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Solar Charge Controller Calculator

Sizes a solar charge controller for your array and battery voltage. MPPT and PWM are calculated differently, because they work differently — pick the type and the calculator uses the right method.

W
V
A

From the panel datasheet. Only used for PWM sizing.

Only used for PWM sizing.

%

Standard practice is 25%, covering edge-of-cloud irradiance spikes above the panel rating.

Controller rating needed
— A

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

Calculated current — A
Nearest standard controller — A
Array that size will take — W
MPPT or PWM? —

The formula

MPPT: amps = array watts ÷ battery voltage × margin PWM: amps = panel Isc × parallel strings × margin

The two types size completely differently. An MPPT converts surplus array voltage into extra charging current, so its output depends on power and battery voltage. A PWM simply connects the array to the battery, so panel current passes through unchanged and the surplus voltage is wasted.

What to watch out for

The difference between the two is the whole decision:

  • MPPT converts excess panel voltage into extra current, recovering 20–30% more energy in cold or low light. Output current is array watts ÷ battery volts. It also allows high-voltage strings, which means thinner cable from the roof.
  • PWM connects the array almost directly to the battery, dragging panel voltage down to battery voltage and wasting the difference. Output is simply the panel current. Cheap, and fine for small 12 V systems with matched panels.
  • The 25% margin is not optional. Edge-of-cloud effects can briefly push an array above its rated output, and a controller sized exactly will trip or fail.

Note the leverage of battery voltage on an MPPT: 2,000 W into a 12 V bank needs 167 A of controller, but into 48 V only 42 A. That is a large cost difference for the same array.

Frequently Asked Questions

What size charge controller do I need?

For MPPT, divide array watts by battery voltage and add 25%. A 2,000 W array on 48 V needs 42 A, so a 50 A controller.

MPPT or PWM?

MPPT for anything above a few hundred watts, or where the array voltage is well above the battery voltage. PWM only for small, low-voltage systems where the cost difference matters more than the 20–30% energy gain.

Why does battery voltage change the controller size?

An MPPT delivers power, not current. The same 2,000 W array needs 167 A at 12 V but only 42 A at 48 V — one reason larger systems run at 48 V.

Can I oversize the array beyond the controller rating?

Often yes, within the manufacturer's stated limit — the controller simply clips output on the brightest days. It can improve winter yield, but only do it where the datasheet explicitly allows it.

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