How to use the calculator
- Tick what runs at the same time Check every 120 volt appliance you want on together. The tool starts with one 13,500 BTU rooftop AC, the microwave and the fridge on electric ticked.
- Check the running watts Each row holds a running wattage from a maker spec. If your appliance label shows a different figure, type yours in. Rows you add with “Add your own” count running watts only, with no start-up extra.
- Tick the soft starter box if you have one A soft starter is a small module wired into the rooftop AC that eases the compressor on, so it pulls less power at the moment it starts.
- Read both numbers The result shows running watts and starting (peak) watts, then lists which generators from our list are big enough and which are too small. A generator has to cover both numbers, the running watts and the starting watts.
How the calculator works
Running watts are the sum of every ticked row. Starting watts are the running watts plus the single biggest start-up extra among the ticked items. The tool assumes only one thing starts at a time, so it adds the largest extra, not all of them.
The rooftop AC is the only default item with a start figure: 3,500 W. That number is not a measured surge. It comes from Dometic’s own generator guideline in its rooftop AC manual, which calls for a 3.5 kW generator for one unit and 5 kW for two. Dometic says the guideline comes from what people saw in actual use. The AC’s running watts come from the same manual: the Brisk II 13,500 BTU (B57915) is rated 12.4 A for the compressor plus 2.5 A for the fan, and 14.9 A x 115 V is about 1,714 W.
Ticking the soft starter cuts 65% off the part of the AC’s start figure that sits above its running watts. Micro-Air (EasyStart Breeze) and Coleman-Mach (Soft Start) both claim “up to 75%” less starting current, depending on the AC model. “Up to” is the best case, so the calculator assumes 65%, a cautious figure below the maker claim.
A generator passes only if its running rating covers your running watts and its peak rating covers your starting watts. Honda and Champion publish both figures. Cummins Onan publishes only rated watts for its QG generators, so for Onan models the calculator treats rated watts as peak watts. If your running watts pass 3,600 W, the tool warns you: Southwire rates a 30 amp RV connection at 30 A and 120 V, 3,600 W maximum.
Worked example with the defaults. AC 1,714 W + microwave 1,500 W (Furrion FMCM15AA-BS) + fridge 300 W (Norcold N611V on electric) = 3,514 W running. The AC’s extra is 3,500 minus 1,714 = 1,786 W, so starting is 3,514 + 1,786 = 5,300 W. The tool rounds up to the next 100 and tells you to look for at least 3,600 running watts and 5,300 starting watts. Only the Onan QG 5500, Honda EU7000iS and Onan QG 7000 pass.
Now tick the soft starter. The AC’s extra drops to 1,786 x 0.35 = 625 W, and starting falls to 3,514 + 625 = 4,139 W. The same three generators are still the only fits: the Onan QG 4000 is 139 W short at start, and the Champion 4500 is 14 W short on running watts.
Generator ratings the calculator compares
These are the makers’ published ratings on gasoline. For Onan, the starting figure in brackets is what the tool assumes, since Cummins publishes none.
| Generator | Running watts | Starting watts | Runs the default load? |
|---|---|---|---|
| Honda EU2200i | 1,800 | 2,200 | No |
| Onan QG 2800i | 2,800 | Not published (2,800) | No |
| Honda EU3000iS | 2,800 | 3,000 | No |
| Champion 3400 dual fuel inverter (100396) | 3,100 | 3,400 | No |
| Champion 4500 dual fuel inverter (201278) | 3,500 | 4,500 | No, 14 W short running |
| Onan QG 4000 | 4,000 | Not published (4,000) | No, short at start |
| Onan QG 5500 | 5,500 | Not published (5,500) | Yes |
| Honda EU7000iS | 5,500 | 7,000 | Yes |
| Onan QG 7000 | 7,000 | Not published (7,000) | Yes |
Check the exact model: Champion’s other 4500 W dual fuel inverter, the 201531, is rated 3,250 running watts on gas. Propane lowers output too. The 201278 drops to 3,150 running watts on propane, and the LP version of the Onan QG 4000 is rated 3,600 W. Dometic adds that generators lose power at high altitude and from lack of maintenance; our RV generator repair guides cover the upkeep.
What size generator do you need for an RV air conditioner?
The rooftop AC decides the generator size, because it is the one load in the calculator that needs extra power on top of its running watts to start. Run the AC alone through the calculator and you get 1,714 W running and 3,500 W starting. The smallest generators in our list that pass are the Champion 4500 (201278) on gasoline and the Onan QG 4000. Add a soft starter and the AC’s start figure drops to 1,714 + 625 = 2,339 W. Then the Onan QG 2800i, Honda EU3000iS and Champion 3400 pass as well.
Our view: if you want a generator in the 2,800 to 3,000 W class to carry the AC, plan on a soft starter and keep the microwave off while the AC runs. If the generator trips or quits the moment the compressor kicks on, start-up demand is the likely reason. Our guide to an RV AC that keeps tripping the breaker walks through it, and Onan generator keeps shutting off explains the overload codes on built-in units. For running the AC from batteries, use our inverter size calculator.
Tools and parts for this fix
Only what this guide uses. For parts, match the part number in your unit's manual before you order.
Champion 4500W dual fuel inverter generator
Its 3,500 running watts cover one 13,500 BTU AC plus small loads when the calculator lands there.
Check price on Amazon
Micro-Air EasyStart Breeze 399 soft start
Lowers the AC's starting surge so the calculator's starting watts, and the generator you need, drop.
Check price on Amazon
Camco 30A to 50A dogbone adapter
Plugs a 50 amp RV cord into the generator's 30 amp outlet when one generator is enough.
Check price on Amazon
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Common questions
What size generator do I need to run everything in my camper?
For a 30 amp camper, a generator with at least 3,600 running watts covers everything the RV can draw, because a 30 amp connection tops out at 3,600 W. With one AC in that mix, the calculator’s math puts starting watts at up to 3,600 + 1,786 = 5,386 W, which the Onan QG 5500, Honda EU7000iS and Onan QG 7000 meet. Tick only what you really use together and you may get a smaller answer.
Can I run my RV AC with a 2000 watt generator?
Not by our numbers. A Dometic Brisk II 13,500 BTU runs at about 1,714 W, which fits, but its start figure is 3,500 W, or 2,339 W with a soft starter. Both are more than 2,000 W. Add the 300 W fridge and the running load alone passes 2,000 W.
Will a 2200 watt generator run an RV air conditioner?
The calculator says no for the Honda EU2200i. Its 1,800 running watts cover the AC’s 1,714 W with 86 W to spare, but its 2,200 W maximum is below even the soft-start figure of 2,339 W. The 3,500 W start figure is a guideline, not a measured surge, so your AC may do better, but we would not plan a trip around it.
Will a 5000 watt generator run a 30 amp RV?
Yes, if 5,000 W is its running rating, since that is more than the 3,600 W a 30 amp RV can take. Check which number the 5,000 is: Champion’s 4500 model, for example, is named for its starting watts and runs at 3,500 W. With the default load the calculator wants 5,300 starting watts, so a generator with a 5,000 W peak falls 300 W short unless the AC has a soft starter, which brings the need down to 4,139 W.
What size generator do I need to run 2 AC on my RV?
Look for at least 3,500 running watts and 5,300 starting watts. Two Dometic Brisk II units, 13,500 and 15,000 BTU, add up to 1,714 + 1,760 = 3,474 W running, and the AC start extra brings starting to 5,260 W, in line with Dometic’s 5 kW guideline for two units, so the Onan QG 5500, Honda EU7000iS and Onan QG 7000 pass. With soft starters on both, the Champion 4500 (201278) also passes, but with just 26 W of running watts to spare. On a 30 amp RV the two ACs leave only 126 W of the 3,600 W limit, so everything else stays off; our 30 amp load calculator shows the same limit in amps.