Generator Size Calculator
Determine the right generator size for backup power by entering appliance running watts and surge multipliers.
| Appliance | Running Watts | Surge Multiplier | Surge Watts |
|---|
Running Watts vs. Surge Watts
Every motor-driven appliance (refrigerators, pumps, AC units, power tools) needs extra power to start up. This startup spike — called surge or starting watts — can be 1.5× to 3× the running wattage and lasts only a fraction of a second. Your generator must handle the highest single surge load plus the running load of all other appliances simultaneously.
The 20% safety margin built into the recommended size accounts for efficiency losses, fuel mixture variation, and the fact that running a generator at or near 100% capacity shortens its lifespan significantly. Most manufacturers recommend loading generators to no more than 80% of rated capacity.
When in doubt, size up. A generator that's slightly oversized is far better than one that trips under load. For whole-home backup, consult a licensed electrician for a proper load calculation.