BatteryBankCalc Tip

Solar charge controller calculator

An MPPT charge controller is rated by the charging current it can deliver to the battery. That current is roughly the array watts divided by the battery voltage, and a 25% safety margin is commonly added for cold, bright days when panels produce more than their rating. BatteryBankCalc uses the panels it suggests for your loads, or you can change the panel size. Doubling the battery voltage halves the controller current.

    Daily loads
    LoadWattsHours/dayQtyWh/day
    Solar and inverter settings

    Saved you some time? Leave a tip, entirely optional.

    Also check the controller's maximum PV input voltage against the open-circuit voltage of your panel string at the coldest expected temperature.

    How the battery bank calculator works

    BatteryBankCalc adds watts × hours × quantity for every load to get watt-hours per day. Battery watt-hours = daily Wh × days of autonomy ÷ (usable depth of discharge × inverter efficiency), and amp-hours = that ÷ system voltage. Batteries in series = system voltage ÷ battery voltage; strings in parallel = amp-hours needed ÷ battery amp-hours, rounded up. Inverter = sum of all load watts × (1 + headroom), rounded up to a standard size. Solar array = daily Wh ÷ inverter efficiency ÷ (peak sun hours × system efficiency); controller amps = installed panel watts ÷ system voltage × 1.25.

    Usable capacity by battery type

    BatteryTypical usable depth of dischargeUsable Wh from 12 V 100 Ah
    Lithium iron phosphate (LiFePO4)80%960 Wh
    AGM / gel lead-acid50%600 Wh
    Flooded lead-acid50%600 Wh

    Batteries in series for each system voltage

    System12 V batteries in seriesCurrent for a 1,000 W load
    12 V183.3 A
    24 V241.7 A
    48 V420.8 A

    Depth of discharge, inverter efficiency (90%), solar system efficiency (80%) and the 1.25 controller factor are common planning figures, not product specifications; follow your battery, inverter and controller manuals, and size cables and fuses for the current. A licensed electrician should connect any system to house wiring. Results are estimates.

    Frequently asked questions

    What size charge controller for 800 W of solar?

    800 W ÷ 24 V × 1.25 = about 42 A, so a 50 A controller on 24 V, or about 21 A (a 30 A controller) on 48 V.

    MPPT or PWM?

    MPPT controllers harvest noticeably more energy, especially with higher-voltage panels and in cold weather; PWM is cheaper and fine for very small 12 V systems with matching panels.

    Can I oversize the array?

    Many MPPT controllers allow it and simply limit the output current; check the manual.

    Is BatteryBankCalc free?

    Yes, with no sign-up or ads.

    Are my loads saved?

    Yes, in this browser only.

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