How is the size of a PV system expressed (what is kWp)?

PV systems are typically rated in kWp - kilowatt (peak) - the combined installed capacity of all the PV modules installed. All modules are supplied with a Wp - Watt (peak) - rating; 1kWp equals 1,000Wp. The Wp rating of a module expresses the rated output of that module under "standard test conditions" (stc) - internationally agreed temperature and sunlight conditions. To enable comparison, all modules are tested (and rated) using the same stc conditions.