Nominal power (or peak power) is the nameplate capacity of photovoltaic (PV) devices, such as solar cells, modules and systems. It is determined by measuring the electric current and voltage in a circuit, while varying the resistance under precisely defined conditions. The nominal power is important for designing an installation in order to correctly dimension its cabling and converters.[1]
Nominal power is also called peak power because the test conditions at which it is determined are similar to the maximum irradiation from the sun. Thus this quantity approximates the theoretical maximum production of the panel on a clear sunny day with the panel oriented perpendicular to the sun. The nominal power is generally not reached under actual radiation conditions. In practice, actual conditions will allow for approximately 15-20% lower generation due to the considerable heating of the solar cells.[2]
Moreover, in installations where electricity is converted to AC, such as solar power plants, the actual total electricity generation capacity is limited by the inverter, which is usually sized at a lower peak capacity than the solar system for economic reasons. Since the peak DC power is reached only for a few hours each year, using a smaller inverter allows to save money on the inverter while clipping (wasting) only a very small portion of the total energy production. The capacity of the power plant after DC-AC conversion is usually reported in WAC as opposed to WDC or Watt-peak (Wp).
future was invoked but never defined (see the help page).