Solar Panel Performance Explained

A solar panel's wattage tells you what it produces under standard laboratory conditions, not what it will produce on your roof. Real output depends on how much light reaches the panel, its temperature, the direction and pitch of the roof, shading, and the rest of the system. Here is what each of those means in practice.

What the wattage on a panel means

Panels are rated in watts under Standard Test Conditions: 1,000 watts of light per square metre, a cell temperature of 25°C and a defined light spectrum. Most domestic panels fitted today are rated somewhere around 400 to 500 W each, using half-cut cell designs that are more tolerant of partial shade than older full-cell panels.

Those test conditions are a fair comparison between panels, but your roof rarely matches them. In the UK, full-strength sunlight on cool panels happens for a limited number of hours a year, so a panel spends most of its time producing well below its rating. That is normal and is already accounted for in a proper generation estimate.

Efficiency, and why it matters on London roofs

Efficiency is how much of the light hitting a panel becomes electricity. Most mainstream panels sold today convert more than 20% of it. Higher efficiency means more output from the same area, which matters most where roof space is tight, as it is on most London terraces.

What affects real-world output

• Light: more direct sunlight means more output. Panels still generate on cloudy days, but much less.

• Temperature: panels are slightly less efficient when hot. A bright, cool spring day can beat a hot summer one.

• Orientation and pitch: south-facing roofs produce the most over a year, but east and west roofs work well too. See the best direction and angle for solar panels.

• Shading: chimneys, trees and neighbouring buildings can cut output, sometimes by more than the shaded area suggests. Optimisers or careful string design can reduce the effect.

• The rest of the system: the inverter converts DC to AC with a small loss, and a battery loses a little in charging and discharging. Good modern equipment keeps these losses small.

How much will your panels produce?

For an independent first look, the free Global Solar Atlas and the EU's PVGIS tool both estimate output per kW of panels for any location. What that means for your home depends on your system size, how much of the generation you use yourself and your tariff. That is why we model your roof before quoting rather than promising a figure.

Keeping performance up

• Watch your monitoring app. A sudden drop compared with the same time last year is worth looking into. See monitoring your solar panels through an app.

• Keep birds out. Pigeons nesting under panels cause mess and can damage cabling. See pigeon-proofing solar panels.

• Get faults checked. If output stops or drops sharply, start with what to check when solar panels are not generating.

If your output has dropped compared with previous years, read why your solar might be generating less than it used to.

If your system is not performing as it should, book a solar system inspection. If you are planning a new system, request a quotation and we will model the output for your roof.