How many solar panels do I need?

Updated 26 October 2026 · Birmingham Solar

Start with kWh, not your monthly payment.

Find the annual electricity consumption on your bill or supplier account. A payment in pounds includes tariff rates and may include a balance adjustment, so it is a poor way to size a system. If you only have part of a year, remember that demand may change with the seasons.

Next, consider when the electricity is used. A household away during the day may export more than a household with similar annual consumption that regularly uses appliances while solar is generating.

Convert panel ratings into system capacity.

Multiply the number of panels by the rating of each panel, then divide by 1,000 to express the result in kWp. Ten panels rated at 450 W give a 4.5 kWp array. Twelve of the same panels give 5.4 kWp. These are examples of the calculation, not recommended packages.

kWp describes nominal array capacity. kWh describes energy produced or used over time. A 4.5 kWp system will not generate 4.5 kWh every hour: daylight, weather, orientation and other losses change the output.

Check the roof before fixing the number.

Panel dimensions vary. A roof plan needs to allow for the actual products, roof edges, obstructions, access and the mounting arrangement. A simple measurement of total roof area can overstate how many panels will fit.

Shading can matter more than a small difference in nominal capacity. Ask for the layout and generation assumptions, including how obstacles and any split roof orientations have been treated.

Do not size solar and storage using the same number.

Panels are rated in power; battery storage is described by usable energy capacity and output power. The storage you need depends on the surplus available to charge it and the demand it can serve later. A bigger array does not automatically justify a bigger battery.

Use a calculation to compare scenarios.

Try the solar power calculator with different array sizes and usage patterns. Watch the grid import and export results as well as the headline benefit. More generation that is mostly exported has a different value from generation used at home.

Treat the result as an initial comparison. A property assessment is needed to establish the roof layout, inverter arrangement, electrical work and a site-specific generation estimate.