The term peak power (Wp) is used in solar technology to evaluate the performance of solar cells. It is the optimum peak power that a solar cell or photovoltaic system achieves under location-dependent criteria and specified test conditions.
When determining peak power, the test conditions, or Standard Test Conditions( STC), should correspond to the actual environmental conditions when operating an installed solar cell, but they represent idealized environmental conditions. Peak power refers to solar irradiance, solar cell temperature, and atmospheric density. It is expressed in watts peak (Wp), kilowatts peak (kWp), or megawatts peak (MWp). The standard test condition (STC) is solar irradiance of 1,000 W/m2, ambient temperature of 25 °C, and wind speed of 1 m/s.
More realistic values for ambient temperature are obtained with Normal Operating Cell Temperature (NOCT) and show that with NOCT rating, cell performance is lower than that indicated by peak power.