RV Generator Size Calculator

What size generator do I need to run an RV air conditioner?

Two numbers, not one. A rooftop unit runs at 1,000–1,667 W across the 9,000–15,000 BTU sizes at a typical efficiency, more as it ages. For the start, the usual planning convention puts the compressor at about 2× its running draw for a fraction of a second while it comes up to speed, and this page uses that 2× when it has nothing better; the locked-rotor amps (LRA) on your unit's data plate are what decide it, and the tool uses them when you enter them. A generator has to clear the start with its surge rating and the running load with its rated (continuous) watts, with whatever else is on already counted in both. Computed by the engine for a bare rooftop unit at sea level, nothing else running:

Running watts at a typical rooftop EER of 9 (BTU per watt); starting surge 2× running without a soft start, about equal to running with one; class includes 10% running headroom. Your data plate's running amps and LRA beat these.

That is why the answer to "will a 2,000 W generator run my 13,500 BTU AC" is both yes and no: it can carry the 1,500 W running load with room to spare, and it cannot supply the 3,000 W the compressor asks for in the first half-second. A soft start closes exactly that gap, which is the next section.

Will a soft start let a smaller generator run the AC?

A soft start (Micro-Air EasyStart, SoftStartRV and similar) ramps the compressor up over a few hundred milliseconds instead of slamming it on, cutting the inrush by 65–75%, and it is usually the cheapest fix on this page: a 13,500 BTU unit that needed 3,000 W to start needs about 1,500 W. That moves it from the 3,200 W class down to the 2,200 W class, the class of Honda's EU2200i at 46.5 lb dry, a generator you can lift into a truck bed and run quietly at a campsite. On a rig that already owns a 2,000–2,200 W inverter generator, a soft start is a smaller purchase than the next generator class every time the math says the running load fits, and the result above says so when it does.

Does altitude change what size generator I need?

Cummins' own spec sheets for its Onan QG 5500 and QG 4000 RV generators say output "decreases 3.5% for every 1000 ft", more than most people expect, and this page applies that rate to every generator; a carbureted engine run high for long also wants the maker's high-altitude jet kit. Dual-fuel generators give up another 10% on propane: Champion lists its 201122 at 1,850 W running on gasoline and 1,665 W on propane. What two common generators deliver:

Rated / surge watts delivered; 3.5% per 1,000 ft (the Cummins figure), 10% for propane. Elevation of common boondocking country: Moab 4,000 ft, Flagstaff 7,000 ft, Leadville 10,000 ft.

At 6,000 ft a 2,200 W-class generator makes 1,422 W, less than a 13,500 BTU unit draws running, so the soft start that saved it at sea level can't save it in Colorado. The tool applies both derates to your generator and, when sizing a new one, inflates the nameplate you need so the derated output still clears the load where you camp.

Can I run the AC and the microwave at the same time?

A 3,000 W-rated open-frame running a 13,500 BTU unit has about 1,500 W spare, so a 1,200 W microwave fits while the AC is running. On the generator you own, the running total has to fit under its rated watts, and the order matters. Start the microwave first and the AC's 1,500 W start surge on top of 2,700 W trips it. The "also running" list above is there because the fridge on 120 V, the converter topping up the bank and the water heater's element are all drawing before anyone touches the microwave. Tick them and the result counts them in both the running and the starting check.

What size generator for a 30 amp vs 50 amp RV?

The service caps what the rig can take, whatever you need. A 30 A rig's inlet passes 3,600 W, so anything above a 4,500 W class is wasted through the cord, because the panel can't use it. A 50 A rig has two legs at 6,000 W each; a portable generator feeds it through a 50-to-30 adapter at 3,600 W, and running two ACs from it means a 50 A-capable unit in the 7,000 W-plus class, or two paralleled inverter generators with a 50 A parallel kit. The tool flags a recommendation that exceeds your inlet, because at that point the answer is a soft start, or running one thing at a time. The shore-power calculator shows what runs together on each service.

Inverter or open-frame?

For a rig with electronics, an inverter generator: clean sine-wave power, half the noise, and an engine that throttles down when the AC cycles off (the generator fuel calculator prices that difference by the gallon). Open-frame units win on watts per dollar and matter when you need a 3,500 W class or more and can park it far from the door. After curfew, the same AC on the bank is the AC-on-batteries question. Either way, treat the "also running" list as the size, buy the class the result names for your altitude, and spend the money you saved on the soft start.