What size generator for an RV: the loads, the derates, and why no maker gives you a formula

The question has a real answer for your coach and no published answer for coaches in general, and it is worth understanding why before doing the arithmetic, because it explains the shape of every table on this page.

What each maker will and will not give you

Coleman-Mach makes air conditioners and refuses to size a generator for them. Its own FAQ says: "We are aware that the conditions under which our air conditioners might be operating may vary considerably from these design parameters. For this reason, we cannot assist in sizing a generator for you." What it gives instead is one piece of arithmetic, covered below, and the instruction to read the running amps off your own unit.

Dometic publishes a minimum and calls it a guideline. Its Penguin II instructions carry a minimum generator size per unit, and the footnote attached to that column is the important part: "Dometic, LLC gives GENERAL guidelines for generator requirements. These guidelines come from experiences people have had in actual applications."

Cummins publishes recommended ranges and frames them as guidance. Its published list opens by calling itself "some high-level guidance" and closes with the note that the ranges "could change based on your specific equipment."

Why the arithmetic is left to you

Because the load is a property of the coach rather than of the generator. Two motorhomes with the same air conditioner differ by the charger behind it and the altitude they camp at, and neither maker can see either. No maker publishes a margin percentage either, so this page does not invent one. What follows is the makers' own figures and the order to put them in.

The air conditioner, running

Onan publishes two ranges for the same load, and it is worth knowing which is which. Its appliance table lists an air conditioner at 1,200 to 2,400 watts and 10 to 20 amps, while the sentence about startup pairs its multiplier with 1,400 to 2,400 watts. Both are Onan's, and the second is the one the startup arithmetic belongs to.

The air conditioner makers publish it per unit rather than as a range, which is the figure to use if you have the model. Dometic's Penguin II table gives compressor running amps of roughly "10.5" to "12.6" depending on the model, and Coleman-Mach tells you where to find yours: "Find the running amp draw for your system (found on the data tag of the unit)" Reading that tag beats any table here.

The air conditioner, starting, which is the one that catches people

This is where a plan that adds up on paper fails in the campground. Onan states that a compressor can draw three to four times its running watts during startup, and puts the consequence in one sentence worth quoting: "Too much baseload can prevent air conditioners from starting."

Coleman-Mach gives a different multiplier, and it is the only sizing arithmetic it publishes: "To calculate the max potential startup: Find the running amp draw for your system (found on the data tag of the unit) and multiply it by 2.5." Then it tells you to take the result to a generator maker, which is the sentence before the refusal above.

Two makers, two numbers for the same physical event. Onan says three to four times; Coleman-Mach says two and a half. This page does not average them. If you are choosing a generator, the larger figure is the one that will not disappoint you, and the air conditioner's own locked-rotor amps are the figure that leaves the arithmetic behind entirely. Dometic publishes them per model in its Penguin II table, in the fifties and sixties of amps, and yours is on your own unit.

The converter, and the charger that hides behind it

Onan calls the charger an invisible load and the word is well chosen, because it is working whether or not anyone asked it to: "Battery chargers are activated automatically and can draw a large load (up to 3000 Watts). Manage electrical loads by adjusting battery charge rates to best suit your needs." Its footnote is blunter still: "Battery chargers can be a source of significant load and will be on whether they are connected to shore power or the generator set."

Its appliance table lists the charger and the converter separately, "Battery Charger Up to 3000" and "Converter 500-1000", which is the range between a modest converter and a large inverter charger.

The converter makers publish what the unit actually pulls from the wall, which is the figure the generator sees. Progressive Dynamics gives its PD9300 series by output size: 500 watts at 30 amps, 725 at 45, 1,000 at 60, and 1,300 at 80. A 60 amp charger is therefore a 1,000 watt load before anything else is switched on.

Everything else with a big element in it

The other loads are smaller and worth adding because they are the ones people forget. Suburban publishes its water heater element at "TOTAL CONNECTED WATTS = 1440 - AMPS = 12", which is more than half of a 2,000 watt generator on its own. Cummins' own appliance table puts a coffee pot at 900 to 1,200 watts and a microwave at 750 to 1,500. And the makers' rule for the total is stated plainly: "If the sum of the loads exceeds the generator set power rating, the generator set will shut down or its line circuit breakers will trip."

Read your own data plate rather than the table

Every figure above is a range because the makers cannot see your equipment. The running amps or watts are printed on your air conditioner's data tag, your charger's rating is on its case and in its manual, and the water heater's element wattage is in its own documentation. Five minutes with a torch beats every table here, and it is the step the makers assume you have already done.

The worked example, using only published figures

One 15,000 BTU air conditioner, one 60 amp converter charger, and nothing else running. Running load: the air conditioner's own figure, call it 1,700 watts, plus the Progressive Dynamics figure for a 60 amp unit, 1,000 watts. That is 2,700 watts of steady load, which a 3,000 watt generator would carry until the compressor starts.

Starting is where it goes wrong. Apply Onan's multiplier and the same unit wants three to four times its running watts for the moment of startup, so the compressor alone asks for something in the region of 5,100 to 6,800 watts at that instant, and the charger's 1,000 watts sits on top of it. This is exactly the case Onan describes, and its own remedy is to manage the load rather than to buy a bigger machine: turn the charger's rate down, or let the charger finish before the air conditioner starts. The spec sheet for one of its small diesels rates it for one conventional 15,000 BTU air conditioner plus up to 1,000 watts of additional load at 100 degrees and 500 feet of altitude, which is a narrower promise than a wattage figure alone suggests.

Onan's table and the 3.5 percent rule

A generator does not make its nameplate power at altitude or in the heat, and Onan publishes both derates rather than leaving you to discover them. The general rule: "Power decreases approximately 3.5% of rated power each 305 m (1000 ft) of increase in elevation." For temperature: "As ambient temperature increases, rated generator set engine power decreases approximately 1% for every 5.5 °C (10 °F) above 25 °C (77 °F)." Those two bases do not agree with each other: the blanket percentage derates from sea level, while the table below holds full rated power to 3,000 feet and steps down only from there. Both are Onan's, and the table is the one to use where it applies to your model.

What happens to the same generator at 6,000 feet

The manual carries a worked table for a 7.0 kW set, which is easier to use than the percentage: "Up to 914 m (3000 ft) | 7000 Watts", "1219 m (4000 ft) | 6755 Watts", "1524 m (5000 ft) | 6510 Watts", and then 6510 watts minus 245 watts for every further 1,000 feet. So the same generator is rated 7,000 watts up to 3,000 feet and 6,265 at 6,000, before the temperature derate is applied on top of it. Note which basis that comes from: it is the table's own step, not the percentage rule applied from sea level, and the two give different answers. A Colorado or Utah summer at that elevation is the case where a generator sized at home stops starting the air conditioner on the road.

Cummins' wattage table, and the two pages that disagree

Cummins publishes recommended wattages by coach class and air conditioner count, and it publishes two versions of them that do not match. Its news article gives "Class B Van with one AC unit: 2,000 to 3,600 watts" and "Class C RV with one AC unit: 2,800 to 4,000 watts". Its onboard generator guide gives "Class B Van with one AC unit | 2,500 to 2,800 watts" and "Class C RV with one AC unit | 3,200 to 4,000 watts". Both tables agree on the larger coaches: "Class A RV with two AC units (15,000 BTU each): 5,500 to 8,000 watts".

Same maker, two pages, different numbers for the same class. The two tables agree on the larger coaches and differ on the smaller ones, and the overlap is what both point at.

There is a second way to approach the same question, and it is Cummins' own: match the generator to the shore service the coach already has. "A 30-amp shore cord is rated at 120 V. Therefore, the max total power available is 3,600 watts." If the coach is built for 30 amp service and everything in it runs on a 30 amp pedestal, a generator of that size reproduces what you already have. Cummins then adds: "you can also get a 4,000-watt generator for these coaches to give additional electrical motor-starting power." Four hundred watts over the shore figure, and the reason given is the motor starting.

Dometic's per-unit minimums, and what "GENERAL guidelines" means

Dometic's table gives a minimum generator size per air conditioner and by number of units: 3.5 kW for one and 5.0 kW for two on the standard Penguin II, and 2.5 kW and 4.0 kW on its high-efficiency models. A reader who quotes one of those as "what Dometic says" is wrong for half the range, because the 2.5 against 3.5 kW difference is the efficiency class of the unit rather than a rounding difference.

Onan's full-line brochure answers the question a different way, with a per-model air conditioning capacity: its QG 4000 is rated for one 15,000 BTU unit, the QG 5500 for two, and the QD 10000 for three. That is the closest any maker comes to answering the question directly, and note what it is not: the smallest units are rated for a high-efficiency air conditioner, which the brochure marks "HE".

It will not give you a single number, because no maker does, and inventing one would be the same defect as a made-up quote. It will not give you a margin percentage, because no maker publishes one either, and the phrase "add 20 percent" that appears in forum answers is nobody's documented rule.

What it will do is hand you the figures and the order: your own appliances' wattages, the startup multiplier from the air conditioner maker whose unit you have, the charger load, and the altitude derate for where you camp. That is more work than reading a table and it is the only version of the answer that survives contact with your own coach.

Two operational notes from Onan that save a generator rather than size one. Bring large loads on after the generator is running rather than before, and let it settle before shutting down. And if the arithmetic comes out close, the honest answer is load management rather than a bigger machine: turning a charger's rate down or delaying it is free, and Onan documents automatic systems that do exactly that.

If the generator is already fitted and misbehaving rather than being chosen, that is the generator fault guide. The loads side is the air conditioner guide and the converter guide. For the makers' own documents behind this page, the manuals section has a power and electrical page.