Generator Size Calculator
Size a backup generator based on the appliances and circuits you actually want to keep running.
Generator size needed
- Recommended generator size
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How this calculator works
A generator needs enough capacity for the combined running watts of everything you want powered at once, plus the surge (startup) watts of the single largest motor-driven appliance turning on — motors like well pumps, refrigerator compressors, and AC units draw a brief spike well above their steady running wattage when they start. This calculator sums the running loads and adds only the biggest surge spike, since appliances rarely all start at the exact same instant, then adds 20% headroom and rounds up to a common commercial generator size.
Where these numbers come from
Typical running and surge wattage figures for common household appliances are drawn from generator manufacturer sizing guides (Generac, Honda, Champion) and are representative averages — your specific appliance's nameplate rating (often on a label on the unit itself) is more accurate than a generic average if you have it.
Frequently asked questions
Why does surge wattage matter more than running wattage?
A generator that can handle the combined running load might still stall or trip when a well pump or refrigerator compressor kicks on, because that brief startup surge can be 2-3x the appliance's steady running wattage.
Do I need to add up every appliance's surge watts?
No — appliances rarely all start at the exact same moment, so this calculator only adds the single largest surge on top of the total running load, which reflects realistic worst-case timing rather than an unrealistically pessimistic sum of every surge at once.
Should I get a bigger generator than this recommends?
The 20% headroom built into the recommendation covers normal variation, but if you plan to add appliances later or want quieter, lower-load running, sizing up further doesn't hurt and gives more flexibility.
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