Thursday, June 8, 2023


What size and type generator do I need for my RV?

Dear Dave,
What size of a generator do we need for our RV? It’s a Montana High Country. Should it be an inverter generator or just a portable non-inverter? And can we get one that is wired in to switch auto? —Debra, 2021 Montana High Country

Dear Debra,
The size of generator depends on how many 120-volt components you wish to run at any given time. You will need to calculate the power requirements for items like the refrigerator, roof air conditioners, microwave, and any appliance or components plugged into outlets. You will also need to factor in the converter/charger or inverter/charger if you have one of these.

Watts used by appliances

Typically a 13,500 btu roof air conditioner will pull 2800 watts during startup and 1800 watts in run mode. You can get by with a 2500- to 2800-watt generator. However, I would recommend using a SoftStartRV™ installed on the air conditioner to reduce the startup draw. This size generator is the minimum and might be too small if you are planning to run other 120-volt components—especially if your roof air conditioner is a larger 15,000 btu one.

A residential refrigerator can run 1,500 watts, a microwave anywhere from 1,000-1,500 depending on the size, a TV 300-400, and a single-serve coffee maker draws 1,200 watts! Keep in mind, all of these will not be running at the same time. However, it is difficult to determine when they will cycle on and off during the day. Typically, if you are only going to run one roof air conditioner and a few appliances, you would be OK with a 3,500-watt generator.


If you are considering a portable, you definitely want a generator/inverter that will produce a pure sine wave rather than the industrial generators you find at home improvement locations or discount box stores. These produce raw power designed for power tools and lights and will ruin microprocessors in some of your electronic components. If you are looking at a permanently mounted generator such as Onan, it is already set for proper power.

Automatic Transfer Switch (ATS)

I do not know of any generators that have an automatic transfer switch built in, but rather an ATS box in the service compartment. If you are going with the permanently mounted version, you can get a Progressive Dynamics or Southwire ATS and run the power from the generator to the ATS as well as the shoreline cord, and then the power from the ATS to the distribution center. Currently your rig has a shoreline cord that is wired directly to the distribution center. If you go with a portable generator, I do not see a good way to have an ATS as you would still need to plug a cord into the generator manually.

An ATS is simply a box that switches the power source. It is set on default to provide power when available from the generator line coming in and switches internally when it senses power from the shoreline cord, which then creates an open circuit from the generator.

If no ATS

If you did not have an ATS, you would physically plug the shoreline cord into a “J” box that is an outlet wired directly from the generator.

Here you see the box on the left is the junction box with the shoreline cord wired into it, and then it is wired to the distribution center, which is similar to what you have now. The outlet to the right is the “J” box that has a line coming from the generator. When you plug the shoreline cord into the “J” box, power comes from the generator to the shoreline cord, through the junction box, and to the distribution center, just like if you were plugged into a campground source.

Maybe some of our readers have found something that automatically switches for a portable generator. However, as I stated earlier, you still need to remove the generator, start it up and plug some type of cord/line into it so an ATS would not be needed.

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Dear Dave,
It seems my two 6v in-house batteries only charge through the solar controller. The battery voltage is fine during the day but they drain during the night while parked with no use. That occurs whether it’s plugged into shore power at 30 or 50 amps. I also plug into standard house power 115v at home to maintain the batteries, but it doesn’t work. Help. —Reno, 2022 Thor Magnitude BT36

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Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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Dave Solberg
Dave Solberg
Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.


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Gary Stone
2 months ago

We’ve been happy with a 2200 watt gen. We installed the easy start device to our AC and it runs the AC no problem. Don’t expect to run everything at the same time, though. Also gens lose about 3% of their power roughly with every 1000’ of elevation gain, so don’t expect the same performance you got at the ocean at 8,000 feet. A 3500 watt gen will run just about everything, but consider they often weigh more than 100 lbs…rough on senior backs! 😄

Thomas D
2 months ago

Do you have a space for a genny that’s permanent like many fifth wheel s have. Already piped for propane? I have an onan which i love. Only runs at 1800 rpm, very quiet. The little ones they use today run at 3600 and although not as noisy as a contractor genny, sound like their little hearts are going to explode. btw, it’s 4000 watt!. Don’t consider anything smaller

2 months ago

Some of the power usage numbers are way over stated. Newer refrigerators only use 300-600 watts, may pull up to 1000 watts on startup. TV’s are in the 50-100 watt range.
Electric heaters, coffee makers, AC and microwaves are biggest energy users.

2 months ago

I have a 2500 watt Inverter that does what I need. I used to have a little Honda that put out around 360 watts. You could run 6 – 60 watt light bulbs but really nothing else in the 120 volt range. I’m sorry I sold it as it would be all I need (it also had a 12 volt outlet) to keep my battery from draining down when parked overnight at a big box store and running my furnace. It was so quiet you could have it between two lawn chairs and still carry on a conversation without raising your voice. I no longer see it in the Honda line up. And at about 25 lbs with a 12 hour run time on a litre of gas I’m sorry I sold it.

2 months ago

Generators are big and bulky. You may want to consider a DC/DC charger. About 3 lbs, half the size of a shoebox and can pump 50 amps into your batteries. We are very happy with our REDARC unit.

2 months ago

Buy bigger, you will not regret it. Buying on the edge of your needs will result in future sorrow.

2 months ago
Reply to  Tom

Agree. Buying on the edge or too small will lead to frustrations when the generator gets overloaded and shuts down!

Jesse Crouse
2 months ago

As far as inverter or bargain brand- No one else needs to hear that cheap piece of crap because you won’t buck up.

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