MWp
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 power is important for designing an installation in order to correctly dimension its cabling and converters. If the available area is limited the solar cell efficiency and with it the nominal power per area (e.g. kW/m2) is also relevant. For comparing modules, the price per nominal power (e.g. $/W) is relevant. For a given installation's physical orientation and location the expected annual production (e.g. kWh) per annual production assuming nominal power i.e. the capacity factor is important. With a projected capacity factor the price per projected annual production (e.g. $/kWh) can be estimated for a given installation. Finally, with a projected value of the production, the amortization of the cost of an installation can be estimated.
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