Why motors surge
A motor at rest needs extra current to get turning. Fridge and freezer compressors, sump and well pumps, furnace fans, air conditioners and power tools all do this. Heaters, lights, toasters and electronics don’t: their starting watts equal their running watts.
How to size for it
- Add the running watts of everything you want on at the same time.
- Find the one appliance with the biggest jump from running to starting watts, and add that jump.
- Add about 20% headroom.
Example: fridge (400 run / 1,200 start), 1/2 HP sump pump (1,050 / 2,150), 1/3 HP furnace fan (700 / 1,400), five LED bulbs and a router. Running is 2,245 W. The sump pump has the biggest jump (+1,100 W), so peak is 3,345 W. Plus 20% is 4,014 W: a 4,500 W class generator.
Don’t add every starting surge together. Motors rarely start at the same instant, and adding them all oversizes the generator.
The biggest start-up jumps
| Appliance | Running | Starting | Starting ÷ running |
|---|---|---|---|
| Mini fridge or cooler | 100 W | 600 W | 6.0× |
| Gas clothes dryer | 700 W | 2,200 W | 3.1× |
| Refrigerator (full-size) | 400 W | 1,200 W | 3.0× |
| Air compressor 2 HP | 2,500 W | 7,500 W | 3.0× |
| Air compressor 3 HP | 3,500 W | 10,500 W | 3.0× |
| Air compressor 1 HP | 1,600 W | 4,500 W | 2.8× |
| Furnace fan 1/2 HP (gas or oil) | 875 W | 2,350 W | 2.7× |
| Dishwasher (cool dry cycle) | 216 W | 540 W | 2.5× |
| Motor 1 HP (capacitor-start capacitor-run) | 1,000 W | 2,300 W | 2.3× |
| Motor 3/4 HP (capacitor-start capacitor-run) | 850 W | 1,900 W | 2.2× |
Running and starting values for all 86 appliances are in the appliance wattage chart.
On a power station
Power stations list continuous output and surge output. The surge figure has to cover your biggest start, just like a generator’s starting watts. Some makers don’t publish surge output; for those, the planner says it can’t confirm a motor will start.
Last reviewed 2026-09-26. Sources · How we calculate