Off-Grid Solar Calculators

Inverters and charge controllers

MPPT Charge Controller Sizer

Size an MPPT from array watts and battery voltage. Charge current is P ÷ V, then add 20% headroom. A separate Voc check uses 1.25 × STC string voltage as a cold-weather shortcut. Output amps are not the same as module Isc.

Inputs

W

Nominal battery voltage the controller charges into.

V

Results

Charge current at STC
33.3A
Controller current with 20% headroom
40.0A
Controller Voc with 25% margin
50V

Recommended gear

Engineering methodology

Charge current I = array watts ÷ battery volts (from P = V × I). Minimum controller current = 1.20 × I (20% headroom). Minimum Voc rating uses 1.25 × STC string Voc as a conservative silicon shortcut; the module temperature coefficient is more accurate for cold weather.

Charge current is not panel Isc

MPPT output current is about array watts ÷ battery volts (P = V × I). A 400 W array into 12 V is about 33 A at the battery even if module Isc is 10 A on a high-voltage string. This tool then adds 20% headroom.

Voc headroom on cold mornings

STC Voc is at 25 °C cells. The coefficient is negative, so cold raises open-circuit voltage. The 1.25 × Voc shortcut is conservative silicon practice; the datasheet coefficient and record-low ambient are more accurate. Never exceed the controller’s max Voc.

Frequently asked questions

Why size an MPPT 20% above array watts divided by battery voltage?

Controllers are sold by output (battery) amps. Irradiance, cold Vmp and rating tolerances can push current above the simple P/V figure. 20% headroom is a common planning margin, not a substitute for the controller’s datasheet.

Is 1.25 × Voc enough for the coldest morning?

It is a shortcut. NEC 690.7 expects the datasheet Voc temperature coefficient and the lowest expected ambient. Never exceed the controller’s max Voc even briefly.

Is MPPT current the same as panel Isc?

No. Output current is roughly array power ÷ battery voltage. A 400 W array into 12 V is about 33 A at the battery, even if module Isc is 10 A on a high-voltage string.

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