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MPPT charge controller size calculator.
Array watts and battery voltage in, minimum controller amps out. Check voltage separately with the series calculator.
Inputs
Controller size
Enter your numbers.
What size charge controller do I need?
An MPPT controller's job is to turn array power into battery current, so the charge current is roughly array watts / battery voltage. A 1,200 W array into a 24 V bank is 50 A. Add headroom for cold, bright conditions and pick the next standard controller size up. This tool uses a 25% headroom default as a convention; it is not a published standard. Separately, the maximum source circuit current is panel Isc x 125% (EC&M, Mike Holt on NEC 690.8), which you compare with the controller's maximum PV input current. Voltage is the other limit; use the series calculator.
Worked example. 1,200 W at 24 V = 50 A. With 25% headroom, 62.5 A, so a 70 A or larger controller. Raising the bank to 48 V halves the current to 25 A (31.3 A with headroom).
Frequently asked questions
What size MPPT charge controller for 1000 watts of solar?
About 1,000 / battery volts: 83 A at 12 V, 42 A at 24 V and 21 A at 48 V, before headroom. A higher battery voltage allows a smaller, cheaper controller.
Why divide by battery voltage and not panel voltage?
An MPPT controller converts the panel's higher voltage down to battery voltage, so power stays about the same and current rises on the battery side.
Do I still need to check voltage?
Yes. The controller also has a maximum PV input voltage. Use the solar panels in series calculator to keep cold-weather string voltage under it.
Can I oversize the array?
Many controllers clip extra power safely, but the rules differ by model. Follow the manufacturer's maximum array watts and current for your battery voltage.
Sources
Checked
- EC&M (Mike Holt): Article 690, Solar Photovoltaic Systems, Part 2 (2016)
- U.S. EIA: Measuring electricity
Amps = watts / volts is Ohm's law style power arithmetic (EIA units). The 125% Isc figure is from the EC&M article on NEC 690.8 (2016). The headroom percentage is a placeholder convention. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Planning estimate only. Always follow the controller's datasheet limits for voltage, current and maximum array watts, and your local electrical code.