If several motors might start together, such as a well pump and a sump pump after a power cut, add their surges or stagger them by switching them on one at a time.
How the generator calculator works
GeneratorSize multiplies each appliance's running watts by its quantity and adds them up. For every appliance it works out the extra starting surge (starting watts − running watts) and adds only the largest one, assuming one motor starts at a time. The recommended running rating is the running total plus your headroom, rounded up to 10 W. Current is watts ÷ 120 V, or ÷ 240 V for a 120/240 V generator with the load split evenly between its two legs.
Biggest starting surges in the list
| Appliance | Running W | Starting W | Extra surge |
|---|---|---|---|
| Central AC 3 ton (240 V) | 3,500 | 10,500 | 7,000 |
| Air compressor 1 HP | 1,600 | 4,500 | 2,900 |
| Window AC 10,000 BTU | 1,200 | 3,600 | 2,400 |
| RV air conditioner 15,000 BTU | 1,800 | 3,500 | 1,700 |
| Furnace fan ½ HP (gas or oil furnace) | 800 | 2,350 | 1,550 |
| Refrigerator / upright freezer | 700 | 2,200 | 1,500 |
| RV air conditioner 13,500 BTU | 1,500 | 3,000 | 1,500 |
| Garage door opener ½ HP | 875 | 2,350 | 1,475 |
Generator types by size
| Running watts | Typical type |
|---|---|
| Up to 2,500 | small inverter generator (about 2,000–2,500 W) |
| 2,500 – 4,500 | mid-size inverter or portable generator |
| 4,500 – 12,000 | large portable generator |
| Over 12,000 | home standby generator |
Wattages are typical figures of the kind published in generator makers' wattage charts; real appliances vary with age, size and efficiency, so use the data plate where you can. Have an electrician install any transfer switch or interlock, run generators outdoors away from windows and doors (carbon monoxide), and use a CO alarm indoors.
Frequently asked questions
Do I add up all the starting watts?
No. Add all running watts plus the single largest extra starting surge (starting minus running watts) of one appliance.
What happens if the surge is too big?
The generator's voltage drops, the motor may stall or fail to start and the generator's breaker may trip.
How much headroom should I leave?
About 10–20% above the running total keeps the generator from running flat out and leaves room for small extra loads.
Is GeneratorSize free?
Yes, with no sign-up or ads.
Is my list saved?
Yes, in this browser only.