Use this RV generator size calculator to find the generator your RV needs. Pick your air conditioner setup, add the watts of everything else that runs with it, choose your altitude, and see the starting watts to shop for.
How to Use the Calculator
Pick your A/C setup, then type the watts of everything else that runs at the same time. Choose your camping altitude last. The result shows the generator size to shop for, in starting watts, plus the running and starting watts behind it.
Starting Watts vs. Running Watts
A roof A/C needs a big jolt for a second or two to start its compressor, then settles down. That jolt is the starting watts, and your generator has to cover it. Published numbers vary by brand and model, so the calculator uses cautious ones.
Micro-Air lists about 1,500 running watts and up to 2,800 starting watts for a 13,500 BTU unit, and about 1,800 running and 3,500 starting for 15,000 BTU. The calculator counts 1,500 running and 3,500 starting for the smaller unit, and 1,800 running and 4,500 starting for the larger one. With two A/C units, it assumes they start one at a time.
How the Calculator Sizes Your Generator
It adds your other watts to your A/C running watts to get your running load. It also works out your starting load, which is your other watts, plus any A/C that is already running, plus one A/C starting. Both numbers feed the answer.
The generator must cover the starting load, and your running load should stay at or under 80 percent of its rating. The calculator takes the larger of the two, adds your altitude cushion, and rounds up to the next 500 watts. Micro-Air also adds a 20 to 30 percent safety margin to its own sizing, so a cushion is normal practice.
Find Your Other Watts
Add up what runs while the A/C runs: the fridge, lights, TV, and a phone charger or two. The calculator starts you at 600 watts as a planning number. A microwave, coffee maker, or hair dryer can each pull 1,000 watts or more, so count them if you use them with the A/C. If a label lists amps instead, multiply by 120.
Generator Size by A/C Setup
Here is what the calculator gives with 600 other watts at low altitude. Each line shows the starting watts and the generator size to shop for.
- One 13,500 BTU A/C: 4,100 starting watts, so shop for 4,500 W
- One 15,000 BTU A/C: 5,100 starting watts, so shop for 5,500 W
- Two 13,500 BTU A/Cs: 5,600 starting watts, so shop for 6,000 W
- Two 15,000 BTU A/Cs: 6,900 starting watts, so shop for 7,000 W
- No A/C: 600 running watts, so a 1,000 W generator is enough
Altitude and Heat Cut Power
RV Travel says engine power decreases 3.5 percent for every 1,000 feet above 500 feet, and 1 percent for every 10 degrees above 85°F. Its authors lost about 18 percent at 5,000 feet and 105°F, and their generator could no longer run the A/C.
The calculator adds a 10 percent cushion from 2,000 feet, 20 percent from 5,000 feet, and 30 percent from 8,000 feet. For one 13,500 BTU A/C and 600 other watts, that moves the answer from 4,500 W to 5,000, 5,500, and 6,000 W. These steps are planning approximations, so add more if it will also be hot.
Soft Starters Can Shrink the Size
A soft starter cuts the A/C’s startup surge. Micro-Air says its EasyStart reduces starting current by 60 to 75 percent on most rooftop units, which lets a 2,000-watt generator start one. Manufacturers typically advise 4,000 watts or larger without it, according to RV PRO.
The calculator does not count a soft starter, so treat its answer as the no-help number. Check your A/C and generator makers’ figures before you downsize.
Check Your Plug
RV Travel says a 30-amp RV service is 3,600 watts and a 50-amp service is 12,000 watts. Your RV cannot draw more than its service allows, so a bigger generator does not give a 30-amp RV more power. Two A/C units usually mean a 50-amp RV.
Your Next Step
Count the watts you run with the A/C, pick your altitude, and note the answer. If you only need lights, a fridge, and electronics, check our battery runtime calculator and solar calculator first, because a quieter setup may be enough.