Generator Size Calculator

Estimate the generator size needed to run multiple loads, accounting for motor startup surge.

Generator Size
— starting watts
Handles startup surge.

Usage Tip

Stagger when you switch on big motor loads like a well pump or AC so their surges do not stack.

THE MATH
running watts = sum of all loads
starting watts = running + (largest load × (surge − 1))
A generator must supply the running watts of everything at once, plus the extra surge motors draw at startup. The surge applies to the single largest load.
Enter your loads in watts and a motor surge multiplier (about 3 for motors, 1 for purely resistive loads).
Size the generator to the starting-watts figure.
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.