What Size Generator for an RV Air Conditioner? (Altitude and Heat Included)

Dometic publishes the answer and almost nobody quotes it: a minimum generator size of 3.5 kW for one rooftop air conditioner, 5.0 kW for two. That figure is identical for every model in both of its specification sheets. Then subtract what altitude and heat take off the generator — about 3.5% per 1,000 feet and 1% per 10°F above 60°F — because that is the part no sizing chart does.

What Dometic actually publishes

Not starting watts. Its specification sheets carry a column headed “Installer Supplied Minimum Generator Size, 1 Unit / 2 Units”, and the entry is 3.5 kW / 5.0 kW.

The striking part is what that figure does not do: it does not move. Across the Penguin II, the Penguin II High Capacity and the Brisk II Evolution sheets — 13,500 and 15,000 BTU, compressors from 52 to 70 locked rotor amps — every single row says 3.5 kW for one unit.

Meanwhile the ranking pages grade their answer by BTU: 13,500 gets “2,800 to 3,500 starting watts”, 15,000 gets “3,300 to 4,000”. They are differentiating on a column the manufacturer does not differentiate on, and they disagree with each other about where the bands fall.

Why nobody can give you an honest “starting watts” figure

Because the manufacturer does not publish one. What Dometic publishes is Locked Rotor Amps — the current the compressor would draw with the rotor stalled. On the models we read: 52.0, 61.0 and 63.0 A within 13,500 BTU Penguin II units alone, and 66.0 and 70.0 A on the 15,000 BTU Brisk II Evolution.

Turning amps into watts needs a power factor, and the power factor at locked rotor is low, variable and not published by anyone. Multiply 61 A by 115 V and you get about 7,000 — a number that is apparent power, not watts, and that would send you to buy a generator twice the size Dometic asks for.

So this page carries locked rotor amps as amps, and never converts them. Two 13,500 BTU air conditioners can differ by 11 locked rotor amps, which is the real reason BTU-keyed charts cannot agree with each other.

The number on the box is not the number you need

A generator advertised as “4,500 watts” carries 3,500 running watts. The 4,500 is the starting figure — a brief surge rating, not a continuous one.

Dometic does not say whether its 3.5 kW is continuous or peak. This page reads it as continuous, because that is how a generator’s size is conventionally rated and because it is the conservative reading of a figure the maker already calls a guideline. On that reading a 4,500-watt generator meets the minimum for one air conditioner exactly — 3,500 against 3,500 — at sea level, at 60°F, with nothing else running.

That is a margin of zero, and the next two sections spend it.

Altitude: 3.5% per 1,000 feet, and that is the good news

Two makers publish the same figure in unrelated documents. Honda’s EU2200i manual and Westinghouse’s iGen4500 manual both put it at about 3.5% of output for every 1,000 feet above sea level.

Honda adds the sentence that matters:

Even with carburetor modification, engine horsepower will decrease about 3.5% for each 1,000-foot (300-meter) increase in altitude. The effect of altitude on horsepower will be greater than this if no carburetor modification is made.

The 3.5% is the best case. Honda specifies a dealer carburetor modification above 2,000 feet, and says the loss is worse without one — while publishing no figure for how much worse. Most owners have never had that modification done, so the calculator’s numbers above 2,000 feet are optimistic and say so.

At 5,000 feet that is 17.5% gone. A 3,700-watt generator is down to 3,053 W before anything else happens.

Heat: the day you need it is the day it is weakest

Westinghouse publishes a second derate, and it is the only maker in these records that does: about 1% for every 10°F above 60°F ambient.

This is the one that catches people, because it runs the wrong way round. You need the air conditioner on a 95°F afternoon — which is exactly when the generator has lost 3.5% of its output.

Three of the four generators in our records publish no temperature figure at all. The calculator applies Westinghouse’s to all of them, which makes the silent three look worse than their sheets suggest — that is deliberate, and it is a comment on the documentation rather than on the machines. The one maker that tells you is not the one to penalise.

When nothing fits: the soft starter

The published lever is a soft starter, which reduces the compressor’s inrush. Micro-Air states up to 75% on the EasyStart Breeze product page and 50 to 70% in its FAQ — two figures from one maker, so both are given here.

What nobody publishes is the number you actually want: the minimum generator size for a soft-started air conditioner. Dometic’s 3.5 kW is stated without reference to soft starters, and Micro-Air quotes a current reduction, not a generator rating. The calculator therefore names the part and refuses to compute a smaller generator from it. Anyone who gives you that number has made it up.

Model	Running	Starting	RV outlet	Altitude derate	Temperature derate	Buy
Honda EU2200i	1.8 kVA rated (continuous)	2.2 kVA maximum	None — no TT-30R; 120 V 20 A household receptacles only	About 3.5% of engine horsepower per 1,000 ft, and greater than that without the dealer carburetor modification Honda specifies above 2,000 ft	Not published by Honda	Check price(opens in a new tab)
Champion 4500W	3500 running watts	4500 starting watts	120 V 29.2 A RV outlet (TT-30R), plus a 5-20R duplex and a 12 V DC outlet	Not published by Champion on its product page	Not published by Champion on its product page	Check price(opens in a new tab)
Westinghouse iGen4500	3700 running watts	4500 peak watts	TT-30R 30 A RV outlet	About 3.5% of maximum power per 1,000 ft above sea level	About 1% per 10°F (6°C) above 60°F (16°C) ambient	Check price(opens in a new tab)
Champion 6500W	5250 running watts	6500 starting watts	120 V 21.9 A RV outlet (TT-30R), plus an L14-30R, two 5-20R GFCI duplex, 12 V DC and USB	Not published by Champion on its product page	Not published by Champion on its product page	Check price(opens in a new tab)

Where this goes wrong

• Sizing on the starting watts printed on the box. 4,500 on the label is 3,500 running, and Dometic’s minimum is asking for the running figure.
• Sizing by BTU. Dometic gives 13,500 and 15,000 BTU units the same 3.5 kW, while their locked rotor currents run from 52 to 70 A. The BTU number is not the one that decides it.
• Ignoring altitude. At 5,000 feet a generator has lost 17.5% before you switch anything on, and above 2,000 feet Honda’s own figure assumes a carburetor modification you probably have not had done.
• Ignoring heat. 1% per 10°F above 60°F, on the day you need it most.
• Forgetting the rest of the RV. Dometic says in the same footnote that “the total power usage of your RV must be considered” — the converter, the fridge and the microwave are on top of its 3.5 kW. Our shore power calculator works out what else you are drawing.
• Buying a generator with no RV outlet. A unit without a TT-30R needs an adapter before it reaches a 30 amp rig at all, which is why the calculator never recommends one.

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