Solar generation by system size

kW and kWh are different

System size is usually given in kW or kWp. That's the rated peak capacity of the panels under test conditions. Annual generation is kWh. That's the amount of electricity produced over a year.

A 4kW system won't produce 4kW all day. Output moves with weather, season, sun angle, shading and inverter behaviour.

A sensible planning range

For early planning, a rough range of 800 to 1,000kWh per kW installed per year is useful for UK homes. A 4kW system would then sit around 3,200 to 4,000kWh per year before roof-specific checks.

The calculator uses 900kWh per kW as a middle shortcut. It's not a promise. It's a way to get started before you have a survey.

Why estimates differ

Two 4kW systems can perform differently because one may be south-facing and unshaded while another is split east-west with shade from a chimney. Location also matters because UK solar resource varies by region.

Inverter sizing can also change the shape of output. Some clipping on strong summer days isn't automatically a problem if it keeps the connection simple and annual losses are small.

Use the number properly

Once you have an installer estimate, run the calculator with that figure. Then run a lower case. If the system still works financially with a 10% to 20% lower generation number, the decision is less fragile.