PV Production

In Meteonorm, the parameter "PV production" is calculated using Ross's method ¹, for a well-maintained, medium-sized crystalline silicon module and an efficient inverter. The calculation is based on the irradiance, ambient temperature, and a few constants.

The module temperature in °C is calculated using the Ross temperature coefficient that describes how much the module's operating temperature rises above ambient temperature per unit of solar irradiance:

where

• $Ta$ is the air temperature in °C,
• $c_R = 0.03$ is the Ross temperature coefficient in $\frac{K}{W/m^2}$ , and
• $GTI$ is the Global Tilted Irradiance in W/m².

From the module temperature we derive an efficiency factor (the hotter the module, the lower its power output)

where $\alpha = 0.005$ 1/K is an efficiency coefficient.

Finally, with a fixed performance ratio $r=0.82$, we compute the PV production in W/kWp (power output relative to installed capacity):

Because $PV$ is relative to the installed capacity, module efficiency need not be taken into account.

The performance ratio $r$ is a dimensionless metric that indicates how efficiently a PV installation converts available solar energy to usable electricity, compared to what an ideal system would produce under Standard Test Conditions STC ². Example: If a 10 kWp system receives 1 000 kWh/m² of irradiance in a year, the ideal yield would be 10 000 kWh. If it actually produces 8 000 kWh, the performance ratio would be 0.8. What reduces $r$ from 1.0: inverter and wiring losses, soiling, temperature losses (modules running hotter than STC 25°C).

To better reflect the temperature-dependent fluctuations in the modules’ efficiency over the course of the day and year, Meteonorm uses a relatively optimistic (constant) performance ratio of 0.82 and calculates the temperature dependence separately using the $t_C$ formula above.