A 30-amp RV service is 120 volts × 30 amps = 3,600 watts, and a 50-amp service is 120/240 V × 50 amps per leg, up to 12,000 watts.
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 to run an RV air conditioner?
About 3,000 starting watts for a 13,500 BTU unit and about 3,500 for 15,000 BTU, plus everything else that is running. A soft-start kit can cut the starting surge substantially.
Can I run the microwave and the AC together?
Only if the running total stays under the generator's rating; with the starter example that is about 3,600 W, just within a 30-amp RV service.
Are inverter generators better for RVs?
They are quieter and their clean power suits electronics, which is why many campgrounds prefer them.
Is GeneratorSize free?
Yes, with no sign-up or ads.
Is my list saved?
Yes, in this browser only.