Backup Power

Generator Size Calculator

A generator must handle both your steady running load and the brief surge when motors start. This calculator estimates the size you need with a safety margin.

Your details

W

Sum of everything running at once.

W

Extra watts the biggest motor needs to start (fridge, pump, AC).

%

Headroom so you never run at 100%. 20% is sensible.

Results

Recommended capacity
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Recommended capacity
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Last updated June 30, 2026. Results update automatically as you type.

How it works

Recommended capacity = (running watts + largest surge) × (1 + safety margin ÷ 100). Sizing for the surge plus headroom stops the generator stalling when a motor kicks in and avoids running it flat out.

Frequently asked questions

Why add a surge figure?

Motors (fridges, pumps, AC) briefly draw 2–3× their running watts when starting. The generator must cover that spike, not just the steady load.

Why a safety margin?

Running a generator constantly at full capacity shortens its life and risks overload. Sizing 20% above your need gives reliable headroom.

How do I find my running watts?

Add up the wattage of everything you want to power simultaneously. Our appliance running cost calculator can help you find each device’s draw.

These calculators provide general estimates for educational purposes only and are not financial advice. Real-world results depend on factors not captured here. Verify figures independently before making any purchase or financial decision.

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