Generator Sizing Calculator
Buy the right size, not the biggest. Add up what you need to power and we’ll size a generator with proper headroom for motor startup surges.
Compare portable and standby generators
Learn moreWhy startup surge matters
A generator has to handle two loads: the steady running watts of everything plugged in, and the brief starting surge when a motor kicks on — which can be two to three times its running draw. Size for the peak, add headroom, and you won’t trip or stall under load.
How it’s calculated & sources
Peak load = total running watts + the single largest motor surge. Recommended size = peak × (1 + headroom). We then suggest the next common generator size that covers it.
Benchmark: size for running watts plus the largest motor’s starting surge, with ~20% headroom (generator manufacturer guidance).
Results update as you type and are general estimates, not personalized financial, tax, medical or legal advice. Verify with a professional.
Worked example
4,000 running watts plus a 2,200-watt surge is a 6,200-watt peak; with 20% headroom you want about 7,500 watts — a 7.5 kW unit.
Frequently asked questions
Do I add every motor’s surge?
No — motors rarely start at the exact same instant, so add only the single largest surge to the running total. That’s the standard sizing convention.
Portable or standby?
Portables (3–8 kW) cover essentials; whole-home standby units (10–24 kW) run everything automatically. Size to the circuits you actually need during an outage.