Off-Grid Solar Calculators

Inverters and charge controllers

Inverter Sizing Calculator

List running watts of everything that might be on together, then check motor starting. Induction loads such as fridges, well pumps and air conditioners often need 3–7× running watts for a second or two.

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%).

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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