Adjusting Solar Panel Voc for temperature

Adjusting Solar Panel Voc for temperature 2021-09-08

When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature Adjusted Max Voc.

The following is from the Paper:

Panels specs are all given for Standard Test conditions at 25C. However, if the panel is colder than 25C, it will produce a higher Voc. This table from the US National Electric Code shows the level of voltage increase for various temperature ranges:

As you can see, at freezing temperature (0C), there is a 10% increase in voltage and at more extreme temperatures it can be as much as a 25% increase. Many areas in North America and Europe regularly get well below 0 o C and the voltage increase can become substantial.

Meanwhile, the most important not-to-exceed spec on an MPPT Solar Charge Controller (SCC) is the input voltage. If you just use the Voc and do not adjust for temperature extremes for your area, you might burn out the SCC.

NEC Table 690.7(A) is conservative, and you would probably be safe just using it for calculating the Temp Adjusted Max Voc. However , if the panel spec provides a Voc Temperature Coefficient (Sometimes called Beta), the NEC requires you to calculate the Max Voc using beta. Furthermore , since the table is conservative, calculating the Voc from the Temperature Coefficient may allow panel configurations that the table wouldn’t.

The remainder of this paper will describe how to use beta to calculate the Temperature Adjusted Max Voc for a panel
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