Off-Grid Solar Calculators

Inverters and charge controllers

MPPT Charge Controller Sizer

Charge current is array watts divided by battery voltage. Controllers are sold by output amps, so add about 20% headroom. The Voc limit is a separate check — cold mornings raise string voltage.

Inputs

W

Nominal battery voltage the controller charges into.

V

Results

Those inputs are outside what this tool can recommend. Check the notes — a fuse above cable or module rating is a fail, not a size to install. Other tools need values greater than zero and within the stated limits.

Charge current at STC
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Controller current with 20% headroom
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Controller Voc with 25% margin
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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.

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.

More in this cluster

All inverters and charge controllers tools