Your estimated annual benefit
£599a year
Over 25 years that's around £14,976 of bill savings and export income from 3,800 kWh of generation a year.
Bill savings
£263 / year
999 kWh self-used at 26.32p
SEG export income
£336 / year
2,801 kWh exported at 12p
Total annual benefit
£599 / year
Upfront cost and payback
- 4 kWp solar system (0% VAT)
- £6,600
- Total upfront
- £6,600
Payback period
11.0 years
25-year net return
+£7,576
Like the look of £599 a year?
These are estimates. The only way to know your real return is a free quote from MCS-certified installers. Takes about 60 seconds, no obligation.
How we worked this out
Energy prices: Ofgem price cap for 1 October to 31 December 2026 (Direct Debit, GB average): electricity 26.32p/kWh. SEG export rate 12p/kWh (Octopus Outgoing).
Solar generation: Regional yield, south-facing: Midlands 950 (the site-wide figure, our estimate), London and South East 1005, South Wales 1009, North West England 869, Central Scotland 897kWh per kWp a year, scaled by PVGIS for London, Cardiff, Manchester and Edinburgh against Birmingham. Each is one city's figure, so pick the nearest. On PVGIS, south-east or south-west costs about 7%, east or west 22%, north 46%.
Self-consumption: BRE's SAP 10.2 self-use formula, fitted to measured homes, for your system and yearly use, month by month (we spread the solar by PVGIS's Midlands months and your use by SAP's seasonal pattern). Against BRE's own worked example it comes out slightly high. With a battery we assume 8 kWh usable. The rest is exported at the SEG rate.
Install costs: DESNZ's mean for home installs of that size, with 0% VAT until 31 March 2027; a battery at the middle of the Energy Saving Trust's range, £6,500 (it gives no size).
Lifetime: 25 years, the Energy Saving Trust's "25 years or more" for panels, with one inverter replacement at £800 around year 12 (the Energy Saving Trust's figure and timing). A battery may need replacing within that, which isn't allowed for.
Estimates only. Real-world returns depend on shading, panel quality, inverter performance, your tariff, and how much usage you can shift to daytime. Get an MCS-certified roof survey for a firm quote.
The Short Answer
In our view, if the payback comes in comfortably inside the time you expect to stay, it's worth getting quotes.
At the calculator's defaults, a 4kW south-facing system in the Midlands makes about 3,800 kWh a year and is worth about £599: £263 off your bill and £336 in export payments. At £6,200 to £6,600 on DESNZ's figures, that's a 10 to 11 year payback. The Energy Saving Trust puts paybacks at 9 to 12 years by location.
How We'd Use the Result
- Set how long you'll stay against the payback. If you're likely to move before it pays back, we wouldn't buy on the numbers alone.
- Run it with and without a battery. The battery adds its full cost up front, and on our figures it lengthens the payback; our battery guide looks at when one does pay, such as on a cheap overnight tariff.
- Hold your quotes up to the cost line. It's DESNZ's average for a home install of that size. If a quote is well above it, we'd ask the installer what's in it before signing.
- Pick the roof face honestly. East or west still works; on PVGIS north loses about 46%, so we wouldn't put panels on a north roof.
By Area
| Area | kWh per kWp a year (south-facing) | 4kW system |
|---|---|---|
| London and South East | 1,005 | 4,020 kWh |
| South Wales (Cardiff) | 1,009 | 4,036 kWh |
| Midlands (Birmingham) | 950 | 3,800 kWh |
| North West England (Manchester) | 869 | 3,476 kWh |
| Central Scotland (Edinburgh) | 897 | 3,588 kWh |
Our central England figure (950) scaled by the European Commission's PVGIS model for each area. On PVGIS a south-east or south-west roof makes about 7% less than south, east or west 22% and north 46%.
How It Works
- Use at home: BRE's SAP 10.2 self-use formula for your system and yearly use. At the defaults (4kW in a home using Ofgem's typical 2,500 kWh) that's 26% of generation without a battery and 56% with one (8 kWh usable, our assumption), each kWh saving 26.32p at the price cap.
- Export: the rest at 12p on Octopus's Outgoing tariff.
- Cost: DESNZ's mean for home installs by size, with 0% VAT until 31 March 2027. For 4kW that's the £6,600 mean, so the calculator shows the longer end of the payback above.
- Inverter: one replacement at £800 around year 12 (the Energy Saving Trust's figure and timing). It's counted in the payback if it falls due first, and always in the 25-year return.
More in our guide to whether solar is worth it and solar costs guide.