How big are commercial solar panels, how efficient are they and how much do they produce?

Most commercial panels are roughly 1.7 to 2.4 metres long and about 1.1 metres wide. The larger formats carry higher wattages. Always check the exact dimensions on the manufacturer's datasheet, as they vary by model.

Mainstream monocrystalline panels sold today typically convert around 20 to 23 per cent of the sunlight that hits them into electricity. Higher efficiency means more output from the same roof area, not necessarily a better return.

In the UK, each kilowatt-peak (kWp) of panels typically produces somewhere around 800 to 1,100 kWh a year, depending mainly on location, orientation, tilt and shading. A panel's yearly output is its kWp rating multiplied by that local figure.

Divide the system size you need by the panel wattage. For example, a 100 kWp system using 500 W panels needs about 200 panels. The right system size depends on your daytime electricity use and roof space.

As a rough guide, allow around 5 to 7 square metres of usable roof per kWp, including spacing and access routes. Your installer will confirm the layout after a survey.

Not always. They matter most when roof space is limited. Where space is plentiful, a lower-cost panel can give a similar or better return. Compare cost per kWp and expected output together.