Standby generators are usually sized by a licensed electrician or dealer from a load calculation; load management modules and soft-start kits for air conditioners can allow a smaller unit.
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
What size generator for a whole house?
Many homes with central air land in the 14 to 24 kW range; homes with all-electric heat and appliances need more. Add up your loads here and have an electrician confirm.
Can a generator start a central air conditioner?
Yes if its surge rating covers the compressor start. A 3-ton unit can need around 10,000 watts or more to start; a soft-start kit reduces that a lot.
Do I need 240 volts?
Yes for central air, electric water heaters, ranges and dryers, and many well pumps.
Is GeneratorSize free?
Yes, with no sign-up or ads.
Is my list saved?
Yes, in this browser only.