Solar charge controller sizer

Size a solar charge controller from panel specs, series-parallel stringing, cold-weather voltage rise, battery-bank voltage, and controller limits. Use it to avoid the usual mistake of sizing only from watts while ignoring cold Voc and charge-current headroom.

System inputs

Results update instantly. Use the button only if you want a shareable URL.

Temperature units Fahrenheit Celsius

Controller type Battery-bank voltage Battery chemistry Controller class Controller max PV Voc (V) Controller max charge current (A) Recommended PV power at this battery voltage (W) Panel power at STC (W) Panel Voc at STC (V) Panel Vmp at STC (V) Panel Isc at STC (A) Voc temperature coefficient (% per °C) Panels in series per string Parallel strings Coldest expected ambient temperature (°F) Desired headroom below controller limits (%)

Why this page beats a basic watt calculator

A controller can look fine on watts and still be wrong on voltage. Cold mornings push Voc upward, long MPPT strings change the fit completely, and PWM setups can quietly waste a lot of panel voltage.

Checks cold-weather string Voc, not only nameplate panel watts.
Estimates battery charge current, which is the number most controller output ratings care about.
Flags when PWM is a poor match for higher-voltage residential panels and multi-panel strings.

Default example: this array lands around 150V / 100A MPPT once headroom is included.

What to verify against the real datasheet

Exact max PV open-circuit voltage and any cold-weather derating notes
Recommended PV power by battery voltage, if the manufacturer publishes one
Startup and tracking voltage requirements for the specific MPPT controller
Battery maker limits on maximum charge current, especially for lithium packs

Common questions

Short answers for adjacent search queries and first-use questions.

Why does cold weather matter for solar charge controller sizing?

Panel open-circuit voltage rises as temperature falls, so a string that looks safe on a mild day can exceed the controller's voltage limit on a cold morning.

When is MPPT usually better than PWM?

MPPT is usually the stronger choice when panel voltage sits well above battery voltage or when you are trying to preserve more of the array's available power instead of throwing away excess voltage.

Does controller sizing depend only on panel watts and battery volts?

No. Watts matter, but safe sizing also depends on Voc, Isc, stringing, temperature, controller limits, and the resulting charge current into the battery bank.