A generator that is too small will trip or stall when a motor starts. One that is too large wastes fuel and money. The right size starts with a list of the loads you want to power.
Step 1: List running and starting watts
Motors such as refrigerators, sump pumps and furnace blowers draw two to three times their running watts for a moment when they start. Check nameplates or the manufacturer's data.
- Refrigerator: 700 W running, 2,100 W starting
- Sump pump: 800 W running, 2,400 W starting
- Furnace blower: 600 W running, 1,800 W starting
- Lights 400 W, microwave 1,000 W, TV and electronics 300 W
Step 2: Add the running loads
700 + 800 + 600 + 400 + 1,000 + 300 = 3,800 W running.
Step 3: Account for starting surge
Assume motors start one at a time. The worst case is the largest starting load: 3,800 − 800 + 2,400 = 5,400 W peak.
Step 4: Treat the margin as a planning allowance, not a generator selection
A 25% planning margin gives 3,800 × 1.25 = 4,750 W for the running-load screen. This is not enough to select a generator. Do not divide the starting load by a generic surge multiplier: generator motor-start capability depends on alternator transient performance, engine response, motor locked-rotor kVA, starting method, allowable voltage/frequency dip, load sequence and site derating. Use the generator manufacturer’s sizing software or transient curves and verify the complete load list with a qualified designer.
Step 5: Compare kW and kVA ratings
Check both the generator’s continuous kW and kVA ratings and the manufacturer’s motor-starting/transient capability. A simple kW-to-kVA conversion at a nominal power factor does not establish that the generator can start a particular motor.
Safety and code
- Use a transfer switch or interlock; never backfeed a panel (NEC 702.5 for optional standby systems).
- Run portable units outdoors and well away from windows and doors to avoid carbon monoxide.
- Derate for altitude and high temperatures, which reduce engine output.
Try the calculators
For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.