RV Air Conditioner Generator Calculator (13.5K & 15K BTU)
Data reviewed ·how we calculate
The roof air conditioner is the whole generator conversation in RV life: everything else in the rig — lights, fridge on propane, phone charging, water pump — is a rounding error next to it. A 13,500 BTU unit runs at about 1,500W and demands roughly 3,000W at compressor start, which is precisely why the classic pairing is a 3,500W+ portable or two 2,200W inverters in parallel. Pick your unit’s BTU rating below; the soft-start FAQ covers the upgrade that lets a single small inverter do the job.
Size a generator for this load
Computed on typical values; ranges shown below. 25% headroom applied.
How to size it step by step
Read the roof unit’s model plate (or the spec sticker inside the return-air grille): 13,500 and 15,000 BTU are the two standard sizes; running and starting figures above are the planning ranges.
Size to the start: 3,000W typical for a 13.5K unit. With 25% headroom that is 3,750W — the 3,500–4,500W class, or two 2,000–2,200W inverters with a parallel kit.
Add the rest of the rig honestly: converter/charger (up to ~500W when the batteries are hungry), fridge on AC if not propane (~300W), microwave when used. The RV weekend scenario page runs the full combination.
If you want one small quiet inverter to do it alone: a soft-start kit on the roof unit drops the surge ~60–70%, letting a single 2,200W-class inverter start a 13.5K unit — the most popular electrical upgrade in RVing for good reason.
Pro tips
- Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
- Run the generator physically away from the rig and never under the awning or near a window — RV carbon-monoxide deaths cluster exactly there. Distance and a working CO detector inside the rig are non-negotiable.
- Start the AC with the batteries topped up: the converter’s charging draw can be the few hundred watts that make the compressor start fail on a marginal setup.
The data behind this calculator
| Figure | Value | Source |
|---|---|---|
| 13,500 BTU running / starting | ~1,500W / ~3,000W | Dometic Brisk II / Coleman Mach 3 specs; RV sizing guides |
| 15,000 BTU running / starting | ~1,800W / ~3,500W | Coleman Mach 15 / Dometic specs; RV sizing guides |
| Voltage | 120V (30A RV shore-power systems) | RVIA electrical standards for 30A rigs |
| Soft-start kit effect | starting surge cut ~60–70% | Micro-Air EasyStart 364 manufacturer data (RV application) |
| Altitude derating (generator side) | ~3% output loss per 1,000 ft elevation | Honda/Champion generator manuals (carbureted engine derating) |
Duty cycle: On a hot afternoon the compressor barely cycles off — plan fuel for near-continuous running watts, and remember every restart repeats the full surge. High altitude derates generator output ~3% per 1,000 ft, which bites exactly where people camp.
RV air conditioner generator questions, answered
What size generator do I need to run a 13,500 BTU RV air conditioner?
About 1,500W running and 3,000W starting — with 25% headroom, 3,750W, which is the 3,500–4,500W class. In RV practice that means either one open-frame 3,500–4,500W portable, or the beloved quiet option: two 2,200W inverter generators with a parallel kit, giving ~4,400W combined. A single stock 2,200W inverter will NOT start a stock 13.5K unit; it becomes possible after a soft-start kit is installed on the AC.
Will a soft-start kit let my 2,200W inverter run the roof AC?
For a 13,500 BTU unit, usually yes — this is the canonical use case. A soft-start controller (Micro-Air EasyStart is the standard name) ramps the compressor instead of slamming it, cutting the ~3,000W surge to roughly 1,000–1,300W above running draw per manufacturer data. Running draw (~1,500W) then fits a 2,200W inverter with modest margin — but margin is the operative word: on a 95°F afternoon with the converter charging batteries, you are still near the edge. 15K units want the parallel kit regardless.
Why two paralleled 2,200W inverters instead of one 4,500W generator?
Weight, noise and redundancy. Two 2,200W suitcase inverters weigh ~50 lb each and purr at 50–60 dB; one 4,500W open-frame unit weighs 100+ lb and roars. Campground quiet hours and your neighbors’ goodwill are real constraints. The pair also degrades gracefully — one unit still runs everything except the AC. The price premium over a single big portable is the cost of those advantages, and most RVers pay it happily.
How much fuel does RV air conditioning burn per day?
The honest number that shapes boondocking plans: a 13.5K unit at high summer duty cycle averages ~1,200–1,400W, which on inverter generators works out to roughly 3–5 gallons of gasoline per 24 hours of cooling. A weekend of round-the-clock AC is a 10-gallon commitment. Mitigations that actually work: park in shade, run the AC hard midday and coast on fans at night, and insulate the roof vents — each trims real gallons.
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