It may seem counter-intuitive, but solar panel efficiency is affected negatively by temperature increases. Photovoltaic modules are tested at a temperature of 25 degrees C (STC) – about 77 degrees F., and depending on their installed location, heat can reduce output efficiency by 10-30%. As the temperature of the solar panel increases, its output current increases exponentially, while the voltage output is reduced linearly. In fact, the voltage reduction is so predictable, that it can be used to accurately measure temperature.
As a result, heat can severely reduce the solar panel’s production of power.
Different module designs and different semiconductor compounds all react to temperature – here’s a brief intro into what to expect.
The best way to determine panel’s tolerance to heat is by looking at the manufacturer’s data sheet. There, you’ll see a term called the “temperature coefficient (Pmax.)” This is the maximum power temperature coefficient. It tells you how much power the panel will lose when the temperature rises by 1°C above 25°C. @STC (STC is the Standard Test Condition temperature where the module’s nameplate power is determined).
For example, the temperature coefficient of a solar panel is -.258% per 1 degree Celsius. So, for every degree above 25°C, the maximum power of the solar panel falls by .258%, for every degree below, it increases by .258%. What this means no matter where you are, your panel may be affected by seasonal variations. However, the temperature coefficient also tells you that efficiency increases in temperatures lower than 25°C. So, in hot climates, the efficiency will mainly decrease compared to STC.