What is Kilowatt-peak?
Kilowatt-peak (kWp) describes the maximum output a solar system achieves under so-called Standard Test Conditions (STC): 1000 watts of solar irradiance per square metre, a module temperature of 25 degrees and a defined angle of light incidence. The addition of "peak" means exactly that – a peak. The figure is a laboratory reference value, not the actual continuous yield on your roof. It is what makes systems from different manufacturers and of different sizes comparable in the first place.
In everyday use, the actual electricity yield depends on location, roof orientation, tilt, shading and weather. In Switzerland, a system typically delivers a certain annual yield in kilowatt-hours (kWh) per installed kWp – and this varies considerably between the Mittelland, the Alpine region and Ticino. So kWp measures power (the size of the system), while kWh measures the electricity actually produced.
You need the kWp figure as soon as you plan a photovoltaic system: for sizing, the quotation, the building permit and the calculation of subsidies such as the one-off remuneration. The capacity of a battery storage unit is also often matched to the kWp.
Be careful not to confuse kWp with the annual yield – a larger system is not automatically more economical. Factor your self-consumption, available roof area and a sound yield forecast into the planning, and obtain several quotes from certified solar specialists.