Air Conditioner Sizing Calculator

What size air conditioner do I need for my RV?

Pick the unit whose rated capacity, at the outside temperature you camp in, clears the heat coming in, and not much more, because an oversized unit cools the air before it has pulled the moisture out and cycles off into a clammy coach. The heat has five sources: conduction and leakage through the envelope (the same UA the heater sizing calculator uses, run the other way), sun on the roof, sun through the windows, people, and the fridge and electronics, plus a moisture share that depends on where you are. For a 30 ft standard-build trailer with a slide, at 95 °F in full sun with two aboard, the sum is 14,029 BTU/h, which a 15,000 BTU unit clears and a 13,500 doesn't quite. By length and build:

BTU/h of load → the smallest common unit (9,000 / 11,000 / 13,500 / 15,000 / 18,000) or pair that clears it; 95 °F outside, 75 °F set point, full sun on a white roof, two people, moderate humidity, 8 ft wide. The 11,000, 13,500 and 15,000 sizes are the steps in Dometic's rooftop range; 18,000 is also sold as a single unit. The tool uses your rig and your afternoon.

How many BTU per square foot for an RV?

A common rule of thumb is 60–80 BTU/h per square foot of floor. The worked trailer lands at 58, so the rule is roughly right for a standard build in the sun, and that's the only case it fits. In the shade the same rig is 39 per square foot; a four-season build in the sun is 37. A rule that can't tell those apart is how a rig that needed a 13,500 and some shade ends up with a 15,000. Line by line, the worked rig:

30 × 8 ft, 7 ft sidewalls, one slide, standard build (blended U 0.25, 1 air change/h), 95 °F / 75 °F, full sun, two adults, moderate humidity. Planning factors, stated as such; the test pins this rig against the 60 BTU/ft² rule.

Why can't my RV air conditioner keep up?

When the load crosses the unit's capacity, the thermostat can't help: the compressor runs 100% of the time and the coach settles wherever the heat coming in equals the heat going out. The tool prints that temperature, the answer to "it's set to 72 and reads 79". For the worked rig on a 13,500, by afternoon temperature:

Full sun, two people, moderate humidity, 75 °F set point. "Holds" is the inside temperature the unit can maintain once it runs flat out. The duty column is the number the AC-on-batteries calculator asks you for.

Three fixes, cheapest first. Shade (a tree, an awning, a reflective cover on the sunny windows), because sun is 33% of the load. A higher set point: every degree of ΔT you give up is 391 BTU/h on this rig. And only then a bigger or second unit, sized to the afternoon that failed rather than the record.

Sol-air roof delta 0 / 15 / 30 °F (+15 for a dark roof); window gain 10 / 40 / 80 BTU/h per sunlit ft².

Do I need a second AC in my RV?

You need one when the load passes the largest single rooftop unit, 18,000 BTU/h rated at your afternoon. Single rooftops that size are sold (Fogatti's InstaCool Pro 18,000 is rated at 13 A, 1,550 W), so a load between 15,000 and 18,000 is still one unit, and past it the answer is a pair. On a 35-foot-plus floorplan a single unit at the front leaves the bedroom warm regardless. Then the inlet has to carry two, which is where the second-AC plan meets the 30 A pedestal:

1,500 W and 1,667 W running (EER 9); "fits, hot" = under the 3,600 W breaker but over the 80% continuous rule; "start one at a time" = the second compressor's surge on top of the first running trips it. Soft starts fix the surge, and the 3,167 W of running load stays.

So a second unit on a 30 A rig means a soft start on both, one started at a time, the water heater on propane and the microwave off, or a 50 A inlet. The tool runs your two units against your inlet and says which case you're in; off the grid, the generator-size calculator does the same arithmetic against a generator.

Is a bigger RV air conditioner always better?

Capacity you don't need makes the unit cycle: it hits the set point in minutes, before the evaporator coil has run long enough to wring moisture out of the air, and the coach ends up cool and damp with a compressor that starts over and over through the afternoon. That is why this page sizes with no margin and rounds to the next unit up, never two; why a 15,000 on a 20-footer in Oregon is the wrong purchase; and why, for a lot of rigs that "can't keep up", a shadier site and a higher set point come before the next size. The exception is the inverter-driven rooftop unit, which throttles down instead of cycling and can be sized generously without the penalty, and starts without a surge, which changes the two-ACs-on-30-A table above too.