What Size Generator for RV AC?

The short answer

A 13,500 BTU roof air conditioner draws about 1,400 running watts and spikes to roughly 3,500 W for a fraction of a second when the compressor starts. Size for the spike, not for the 1,400.

That gives four answers, and one of them is probably yours.

• 13,500 BTU, as it came from the factory: a 3,500 W inverter generator. It has 3,000 W continuous and exactly 3,500 W of starting capacity.
• 13,500 BTU with a soft start kit: a 2,200 W inverter generator. The kit cuts the surge to about 1,800 W, and the 2,200 W class clears that.
• 15,000 BTU, as it came from the factory: 1,700 W running and 4,000 W starting, which puts you in the 5,000 W class.
• Two 2,200 W inverter generators in parallel: 3,600 W continuous and 4,400 W starting, which is the popular answer for a 30 amp rig.

The rest of this page is why, plus the two things that quietly change the answer: altitude and the 30 amp service limit.

Why a 2,200 watt generator fails

The number printed on the generator is its starting figure. A "2,200 W" inverter generator is rated 1,800 W continuous and 2,200 W for a moment. Your roof unit wants 3,500 W for that moment, so the generator is short by 1,300 W. The compressor stalls, the engine bogs down, and the overload light comes on.

None of that is about running watts. 1,800 W of continuous capacity would carry a 1,400 W air conditioner all afternoon if it could only get the thing spinning. If the split between those two numbers is new to you, our guide on starting watts versus running watts covers it properly.

Here is every RV load in our database with both figures side by side, plus what an hour of each costs a battery.

Look at the soft start row. Same air conditioner, same 1,400 running watts, surge cut from 3,500 W to 1,800 W. Nothing else on that list has anything like that spread between what it needs to run and what it needs to start.

What each generator class actually starts

This table checks every generator class we track against the 3,500 W surge of a 13,500 BTU roof unit. Continuous capacity is never the problem here. Starting capacity is.

The 3,500 W inverter class is the smallest single generator that clears it, and it clears it exactly, with no margin at all. That is fine at sea level on a cool morning and marginal on a hot afternoon in the mountains, which is the next section.

A soft start kit changes the answer

A soft start module ramps the compressor motor up over a second or two rather than throwing it straight to full speed. Running watts do not change. The surge does, from about 3,500 W down to about 1,800 W.

Run both versions through the sizing calculator and the recommendation moves from a 3,500 W inverter generator to a 2,200 W one. That is a smaller, lighter, quieter, cheaper generator carrying the same air conditioner, and it is usually a cheaper change than buying the bigger generator. It is also the only way to make a single 2,200 W unit work, because the 1,800 W continuous rating on that class sits just above the air conditioner's 1,400 W draw and nothing else you own will fit alongside it.

30 amps is 3,600 watts, and that is the real ceiling

A 30 amp RV service is one 120 V leg at 30 amps. 30 times 120 is 3,600 W total, for the whole coach. That is the number people forget when they shop for generators.

A 13,500 BTU air conditioner at 1,400 W draws 12 A of that, leaving about 18 amps for everything else. Add the microwave at 1,000 W and the converter at 450 W and you are at 2,850 W, inside the limit but not by much. Add an electric water heater element at 1,400 W on top and the pedestal breaker opens.

This is why two 2,200 W inverter generators in parallel is such a common answer. Together they produce 3,600 W continuous, which is exactly the 30 amp shore power limit, and 4,400 W starting, which clears the 3,500 W compressor surge with 900 W left over. You also get to run one generator alone on a mild day, which is quieter and burns noticeably less fuel, since a second engine idling costs you gallons even when it is barely loaded.

Altitude takes a bite in the mountains

Engines lose power with thin air. The rule of thumb this site uses is 3.5% per 1,000 feet above about 3,000 feet, so nothing happens at a Texas campground and a lot happens in Colorado.

At 8,000 feet the derate is 17.5%. A 3,500 W inverter generator behaves like a 2,890 W one, and it will no longer start a stock 13,500 BTU roof unit. To start that 3,500 W surge at 8,000 feet you actually need 4,240 W of sea level capacity, which is a 5,000 W generator.

The soft start version survives better but is not untouched. At 8,000 feet a 2,200 W inverter generator still clears the 1,800 W surge, but its continuous output falls to about 1,485 W against a 1,400 W load. It runs, with 6% headroom instead of the 20% we plan for, which is the reason to size up a class before you leave for the mountains. The calculator does not ask for altitude, so apply that derate yourself; it does ask which temperature band you will be working in, and a hot afternoon takes its own bite out of the same engine.

Running the roof air from a battery

People ask this constantly, and the honest answer is: for a nap, not for a night.

A 13,500 BTU unit averages about 980 W once its duty cycle is counted, and it needs a battery that can both carry 1,400 W continuously and survive a 3,500 W surge. Most 1,000 Wh class stations are rated 1,800 W continuous with only 2,700 W of surge, so they will not even start it.

A 2,000 Wh class station costing well over a thousand dollars gives you under two hours of cooling. A soft start kit helps a smaller battery start the unit, but it does not change how much energy air conditioning eats. If you want roof air off grid overnight, you are looking at a large lithium house bank and rooftop solar, not a carry-handle power station.

Sizing the whole weekend, not just the air conditioner

The air conditioner is the hard part, but it is not the only part. Our RV weekend list, roof air plus an absorption fridge, water pump, microwave, TV, converter and two phones, totals about 3,350 running watts, peaks near 5,450 W and burns 16.6 kWh over ten hours. That combination calls for a 7,500 W generator.

Almost nobody buys one. They stagger the loads instead: the microwave runs for two minutes with the air conditioner off, the water heater runs in the morning before anyone wants cooling, and the 3,500 W unit covers the whole trip. Build your own list in the calculator and switch items on and off to see which one is setting your peak. If fuel is the question rather than watts, see how long a generator runs on a tank.