How many solar panels do I need?

The number of solar panels you need depends on how much electricity you use. Start by checking your annual usage in kilowatt-hours (kWh). This is printed on your electricity bill or statement. Most UK households use between 2,700 and 4,200 kWh per year.

A small flat or one-person household typically uses 2,000 to 2,700 kWh. A family of three or four uses 3,000 to 4,200 kWh. A large household with electric heating or an electric vehicle can use 5,000 kWh or more.

Your target is to generate enough electricity from solar to cover a meaningful share of that usage. You do not need to cover 100 per cent, because solar generates during the day when you may not be home to use it. A realistic target is 60 to 80 per cent of your annual electricity demand.

A modern solar panel rated at 400 watts produces roughly 340 to 380 kWh per year in southern UK cities like London. In Manchester or Glasgow, the same panel produces around 310 to 345 kWh. The exact figure depends on the angle, direction and shading of your roof.

Panel wattage matters. A 400W panel produces 25 per cent more electricity than a 320W panel in the same conditions. Most installers now fit 400W to 450W panels as standard. Higher wattage panels cost slightly more but mean you need fewer of them for the same total output.

Panel efficiency also matters, though less than people think. A 22 per cent efficient 400W panel produces the same electricity as a 20 per cent efficient 400W panel. Efficiency affects how much roof space each panel takes up, not how much power it generates.

Step one: find your annual electricity usage in kWh from your bill. Let us say it is 3,500 kWh.

Step two: decide what percentage of that you want solar to cover. A realistic target is 70 per cent, so 0.70 times 3,500 equals 2,450 kWh.

Step three: divide by the output of one panel. In London, a 400W panel produces about 360 kWh per year. So 2,450 divided by 360 equals 6.8, which rounds up to 7 panels.

Step four: check you have enough roof space. Each 400W panel measures about 1.9 square metres. Seven panels need roughly 13.3 square metres of usable roof area. That is a strip about 3 metres wide and 4.5 metres long.

Step five: verify the angle and direction. South-facing at 30 to 40 degrees is ideal. East or west works with 10 to 15 per cent more panels. North-facing is not recommended.

Each standard 400W panel measures roughly 1.9 metres by 1.1 metres, covering about 2 square metres. You also need small gaps between panels for mounting and airflow, so budget 2.2 square metres per panel.

A 4 kW system of ten panels needs about 22 square metres of usable roof. A 3 kW system of eight panels needs about 18 square metres. A 2 kW system of five panels needs about 11 square metres.

Usable roof area is less than total roof area. You need to subtract space around the edges, around chimneys, and around any roof vents or skylights. The usable area is typically 70 to 85 per cent of the total roof surface.

If you have limited roof space, higher-wattage panels (450W) let you fit more capacity in the same area. The cost per watt is slightly higher, but the total output per square metre is better.

A battery does not change how many panels you need, but it changes how much of the generated electricity you actually use. Without a battery, a typical household self-consumes 30 to 40 per cent of its solar generation. The rest is exported to the grid for a small payment.

With a battery, self-consumption rises to 60 to 75 per cent. You store daytime surplus and use it in the evening. This means each kWh of solar is worth more, because it displaces electricity you would otherwise buy at full price.

If you plan to add a battery, you might install slightly more panel capacity than your daytime usage suggests. The extra generation gets stored for evening use. But do not oversize the array relative to your total annual usage. A battery that never fills is not earning its keep.

A 2 kW system (5 panels) suits a small flat or a one-person household using 2,000 to 2,500 kWh per year. It generates about 1,800 kWh annually and costs £3,000 to £4,500 installed.

A 3 kW system (8 panels) suits a couple or small family using 2,700 to 3,200 kWh. It generates about 2,700 kWh and costs £4,000 to £5,500.

A 4 kW system (10 panels) is the most common size for UK homes. It suits a family using 3,200 to 4,200 kWh, generates about 3,500 kWh and costs £5,000 to £7,000.

A 5 kW system (13 panels) suits a large household or one with an electric vehicle. It generates about 4,300 kWh and costs £6,000 to £8,500.