Rv Air Conditioner Size Calculator

RV Air Conditioner Size Calculator that picks a rooftop BTU size from RV length and local summer heat, then checks running watts, start surges, shore power, and generator headroom.

RV Air Conditioner Size Calculator for Your Rig

The RV Air Conditioner Size Calculator picks a rooftop unit size from the length of your RV, then checks whether your shore power and generator can actually run it. Sizing is only half the job, because a unit that fits the floorplan can still trip a 30 A breaker or stall a small generator at startup.

RV owners use it when replacing a failed unit, adding a second one, or planning to camp on generator power. The results show running watts, the compressor start surge, shore power headroom and generator headroom side by side, so you can see which one limits your setup.

Enter the length of the living box in feet or meters, converted at 0.3048 meters per foot, then choose how hot your summers run and whether you park in shade or full sun. The Power and Generator panel takes your shore power service, any soft start, the generator’s running and peak watts, and other loads you run while cooling.

The Length Chart Behind the BTU Size

The starting BTU comes from a length chart that circulates widely among RV owners. A member of the Forest River Forums posted the same chart, which runs from 9,000 BTU at 18 feet to two 15,000 BTU units at 35 to 40 feet. Its original source is unclear, so the calculator treats it as a hot-day baseline of about 95°F rather than a maker’s rule.

$$BTU = C(L) \times k_{climate} \times k_{sun}$$

Here C(L) is the chart value for length L, the climate factor is 0.9 for mild summers or 1.1 for desert heat, and the sun factor is 1.12 for full sun on a dark roof. These factors are the calculator’s own adjustments, not published figures, and every electrical result that follows assumes 120-volt power.

A common mistake is measuring bumper to hitch, which can add several feet and push the result up a full unit size. Measure only the enclosed living space, and count a slide-out as part of it, since it adds floor area the unit has to cool whenever it is open.

The adjusted total is rounded up to a standard unit of 9,000, 10,000, 11,000, 13,500 or 15,000 BTU, and anything above 15,000 BTU is split into two or more units. That split surprises owners of mid-size rigs, because a 22-foot trailer in desert heat with full sun needs about 16,600 BTU and comes out as two 11,000 BTU units.

Running Watts and the Compressor Start Surge

For the running load, the RV Air Conditioner Size Calculator uses maker ratings where it has them. Dometic rates its Brisk II 13,500 BTU unit at 1,670 W in cooling mode under the ISO 5151 test, as listed by dealers such as Johnson’s Surplus, which is about 13.9 A at 120 V.

Real draw climbs in heat, and Micro-Air notes that many rooftop units pull 1,700 to 1,900 W on hot days. Starting is the harder moment, because the compressor draws a short surge far above its running current, and the calculator assumes about 45 A for a standard start.

Micro-Air says its EasyStart cuts start current by 65 to 75 percent compared with the compressor’s locked-rotor amps, and the calculator models a soft-started unit at under 20 A. That drop is often what decides whether a small generator can run the unit at all.

Will It Run on 30 A or 50 A Shore Power?

RV electrical expert Mike Sokol explains that 30 A service is a single 120 V leg worth about 3,600 W, while a 50 A pedestal has two separate 50 A legs that add up to 12,000 W. He also notes that a 20 A household outlet is rated for only 16 A of continuous draw.

The calculator applies that same 80 percent limit to every service, since an air conditioner can run for hours. On the default 22-foot setup, the AC and 500 W of other loads draw 2,170 W against a 2,880 W limit on 30 A, which leaves 710 W for everything else.

Two 13,500 BTU units would draw 3,340 W on their own, which is why two units usually mean 50 A service. On 50 A, the calculator splits the units across the two legs and puts your other loads on the busier one, while a 30 A to 50 A adapter adds no power at all.

Generator Math in the RV Air Conditioner Size Calculator

The generator check runs twice. Running watts are compared with the generator’s continuous output, and the start of the last compressor is compared with its peak output while everything else is already running, because that is the moment most small generators trip.

With a 2,800 W running and 3,000 W peak generator, the default setup has 630 W of running headroom. Starting needs 500 W of other loads plus a 5,400 W surge, so the generator falls 2,900 W short even though it can carry the AC once it is running.

Adding a soft start drops the surge to about 2,400 W, so the start needs about 2,900 W and the same generator clears it with 100 W to spare. Micro-Air says an EasyStart lets a Honda EU2000i start and run a 13,500 or 15,000 BTU unit, with strict load management and possibly not above 4,000 feet.

House batteries are a poor substitute for a generator. A 100 Ah lead-acid battery holds about 1,200 Wh, and the Amp Hour to CCA Calculator shows only half of that is usable, which runs one 13,500 BTU unit for roughly 20 minutes.

The RV Air Conditioner Size Calculator accepts lengths from 8 to 60 feet, and generator peak watts must be at least the running watts. Past 40 feet it scales the top of the chart in proportion to length, which is an extrapolation rather than part of the chart itself.

Unit wattages other than 13,500 BTU are estimates, so the label on your own unit should replace them when you have it. Real cooling also varies with insulation, window area, awnings and how early in the day you start the unit, so treat the size as a starting point.

Load Mistakes That Trip Breakers

Entering a generator’s peak rating as its running rating hides the real continuous limit, and on most inverter generators the two differ by several hundred watts. Leaving the electric water heater element out of Other Loads is another common slip, and soft start makers advise running it on gas while cooling on a generator.

Owners on the Airstream Forums also point to the converter charging the batteries as a reason a small generator struggles. Some owners enter the rig’s length with the slide-outs closed and then camp with them open, so check each input against how you actually use the RV.

RV AC Sizing Questions

What size air conditioner does a 30 ft RV need?

The length chart puts a 30-foot RV at about 20,000 BTU at 95°F. That is more than one standard 15,000 BTU unit can deliver, so the RV Air Conditioner Size Calculator recommends two 11,000 BTU units, which also gives you a backup if one fails mid-trip.

The Mild setting drops the need to about 18,000 BTU, and the calculator notes that one 15,000 BTU unit can manage in milder weather. In hot climates, two smaller units also cool a long floorplan more evenly than one large unit.

Can a 2,000 W generator run a 13,500 BTU RV AC?

Usually only with a soft start and nothing else running. A 13,500 BTU unit can run at about 1,670 W, but its start surge overloads a 2,000 W inverter generator without help, and owners report a Honda EU2000i tripping on exactly that start.

The same owners report the unit starting once an EasyStart was installed. Run the water heater and fridge on gas, and turn the converter off while cooling, since every watt they draw comes out of the small margin left for the compressor.

Can you run two RV air conditioners on 30 amp service?

Not reliably, because two 13,500 BTU units draw about 3,340 W while 30 A service tops out near 3,600 W. That leaves almost nothing for lights, the fridge or the converter, and a second compressor starting on top of the first can trip the breaker.

Many owners on 30 A run one unit at a time and switch between the front and rear as needed. Running both units together for a full afternoon is what 50 A service is built for, since each unit can sit on its own 50 A leg.