Generator sizing calculator
Tick what will run at the same time. The calculator adds running watts, then adds only the largest single surge, and recommends a generator with 20 percent headroom.
- Running load
- 1,850 W
- Peak with the largest motor starting
- 3,350 W
- Generator to buy
- 4,500 W running
Only one motor starts at a time, so peak is running load plus the single biggest surge.
Peak plus 20 percent so the generator is not at its limit, rounded up to the next 500 W.
Running watts and starting watts
Anything with a motor or compressor draws far more for the second or two it takes to spin up. A refrigerator running at 700 watts pulls around 2,200 to start. Ignore that and the generator stalls every time the compressor cycles.
You do not add every surge together, because motors do not all start at the same instant. Add running watts for everything, then add the biggest single surge above running for the largest motor. That is the peak the generator has to survive.
| Load | Running W | Starting W |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Sump pump 1/2 hp | 1,050 | 2,200 |
| Well pump 1/2 hp | 1,000 | 2,100 |
| Furnace fan 1/2 hp | 800 | 2,350 |
| Window AC 10,000 BTU | 1,200 | 3,600 |
| Microwave 1,000 W | 1,000 | 1,000 |
| Electric kettle | 1,500 | 1,500 |
| Space heater | 1,500 | 1,500 |
| Circular saw 7 1/4 in | 1,400 | 2,300 |
| Air compressor 1 hp | 1,600 | 4,500 |
| MIG welder 140 A | 3,000 | 4,000 |
| Table saw 10 in | 1,800 | 4,500 |
What a 3,500 watt generator can run
A 3,500 watt running, 4,000 watt peak unit is the common portable size. It comfortably runs a refrigerator, a sump pump, lights, television and phone chargers together, with room for a microwave used briefly.
It will not run that lot plus a 1,500 watt space heater or kettle, and it will not start a well pump while the refrigerator and sump pump are both running. Anything with heating elements is what tips a small generator over.
For a site, the same generator runs a circular saw and a work light, or an angle grinder and a battery charger. It will not run a 140 amp MIG welder or a 1 hp compressor reliably alongside anything else.
Whole house and standby sizing
A standby generator wired through a transfer switch is sized differently, because it has to start whatever the house asks for without anyone managing loads. Add up the running load of everything that could be on, add the largest motor surge, and expect a figure of 12 to 20 kW for a typical house with electric appliances, or 7 to 10 kW if heating and cooking are gas.
Air conditioning is usually the deciding load. A 3 ton unit needs around 5 kW running and 15 kW or more to start without a soft-start kit, and that surge alone sets the generator size for many homes.
