Solar Generation by System Size UK
Annual solar generation is measured in kWh. System size is usually shown as kW or kWp. For early UK planning, a useful shortcut is to multiply system size by an annual yield per kWp, then replace the shortcut with a roof-specific PVGIS or installer estimate.
Quick answer: a cautious UK planning range is about 800 to 1,000kWh per kWp installed each year. A 4kWp system might be modelled at roughly 3,200 to 4,000kWh a year before roof-specific checks.
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 |
kW, kWp and kWh in plain English
kWp is the rated peak output of the panels under test conditions. kWh is the amount of electricity generated over time. A 4kWp system won't produce 4kW all day, and it won't produce the same output in January as it does in June.
This is why annual kWh matters more for savings than the headline system size. The system size tells you the scale of the array. The annual kWh estimate tells you what the array may actually produce.
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 · cost per kWh guide · payback factors