Solar panels in London: what to expect

London receives roughly 950 to 1,000 kWh of solar irradiance per square metre each year. That puts it among the better-performing UK cities for solar generation. A typical south-facing roof at a 35-degree tilt in London can produce between 850 and 950 kWh per kilowatt of installed capacity annually.

For a standard 400-watt panel, that means around 340 to 380 kWh per year. A 4 kW system of ten panels would generate approximately 3,400 to 3,800 kWh annually. That covers a large share of a typical London household's electricity use, which averages around 3,300 kWh per year for a medium consumer.

These figures assume an unshaded south-facing roof. East or west-facing roofs in London still perform well, typically producing 75 to 85 per cent of a south-facing array. A north-facing roof is generally not worth installing on.

The actual output on your roof depends on three things: the angle, the direction it faces, and any shading from chimneys, trees or neighbouring buildings. Postcode averages give a rough picture. Your specific roof may do better or worse. Use the WattSnap scanner to measure your actual roof in seconds.

A typical 4 kW solar panel system in London costs between £5,000 and £7,000 installed. That includes panels, inverter, mounting and labour. Prices have fallen significantly over the past decade. A system that cost £12,000 in 2014 would cost roughly half that today.

London installation prices tend to run slightly higher than the national average, reflecting higher labour costs in the capital. But the higher electricity prices in London partly offset this. London households on standard tariffs pay around 26 to 28 pence per kWh, making each kWh of solar generation worth more in savings.

Battery storage adds £2,000 to £4,000 to the typical system cost. A battery lets you use solar electricity in the evening rather than exporting it, which roughly doubles the self-consumption rate. Whether a battery makes sense depends on how much electricity you use in the evening and overnight.

A 4 kW system in London generating 3,600 kWh per year saves around £950 to £1,000 annually on electricity bills at current prices. With the Smart Export Guarantee, you can earn an additional £80 to £150 per year for surplus electricity exported to the grid.

The payback period for a typical London system is 6 to 8 years without a battery, or 8 to 11 years with one. After payback, the system continues generating for another 15 to 20 years with minimal maintenance costs.

Solar panels degrade at roughly 0.5 per cent per year. A system producing 3,600 kWh in year one will still produce around 3,250 kWh in year twenty. Inverters typically need replacing once during the system lifetime, at a cost of £500 to £1,000.

Most domestic solar installations in London fall under permitted development rights. You do not need planning permission if the panels do not protrude more than 200mm from the roof surface and are not on a listed building or in a conservation area.

If you live in a conservation area, panels are usually permitted on the rear or side of the roof. Front-facing panels may require planning permission. Check with your local borough planning department if you are unsure.

Flats and leasehold properties have additional considerations. You generally need the freeholder's permission to install panels on a shared roof. Some London boroughs have specific guidance for solar on flats and mansion blocks.

London roofs face more shading challenges than roofs in rural areas. Chimney stacks, neighbouring buildings, and mature trees all reduce output. Even partial shading on one panel can affect the whole array if you do not have optimisers.

Micro-inverters or power optimisers help mitigate shading losses. They let each panel operate independently rather than dragging the whole array down to the level of the most shaded panel. This adds about £500 to £800 to the system cost but can recover 10 to 25 per cent of lost output on partially shaded roofs.

The WattSnap scanner can include shading from your actual skyline. You turn on the spot with your phone, tracing the tops of nearby buildings and trees, and the phone's compass and tilt sensor record how high the skyline is in each direction. That gives you an annual estimate with the shading built in. It measures from one point, so it won't show which individual panels are shaded. An installer's survey is still the way to decide on optimisers.