Solar PV DC calculator

Calculate the voltage drop of the DC cable on the solar side and the smallest suitable cross-section, or the open-circuit voltage of the string at the lowest temperature.

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What does the solar PV DC calculator work out?

It examines two things on the DC side of a solar system:

• DC cable: the voltage drop ΔU = 2 · L · I / (γ · A) (copper, operating temperature; L is the one-way length), as a percentage of the DC voltage, the loss in the cable, the smallest suitable cross-section and the longest length within the chosen limit (1, 2 or 3 %).
• String voltage in the cold: the open-circuit voltage of a module rises in the cold: Uoc(T) = Uoc · (1 + β/100 · (T − 25)), where β is the temperature coefficient of Uoc (%/K, negative). The string voltage is the module voltage multiplied by the number of modules; if you enter the permitted input voltage of the inverter, the calculator shows whether the string fits.

What to watch for

Take the module data (Uoc, temperature coefficient) from the data sheet, and estimate the lowest temperature for the place of installation.

Limits of the calculation

For the DC cable it calculates only the voltage drop: it does not check the current capacity of the cable, the type of the solar cable or the protection. The string calculation is simplified with the temperature coefficient. The result is for guidance only; the design of the solar system must be carried out by a qualified professional.

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