HOMER Energy Support

Why is my PV Output higher than the rated number?

Last Updated: Dec 27, 2018 01:50PM MST
Products: HOMER Pro and HOMER Grid

I set the PV to 182KW capacity. 
The derating factor is set to 80%. The inverter is not considered.
However, the PV output showed a maximum PV output of 185kW. Could you explain the results? 


There are several reasons why the output of the PV is higher than the rated number.

The rated power of solar panels is determined under carefully controlled laboratory conditions that simulate optimal operation:

> The solar panel is subject to 1000 watts of radiation per square meter. This is the average peak solar radiation received on the surface of Earth on a summer day with clear skies.
> A room temperature of 25°C is used. This is actually much lower than real operating conditions, due to the fact that solar panels are subject to sunlight all day long.
> The light source has a color spectrum that simulates actual sunlight.

The resulting energy output in watts after carrying out this test will be the nameplate capacity of the PV module, and the efficiency is calculated by dividing this by 1000 W/m2. This means that a solar PV module with an output of 180 W/m2 has an efficiency of 18%.

A very important factor to consider is that the output of solar panels for homes decreases as temperature rises. In real life applications, solar PV modules will heat up due to sunlight, and might not reach the rated output even under peak solar radiation. For this reason, many PV system providers specify an output in kWh/m2/day, which is a more reliable way to estimate the output. The same way goes under colder climates, when temperatures drop, power output increases.

Due to all these factors, the actual PV output will vary and could be significantly higher or lower than what it is rated at.

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