Off-Grid Solar Calculators

Inverters and charge controllers

Inverter Sizing Calculator

Choose inverter continuous and surge watts for mixed loads and induction motors. Continuous = (running watts ÷ efficiency) × 1.25. Surge is the larger of that value or the other running loads plus the largest motor’s start at 3–7× its running watts. Check the DC current, about AC watts ÷ (battery volts × efficiency), against battery C-rate and cable size.

Inputs

Units
W

Everything that can run at once, including the largest motor’s running watts.

W

Resistive loads ≈ 1×. Fridge compressors often 3–5×. Well pumps and A/C can be 5–7×.

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Suggested continuous rating = (combined running watts ÷ inverter efficiency) × 1.25. Suggested surge rating = the larger of that continuous value and the start peak: the other running loads (combined running watts − largest motor running watts) plus largest motor running watts × surge factor. The combined running load must include the motor, so a motor larger than it makes the result invalid. If motor watts are zero, surge equals the continuous rating. Induction motors often need 3–7× running watts for a short start. Efficiency is DC-to-AC conversion (typically 85–95%). Inverter nameplates rate AC output, so dividing by efficiency is deliberate extra margin: the quotient is the DC power drawn. Surge ratings last milliseconds to a few seconds — they are not extra daily energy.

Continuous watts vs surge watts

Continuous rating = (combined running watts ÷ efficiency) × 1.25. Surge is the larger of that value and the start peak: everything else still running plus the largest motor at 3–7× its running watts for an induction start. A fridge or well pump can need several times running watts for a second; a kettle does not.

DC current the battery must supply

Inverter input current is about AC watts ÷ (V_battery × efficiency). A 2,000 W inverter on 12 V at 90% is on the order of 185 A, and more as the battery voltage sags. Check C-rate and cable ampacity; the AC nameplate hides that DC current.

Frequently asked questions

Why do fridges and well pumps need so much inverter surge?

Induction motors draw locked-rotor current at start, often 3–7× running watts for a second or less. Size surge from the nameplate LRA or a measured start, not from running watts alone: LRA × AC voltage gives the start in VA, a safe figure to compare with the surge rating. Surge duration varies by inverter model.

What is the extra 25% on continuous watts?

It is headroom for load growth and heat derating, since inverters should not run at their full rating for hours. Efficiency is a separate step: dividing by η gives the DC power drawn, which adds a little more margin because inverter nameplates rate AC output. Continuous inverter watts = (running watts ÷ η) × 1.25.

Can I add every appliance’s watts together?

Add what can actually run at once, including the largest motor’s running watts. The surge check then puts that motor’s start on top of everything else. Diversity (not everything on together) is real; this tool does not guess your diversity factor.

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