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 motor watts × 3–7. Check the DC current (P ÷ V) against battery C-rate and cable size.

Inputs

W
W

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

Results

Suggested continuous rating
1,111W
Suggested surge rating
3,000W

Recommended gear

Engineering methodology

Suggested continuous rating = (combined running watts ÷ inverter efficiency) × 1.25. Suggested surge rating = the larger of that continuous value and (largest motor running watts × surge factor). Induction motors often need 3–7× running watts for a short start. Efficiency is DC-to-AC conversion (typically 85–95%).

Continuous watts vs surge watts

Continuous rating = (combined running watts ÷ efficiency) × 1.25. Surge is the larger of that value and motor watts × 3–7 for induction starts. 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. 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. Surge duration varies by inverter model.

What is the extra 25% on continuous watts?

Headroom for efficiency already in the denominator, plus load growth and inverter derating. Continuous inverter watts = (running watts ÷ η) × 1.25.

Can I add every appliance’s watts together?

Add what can actually run at once. Then check the single largest motor for surge. Diversity (not everything on together) is real; this tool does not guess your diversity factor.

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