Nominal power (photovoltaic)
The nominal power is the nameplate capacity of photovoltaic (PV) devices, such as solar cells, panels and systems, and is determined by measuring the electric current and voltage in a circuit, while varying the resistance under precisely defined conditions. These Standard Test Conditions (STC) are specified in standards such as IEC 61215, IEC 61646 and UL 1703; specifically the light intensity is 1000 W/m2, with a spectrum similar to sunlight hitting the earth's surface at latitude 35°N in the summer (airmass 1.5), the temperature of the cells being 25 °C. The power is measured while varying the resistive load on the module between an open and closed circuit (between maximum and minimum resistance). The highest power thus measured is the 'nominal' power of the module in watts. This nominal power divided by the light power that falls on a given area of a photovoltaic device (area × 1000 W/m2) defines its efficiency, the ratio of the device's electrical output to the incident energy.
The nominal and potentially maximum power is important to the technician planning an installation in order to choose correctly the size of alternating-current converters and the cross-sectional area of the connecting wires. It is not a good measure with which to compare solar modules and prices. The output in kWh per m2 is much better suited.
The peak power is not the same as the power under actual radiation conditions. In practice, this will be approximately 15-20% lower due to the considerable heating of the solar cells.
The international System of Units does not permit the use of suffixes or additional symbols. Despite this rule, in colloquial language, instead of stating "the nominal power of the module is one kilowatt", it is stated "the module has one kilowatt-peak". The terms watt-peak (Wp), kilowatt-peak (kWp) and megawatt-peak (MWp) are also used. In the context of domestic PV installations, kWp is the most common unit encountered.
...
Wikipedia