Inverter size calculator
Estimate inverter continuous watts, total starting surge, and battery current from simultaneous AC loads, headroom, voltage, and efficiency.
Review your appliance budget, but enter only simultaneous inverter-powered loads here. After estimating current, check the cable run below.
Enter your system assumptions
Formula and assumptions
continuous target = load W × (1 + headroom / 100); surge target = max(continuous target, total peak W); battery A = load W ÷ battery V ÷ efficiency fractionEnter measured AC watts, not daily Wh. Peak is the total load during a start event, including appliances already running. The 25% headroom is an editable planning allowance, not a code rule.
Worked example
For 1,000 W running load, 2,000 W total peak, 25% headroom, 12 V, and 90% efficiency: plan at least 1,250 W continuous, 2,000 W surge, and approximately 92.6 A from the battery at the entered running load.
Frequently asked questions
How do I use watts versus VA?
This tool uses real watts. Compare watts and VA at your load power factor to check both continuous ratings; a VA rating alone is insufficient.
Is any 2,000 W surge rating sufficient?
No. The supported surge duration must exceed the appliance start duration. Check wave shape, temperature derating, battery/BMS peak current, low-voltage behavior, and the manufacturer’s requirements.