The power rating of a solar panel, measured in Watts (W), is calculated under Standard Test Conditions (STC) at a cell temperature of 25°C and an irradiance level of 1000W/m2. However, in real-world use, cell temperature generally rises well above 25°C, depending on the ambient air temperature, wind speed, time of day and amount of solar irradiance (W/m2). During sunny weather, the internal cell temperature is typically 20-30°C higher than the ambient air temperature, which equates to approximately 8-15% reduction in total power output - depending on the type of solar cell and its temperature coefficient. To provide an average real-world estimate of solar panel performance, most manufacturers will also specify the power rating under NOCT conditions or the Nominal Operating Cell Temperature. NOCT performance is typically specified at a cell temperature of 45°C and a lower solar irradiance level of 800W/m2, which attempts to approximate the average real-world operating conditions of a solar panel.
Conversely, extremely cold temperatures can result in an increase in power generation above the nameplate rating as the PV cell voltage increases at lower temperatures below STC (25°C). Solar panels can exceed the panel power rating (Pmax) for short periods of time during very cold weather. This often occurs when full sunlight breaks through after a period of cloudy weather.
Cell temperatures above or below STC will either reduce or increase the power output by a specific amount for every degree above or below 25°C. This is known as the power temperature coefficient which is measured in %/°C. Monocrystalline panels have an average temperature coefficient of -0.38% /°C, while polycrystalline panels are slightly higher at -0.40% /°C. Monocrystalline IBC cells have a much better (lower) temperature coefficient of around -0.30%/°C while the best performing cells at high temperatures are HJT (heterojunction) cells which are as low as -0.25% /°C.
The power temperature coefficient is measured in % per °C - Lower is more efficient
Polycrystalline cells - 0.39 to 0.43 % /°C
Monocrystalline cells - 0.35 to 0.40 % /°C
Monocrystalline IBC cells - 0.28 to 0.31 % /°C
Monocrystalline HJT cells - 0.25 to 0.27 % /°C
The chart below highlights the difference in power loss between panels using different PV cell types. N-type heterojunction (HJT) and IBC cells show far lower power loss at elevated temperatures compared with common poly and monocrystalline PERC cells.