Why multiply Isc by 1.25?

NEC 690.8 requires conductors and devices to be rated for 125 percent of the continuous short-circuit current to handle prolonged high irradiance and edge-of-cloud enhancement. The charge controller's current rating must exceed this grossed-up figure with margin.

How does MPPT sizing differ from PWM?

A PWM controller connects the array nearly directly to the battery, so it sees array current and the array must match battery voltage. An MPPT controller converts excess array voltage into extra current, so its output amperage is array power divided by battery voltage, which can exceed array Isc.

Why apply a temperature factor to Voc?

Solar panel open-circuit voltage rises as temperature falls, and on a cold clear morning it can exceed the nameplate Voc by 10 to 25 percent. The controller's maximum input voltage must exceed the cold Voc of the series string or it can be damaged.

What battery voltage should I enter?

Enter your nominal system voltage of 12, 24, or 48 volts. MPPT output current scales inversely with battery voltage, so a higher-voltage bank lets a smaller-amp controller carry more array power, which is why 48 V systems are common for large arrays.

Does a bigger controller hurt?

No. Oversizing the amperage rating is safe and gives headroom, though it costs more. Undersizing is the danger, as the controller will clip output, overheat, or shut down. Voltage rating, by contrast, must never be exceeded by the cold-array Voc.

Solar Charge Controller Sizing Calculator

A solar charge controller must safely handle both the current your array can produce and the voltage it can reach on a cold day. This calculator applies the NEC 1.25 current factor and a low-temperature voltage correction to size the minimum controller amperage and voltage rating for MPPT or PWM systems.

How it works

For current, the array short-circuit current is the per-string Isc times the number of parallel strings, grossed up by the NEC factor:

Array Isc = module Isc × parallel strings
PWM min amps  = Array Isc × 1.25
MPPT min amps = (Array Isc × Vmp_string × 1.25) ÷ battery V   (approx, via power)

For voltage, the series-string open-circuit voltage is corrected for cold:

String Voc = module Voc × modules in series
Max cold Voc = String Voc × (1 + temp factor)

The MPPT estimate uses array Voc as a conservative stand-in for the power calculation when Vmp is not supplied.

Tips

Pick a controller whose amp rating exceeds the calculated minimum with margin.
Never let cold-morning array Voc exceed the controller’s max input voltage.
Higher battery voltage lets a smaller-amp MPPT controller pass more power.