Solar panel calculator
How much solar panels could generate and save, and how long they take to pay for themselves.
Saving in the first year
£684.00
Pays for itself in 9.7 years
- Generation per year
- 3,600 kWh
- Used at home
- 1,440 kWh
- Exported
- 2,160 kWh
- Saving over 25 years
- £16,112.27
- Net gain after cost
- £9,612.27
- Saving per month
- £57.00
Show workings
- Generation4 kWp × 900 × 1 (direction) × 1 (shade) = 3,600 kWh
- Used at home3,600 kWh × 40% = 1,440 kWh × 25p = £360.00
- Exported2,160 kWh × 15p = £324.00
- First-year saving£684.00
- PaybackSavings (falling 0.5% a year) add up to £6,500.00 after 9.7 years
How it works
Generation = system size (kWp) × typical yield for your area × a factor for which way the roof faces × a factor for shade. A south-facing, unshaded 4 kWp system in the Midlands makes about 3,600 kWh a year.
Savings come two ways: electricity you use yourself (you don't buy it, so it's worth your import price) and electricity you export (paid at your export tariff, such as the Smart Export Guarantee). Using more yourself saves more, which is why a battery helps.
Panels lose about 0.5% of output a year, which is included over 25 years. Energy prices will change; this assumes today's. Inverters often need replacing after 10–15 years.
Worked example
A 4 kWp system costing £6,500 making 3,600 kWh a year, using 40% at home at 25p and exporting the rest at 15p, saves 1,440 × 25p + 2,160 × 15p = £684 a year. That pays for itself in about 9.7 years.
Questions
How much of my solar will I use myself?
Without a battery, typically 30–50%, depending on whether someone is home in the day. A battery usually raises it to 60–80%. Running the washing machine or charging an EV in daylight helps too.
What is the Smart Export Guarantee?
A UK scheme where larger energy suppliers must pay you for electricity you export. Rates vary from a few pence to around 15p per kWh, so compare tariffs.
Do I need planning permission?
Most homes in the UK don't, as panels count as permitted development, but listed buildings and conservation areas have extra rules. An MCS-certified installer is needed for the Smart Export Guarantee.