Solar panels do not work at 100% efficiency. Many people will assume that they do but the efficiency of the panel will depend upon the type of panel and the location, among many other things.

The efficiency of solar panels is easy to measure. All you have to do is look at the amount of sunlight that hits the panel and compare it to the amount of power that comes out of it. So if the panel receives 100 watts of sunlight, but it produces 10 watts of power, then you just turn this into a percentage. In this example the efficiency will be 10%.

Photovoltaic panels will normally have an efficiency of around 10-20%. These are the standard panels that most people use. These generate electricity for use or for selling back to the grid. The other type of panel is the thermal panel. These are used only to heat water and radiators and they generally have a much higher efficiency at 90%, but they do work slightly differently to the standard panels.

Talk to your installer about the efficiency of the solar panels that you have chosen. The assessment on the efficiency of the panel will have taken place in a lab. Remember that this figure is an assessment of the results that were achieved in the lab. The panels that you have will not have been tested on a roof, but some manufacturers will be able to give you a PTC rating – performance test conditions. This will be a better idea of what your panels can achieve when they are actually in use.

Look out for a ‘system PTC rating’. This looks at the efficiency of the inverter too, the equipment that converts the current. A good inverter will have an efficiency rating of between 85% and 95%.