UK Solar Panel Calculator

How this calculator works

You enter roof area, orientation, pitch, shading, region, yearly electricity use, the share used in the daytime, your electricity rate in pence per kWh, system size in kW and whether you want a battery. The calculator assumes 400 W panels of 2 m² each and stops if your roof area is too small for the system size you chose.

Yearly generation is the system size multiplied by a fixed base yield per kW, then adjusted by factors for orientation, region, pitch and shading and an 85% system efficiency. Self-use is the lower of 30% of generation and your daytime usage, and a battery moves part of the exported energy into self-use. Self-used kWh are valued at your electricity rate and exported kWh at one flat export rate that the page labels a Smart Export Guarantee average, which may not match your supplier. Payback divides a fixed system cost for the size band, plus a battery cost per kWh if you add one, by the annual saving.

The region, pitch and shading factors are rounded rules of thumb, not a site survey, so a real design could generate more or less. System costs, battery costs and the export rate are assumed values with no cited source, and the result ignores inverter replacement, maintenance and changes in electricity prices. It is an estimate, not a quote, a free-solar promise or an ECO4 approval.

To see how support for solar works, read the solar panel grants guide or check your eligibility.

Worked example

Example inputs, which are also the calculator's starting values: a 4 kW system on 40 m² of south-facing roof at a 35 degree pitch with no shading, in South England, using 3,100 kWh of electricity a year with 40% of it in the daytime, at 28.5p per kWh, with no battery.

1. Panels: 4 kW ÷ 0.4 kW per panel = 10 panels, needing 10 × 2 m² = 20 m². That fits within 40 m².
2. Generation: 4 kW × 950 kWh per kW × 1.0 (south) × 1.0 (South England) × 1.0 (35 degree pitch) × 1.0 (no shading) × 0.85 efficiency = 3,230 kWh a year.
3. Self-use is the lower of 30% × 3,230 = 969 kWh and 40% × 3,100 = 1,240 kWh, so 969 kWh. Exported: 3,230 − 969 = 2,261 kWh.
4. Value: 969 kWh × £0.285 = £276.165 saved, plus 2,261 kWh × £0.055 = £124.355 export income, giving £400.52, shown as £401 a year.
5. System cost: 4 kW falls in the up to 4 kW band, £6,500. Payback: £6,500 ÷ £400.52 = 16.2 years.
6. Carbon: 3,230 kWh × 0.131 kg = 423.13 kg, shown as 0.4 tonnes a year.

How this is calculated and sources

The 0.131 kg CO₂e per kWh figure is labelled in the calculator's code as the UK grid factor from the DESNZ 2026 factors. The 950 kWh per kW base yield, the 85% efficiency, the 30% self-use cap, the £0.055 export rate (labelled in the code as a Smart Export Guarantee average, with no published source), the £6,500 system cost and the region, pitch and shading factors are assumptions used by this calculator. The 28.5p rate is a starting value you can change. The output is an estimate, not a quote or a design.