MPPT solar charge controller selection calculator

Formulas and assumptions

Open-circuit voltage in frost: Voc(T) = Voc × number of panels in series × (1 + Voc coefficient / 100 × (T − 25 °C)). Example: 2 panels with Voc 37 V and a coefficient of −0.35%/°C at −20 °C: 74 V × (1 + 0.0035 × 45) ≈ 85.7 V. Panel open-circuit voltage (Voc) rises when temperature falls below 25 °C — by the percentage per degree given by the Voc temperature coefficient in the panel datasheet. With Victron BlueSolar panels (−0.35%/°C), at −20 °C this is about 16% more than in the catalogue.

Short-circuit current in heat: Isc(T) = Isc × number of strings × (1 + Isc coefficient / 100 × (T − 25 °C)).

These are the same formulas used by Victron’s official calculator (we checked in its source code), so for the same data the Voc and Isc results are identical.

Controller limits are taken from the datasheets: maximum Voc, maximum PV short-circuit current and rated power for each battery voltage. The 150 V and 250 V controllers also have maximum start-up and operating voltages of 145 V and 245 V respectively. Victron advises leaving at least 10% voltage margin and, in a cool climate, recalculating Voc for the lowest expected temperature. Exceeding the voltage can damage the controller, and such damage is not covered by the warranty.

MPPT RS 450: array Voc must be lower than 450 V and lower than 8 × battery float voltage (for example, 55.2 V × 8 = 441.6 V). Start-up from 120 V, max. 16 A operating current and 20 A short-circuit current per tracker. The array is calculated for one tracker.

Charging start: according to the datasheets, PV voltage must exceed battery voltage by 5 V for the controller to start, and then must not fall below battery voltage + 1 V. That is why we also check panel Vmp in heat.

Power: an array with higher power than nominal will not damage the controller — in strong sunlight the controller will limit power. Victron’s official calculator marks an array above 130% of nominal power as oversized.

The calculator does not take into account voltage drop on PV cables or controller efficiency. The result is a sizing aid — before buying, check the datasheets and the manual for the chosen controller. Details in the guide How to select an MPPT controller for panels.

Sources