Solar Charge Controller Sizing Formulas

Charge controller formulas: the output current of an MPPT controller from the array power and battery voltage, the safety margin and the daily charge.

The formulas

Parray is the array power at standard test conditions in watts, η the efficiency of the controller and the wiring, Vbat the battery voltage, h the peak sun hours of the day and Isc total the sum of the short-circuit currents of the parallel strings. The factor 1.25 is a common margin for an irradiance above 1,000 W/m², and the rules of the code or the maker may ask for another one. The input voltage of the controller must also stay above the open-circuit voltage of the array at the coldest temperature, which the solar string article covers. The sizes in the list, the efficiency and the sun hours are examples.

Worked example: a 3 kWp array on a 48 V battery, MPPT controller

1. I = 3000 × 0.96 / 48 = 60 A
2. Irating ≥ 1.25 × 60 = 75 A, so the next size in the example list of 40, 60, 80 and 100 A is 80 A
3. With 5 peak sun hours, Q = 3000 × 5 × 0.96 / 48 = 300 Ah a day

Worked example: a PWM controller, 4 strings of 10 A short-circuit current

1. Irating ≥ 1.25 × 4 × 10 = 50 A

MPPT current on a 48 V battery by array size

Frequently asked questions

Why is the output current based on the battery voltage?

The controller turns the array voltage down to the battery voltage, and the power stays about the same. A lower battery voltage therefore gives a higher current.

What is the difference between MPPT and PWM?

An MPPT controller converts the array voltage to the battery voltage and can use an array of a higher voltage. A PWM controller connects the array to the battery, so the array voltage must match the battery.

Why add 25%?

The irradiance can exceed 1,000 W/m² for a short time, and the margin keeps the controller within its rating.

Does the controller rating cover the battery limit?

No. The charge current must also stay within the limit of the battery, so check that figure with the battery maker.

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