For your own appliance, read the data plate: watts may be printed directly, or multiply volts × amps. A motor typically needs 2 to 3 times its running watts to start; the locked-rotor amps (LRA) on an air conditioner's plate give its starting current.
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
How many watts does a refrigerator use?
Typically about 700 running watts and 2,200 to start, though modern efficient fridges often run on less.
How many watts does a sump pump need?
About 800 running and 1,300 starting watts for ⅓ HP, and about 1,050 running and 2,150 starting for ½ HP.
How do I find the watts of an appliance?
Look at the data plate or manual. If it lists amps, multiply by the voltage (120 V or 240 V).
Is GeneratorSize free?
Yes, with no sign-up or ads.
Is my list saved?
Yes, in this browser only.