UK Solar Generation by System Size
Annual solar generation is measured in kWh. A quick planning shortcut is to multiply system size by a yield per installed kW, then refine the estimate with roof-specific data from a survey, installer design or PVGIS.
Planning table
| System size | Careful estimate | Middle estimate | Strong estimate |
|---|---|---|---|
| 2kW | 1,600 kWh/year | 1,800 kWh/year | 2,000 kWh/year |
| 3kW | 2,400 kWh/year | 2,700 kWh/year | 3,000 kWh/year |
| 4kW | 3,200 kWh/year | 3,600 kWh/year | 4,000 kWh/year |
| 5kW | 4,000 kWh/year | 4,500 kWh/year | 5,000 kWh/year |
| 6kW | 4,800 kWh/year | 5,400 kWh/year | 6,000 kWh/year |
| 8kW | 6,400 kWh/year | 7,200 kWh/year | 8,000 kWh/year |
What changes the generation estimate?
- Orientation: south-facing roofs usually generate more annually, but east-west layouts can still work well for household use.
- Shading: trees, chimneys, dormers and neighbouring buildings can materially reduce output.
- Pitch: roof angle affects seasonal output and the winter/summer shape of generation.
- Location: southern UK locations tend to receive more usable solar resource than northern locations, all else equal.
- System design: inverter sizing, panel layout and optimisers can affect real production.
How to use this with the calculator
Start with the middle estimate if you only know the system size. Once an installer provides a design, use their annual kWh estimate, then test a lower case by reducing it by 10% to 20%. If the installer’s number is far above the strong estimate, ask how it was calculated.
PVGIS is a useful independent check because it models solar radiation and photovoltaic performance by location. It still needs good input data for roof pitch, direction and system losses.
Next: use the calculator · 4kW example · payback factors