How much electricity do solar panels generate in the UK?

kWp describes the installed panel capacity. kWh describes the electricity generated.

Ten 400 W panels add up to a 4 kWp system. That does not mean they produce 4 kW continuously, or generate the same amount every day.

For annual generation, the calculation is:

Installed capacity × estimated annual output per kWp = estimated annual generation.

Using the same illustrative assumption of 900 kWh per kWp:

These examples assume the same output per kWp. Real roofs can perform differently because of location, direction, pitch, shading and system losses.

Use the installer’s roof-specific estimate when assessing your quote.

A system generating 3,600 kWh a year averages just under 10 kWh a day.

It will not deliver 10 kWh every day.

UK solar output varies with daylight and weather. Summer production is generally much higher than winter production. Panels also work in cloudy conditions, with lower output than in stronger sunshine.

Consider this deliberately simplified example:

These are invented figures to explain the comparison, not predicted monthly yields.

In the summer month, generation exceeds consumption overall. The household can still import electricity at night or when demand exceeds available solar power.

In the winter month, even perfect use of every generated unit would leave at least 350 kWh to obtain elsewhere.

A healthy annual total can therefore sit alongside substantial winter electricity bills. That matters particularly if your heating uses electricity.

Suppose your panels generate 3,600 kWh over a year.

If your home uses 1,500 kWh of that electricity and exports the remaining 2,100 kWh, you have not avoided buying 3,600 kWh from your supplier.

You have:

• Avoided buying 1,500 kWh.
• Exported 2,100 kWh, which may earn an export payment.

Those quantities can have different financial values.

Your tariff and the timing of your consumption determine the benefit. An annual generation figure is useful, but it needs to connect to realistic assumptions about household use and export.

A battery can move some daytime solar electricity into the evening, subject to capacity, power limits and losses.

It does not create additional solar generation, and a normal home battery cannot carry a summer surplus into winter.

If a quote includes a battery, ask the installer to separate:

• The electricity the panels are expected to generate.
• The amount the household is expected to use directly.
• The additional amount expected to reach the household through storage.
• The electricity still expected to be exported.

That makes it easier to judge what the battery adds. Our article on whether home batteries are worth it examines that decision separately.

Ask for the assumptions behind the headline number.

1. 1. Confirm the panel capacity.
   Use the total panel rating in kWp. An inverter’s kW rating is a different figure.
2. 2. Check the roof details.
   The forecast should reflect your location, roof direction, pitch and shading. If panels sit on different roof faces, ask how each has been modelled.
3. 3. Request monthly output.
   Compare it with your monthly electricity consumption—not just your annual total.
4. 4. Check the losses and limits.
   Ask whether the estimate accounts for system losses, inverter constraints and any export limitation that could affect usable generation.
5. 5. Understand the uncertainty.
   A forecast based on historical weather is an estimate. Ask about expected variation between years and whether any guarantee is actually offered.

PVGIS, the European Commission’s free solar estimation tool, provides annual and monthly output estimates using location, system and weather inputs. It can help cross-check a quote, but nearby trees, buildings and roof-specific shading need proper attention.

Compare quotes using consistent assumptions. A higher forecast deserves an explanation of what makes it higher.