How Long Will An RV Battery Run The Furnace? + Calculations

How To Calculate How Long Your RV Battery Will Run The RV Furnace

When calculating how long your RV battery will run the furnace there are few factors that come into play but the main thing you need to figure out it how much power the fan is using and how many amp-hours your RV batteries hold.

Once you know both of these things it’s just a matter of taking the amp hours your battery has and dividing it by the amps the RV furnace uses.

See Also: Best Deep Cycle RV Batteries (AGM, SLA, 12V, 6V) 2020

But there is a catch.

You can take the total amp hours in a brand new battery and divide it by amp usage of your RV furnace and you’re going to get a pretty large number. That number isn’t going to be 100% accurate because it’s not just your furnace that will be drawing power from the RV battery and the temperature outside will also come into play as well as what kind of condition your RV battery is in.

The RV Battery & Temperature

A general rule of thumb for most kinds of deep-cycle lead-acid (flooded, AGM, or GEL) RV batteries is you don’t want to take them below a 50% discharge especially in cold weather. Any kind of lead-acid battery can freeze at 32°F (0°C) when it’s below a 20% charge. In cold weather, batteries will lose charge faster as well so even if you went to sleep with a 100% charged battery by the time you wake up in the morning it will have lost some charge.

Lithium-ion batteries are quite a bit better and can be discharged to as low as 20% before taking any damage.

See Also: 6 Volt vs 12 Volt RV Batteries: The Pros & Cons Of Each

Taking all of this into account when you calculate how long your RV batteries can run the furnace you should be taking the number of amp hours your battery holds and cutting it in half. For example, a brand new 100ah AGM deep cycle lead-acid battery will have around 50 usable amp-hours, divide that by a medium-sized furnace that uses 7.6 amps and you can technically run your RV furnace for 6.5 hours in moderately cold weather without damaging your battery.

That being said most RV batteries no matter what kind will take some damage over time and start to lose their ability to hold as much charge as when they were brand new. If you have old batteries that have been charged and discharged a lot then you will probably have less usable amp-hours. When calculating how long older batteries can run an RV furnace you may want to knock one or two hours off of your final calculation just to be safe.

Other RV Appliances Using Battery Power

Lots of appliances use the 12V battery power in an RV and some may be drawing more power than you think. The lights and water pump are the obvious ones but using them once or twice at night isn’t going to take too much power from your batteries.

One appliance people often overlook is the RV fridge. Even when it’s in LP (propane) gas mode it’s still drawing power from the battery to keep the sensors and the electronic ignition running. A regular 6 cubic RV fridge will use around 0.8 amps every hour. If you fully charge your battery and let it sit for 6 hours before turning the RV furnace on your RV fridge may have taken around 5 amps from the RV battery.

See Also: Best Heated Drinking Water Hose For RV, Campers + DIY Method

If you use the same calculation used above subtracting the power used by the fridge first it knocks a half-hour off of your RV furnace run time. Granted it’s not a ton of power loss and it may be a little unnecessary to add the RV fridge into your calculations but I thought it was worth mentioning.

How To Estimate The Amp Hours Of An RV Battery

If you don’t already know how many amp hours your RV battery holds you can calculate it using the “reserve capacity” number which is usually listed on the top of the battery. Take the RC number and divide it by 2.4

I’ll show you with this RV battery as an example. You can see the RC (reserve capacity) number is 120 below.

You can see the RC number on the top of the battery.

Then I take 120 and divide it by 2.4 and end up with 50. This battery has 50ah total, it’s wired in parallel with another one of the same kind so that means the RV has 100ah of battery power but only 50ah of that is technically safe to use.

How Many Watts/Amps Does The RV Furnace Use?

Different sizes of RVs, travel trailers, 5th-wheels, and truck campers use different furnaces. We RV full-time in a 32-foot travel trailer. The furnace only has 3 ducts it needs to push hot air through so it’s not as large as what would be found in a 5th-wheel.

When I pulled out the manual for our Atwood furnace to see how many amps and watts it uses I found information for not only my model of RV furnace but 6 others as well. We have the 8525-IV Atwood furnace in our trailer, according to the information on the manual the fan uses 7.6 amps which is 91 watts.

Tables For Most RV Furnace Power Usage

Atwood RV Furnace Power Usage

Model #8516-IV or LD8520-IV or LD8525-IV or LD8531-IV or LD8535-IV or LD1522 Low1522 High2334 Low2334 High
BTU16,00020,00025,00030,00034,00015,00022,00023,00034,000
AMPS4.64.67.67.69.84.810.26.513.2
Watts555591911185812279158

Dometic RV Furnace Power Usage

Model #AFMD16AFMD20AFMD25AFMD30AFMD35
BTU16,00020,00025,00030,00034,000
AMPS4.24.27.57.511.1
Watts50509090132

Suburban SF-Q & SF-FQ Series RV Furnaces

Model #SF-20Q/FQSF-25Q/FQSF-30Q/FQSF-35Q/FQSF-42Q/FQ
BTU20,00025,00030,00035,00040,000
AMPS7.07.07.08.09.0
Watts84848496108

Suburban NT-SQ & NT-SEQ

Model #NT-16SQNT-20SQNT-16SEQNT-20SEQ
BTU16,00019,00016,00019,000
AMPS3.13.12.72.7
Watts37.237.232.432.4

Suburban NT Series

Model #NT-30SPNT-34SPNT-40
BTU30,00034,00040,000
AMPS4.57.08.5
Watts5484102

Suburban Park (P) Model

Model #P-40
BTU40,000
AMPS1.5
Watts18

Suburban SH Series

Model #SH-35SH-42
BTU35,00040,000
AMPS8.210.6
Watts98.4127.2

If your model of RV furnace is not found in these tables you can look at your RV furnace manual to find the power usage, google it, or leave a comment below and I’ll help you figure it out.

Final Formula For Calculating How Long An RV Battery Can Run A Furnace

Battery Amp Hours Divided By 2 = Total Safe Battery Amp Hours (unless you are using lithium-ion batteries then you can multiply the amp hours by 0.8 instead)

Total Safe Battery Amp Hours Divided By The Number Of Amps Your Furnace Uses (see table above for amp usage by RV furnace model) = Total Hours The Battery Can Run The Furnace

If you have old batteries and it’s very cold outside you may want to take off one or two hours from your final number just to be safe. Running an RV battery too low in cold temperatures could cause it to freeze which can damage a battery permanently.

See Also: Best Portable Electric Heated Blankets For Camping

Have any questions about calculating how long an RV battery can run a furnace? Leave a comment below.

by Jenni
Jenni grew up in a small town in Idaho. With a family that loves camping, she has been towing trailers since a very young age.

5 thoughts on “How Long Will An RV Battery Run The Furnace? + Calculations”

  1. Your article is wonderfully written & informative. I’m trying to size a solar system with the ability to run our furnace & water pumps, but do not know the model ours is. We have a 2004 Colorado 27RL fifth wheel. The brand is Dometic. All I have is the remote control manual & it doesn’t have any model #s. How do I know what we have?
    Thanks for any help

    Reply
    • I’m asking about the furnace specifically, but if you know anything about the water pump wattage also, great lol!

      Reply
      • Hey Mickie,

        I found an information pamphlet for the 2004 Colorado 5th-wheels and it said your specific model has a 35,000 BTU furnace which makes sense for the size of the RV. It should draw around 11 amps or 132 watts per hour. If you want to find the specific model number you can usually find it on the furnace itself but you sometimes have to take it out to get to the information sticker. I think 132 watts is a really good guess though based on other Dometic furnaces of that size.

        As for the water pump they normally draw around 5-7 max amps which will be around 60-84 watts an hour. That being said an RV water pump isn’t normally run for an hour straight and it’s rarely run at full capacity. I doubt the average RV water pump takes more than 60 watts a day.

        Thanks for checking out the article. If you have any more questions feel free to comment more.

        Reply
  2. I’m confused about the amp draw calculation for the furnace. I presume the numbers are based on running constantly. But, a furnace isn’t run constantly. Obviously, how long it runs depends on the outside temp and thermostat setting. How can I estimate the amount of time the furnace will actually operate?

    Reply
    • Yes, the numbers are based on the furnace running constantly and it’s true that it shouldn’t be running 24/7. The numbers given are mostly to use as a guide to help with understanding how much draw a furnace has on RV batteries.

      Because every RV is different not only in size but insulation quality it can be really difficult to give a one size fits all formula.

      In my own travel trailer that has a giant slide out in the main room, it seems like the furnace runs about 1/3 of the night when the outer temperatures are around freezing and the inside thermostat is set to the lowest temperature which is 55°F.

      If you want to get an exact number the only thing I can suggest is to run your furnace for a night while the trailer is plugged into an electricity usage monitor like this one (click to view on Amazon). It will tell you how much electricity was used and help you get a better understanding of how much the furnace needed to run.

      Reply

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