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 is 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: What Will The RV Battery Run?

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: Best 12 Volt RV Lithium Battery Reviews + How To Charge

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.

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

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.

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.

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


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 get a good estimate using the “reserve capacity” number which is usually listed on the top of the battery.

There are a few different opinions on the best way to calculate a battery’s amp hours using the RC number. Some say you should divide by 2.4 other say divide by 2 and add 16, and others say just divide by 2.

For my battery, the most accurate way seems to by dividing by 2. Note that this is just an estimate and not an exact calculation for finding the AH rating. The best thing to do would be to look up the amp hour rating online or on the specific battery. But if you have no way of finding that information this is just a way to get an idea of what the amp hours are.

Take the RC number and divide it by 2

I’ll show you using my old RV battery as an example. You can see the RC (reserve capacity) number is 120 in the picture below.

Calculating how long this RV battery can run a furnace using the RC number located on the top
You can see the RC number on the top of the RV battery.

I take 120 and divide it by 2 and end up with 60. This battery has 60ah total, it’s wired in parallel with another one of the same kind so that means the RV has 120ah of battery power but only 60ah 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 furnace 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 RV 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 per hour.

See Also: Best Small Portable Propane Heater For Indoor & RV Use


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 (AH/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: When To Charge A Deep Cycle Battery


Conclusion & RV Furnace Run Time Calculation Example

As an example, I’ll use my RV batteries and furnace.

Earlier I found that RV batteries were around 60 ah each and they are connected in parallel which gives me 120 ah in total. They are regular lead acid batteries, which means only about 60 ah can be used safely before causing damage.

I found the safe number of amp hours I could use by taking the total amp hours (120) and dividing them by 2 (120/2 = 60).

Now I need to divide the amp hours I have by the amp that the furnace will use. My furnace manual said the fan would use 7.6 amps of power every hour.

60/7.6 = 7.9

That means in a perfect world our RV batteries would be able to run the RV furnace for around 8 hours.

8 hours isn’t exactly accurate in the real world though.

If you include factors like other RV accessories using power, cold temperatures reducing amp hour capacity, the age of the batteries, and the battery not being fully charged by the time you wake up in the morning the average time the RV batteries can actually run the RV furnace before being damaged might even be half what was calculated.

My suggestion would be to half the hours that were calculated just to be on the safe side. It might seem like overkill and after using your furnace for a couple of days you might find that the run time is either better or worse than what was calculated.

In the end, it’s better to have your RV batteries more charged than not, especially in cold weather.


How We Run Our RV Furnace While Boondocking In Cold Weather

Most people use a generator when boondocking/dry camping in cold weather to run the RV furnace in the cold morning hours or even all night.

This works well if you have a lot of fuel and you don’t mind the generator noise but most campers don’t appreciate the RVer who runs a noisy generator early in the morning or all night long.

Another option and the way we run our RV furnace on cold mornings is by using a portable power station that sits inside our travel trailer on the top bunk.

We use a 30 amp to 15 amp adapter to connect a long extension cord to the 30 amp power cable on the outside of the travel trailer. We use one of the outer storage doors to get the extension cord inside the trailer and to the power station that sits on the bunk.

A portable power station is basically a lithium battery with an inverter built in. The size of power station we had included two 15 amp outlets we could plug the 15 amp extension cord into.

See Also: Best Portable Power Station/Solar Generator For Camping

When plugged in, our travel trailer was basically plugged into shore power. It was a lot like plugging the RV into a wall outlet on the outside of a home.

Our portable power station when fully charged is able to run the RV furnace fan easily for 6 hours and also charge the RV batteries at the same time. The power station has an LED display that tells us the battery percentage so we knew exactly how long we can run our RV furnace before needing to recharge the power station with solar panels.

Our set up has worked well for us for a couple of years now. Even though installing a large RV battery bank and an inverter might be the better way to go for full-timers, our simple extension cord and portable power station set up has worked very well and it was super easy to install.

I highly recommend this easy solution to weekend campers and full-time RVers who maybe don’t have the budget or the know how to install an inverter and a large RV battery bank.

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.

13 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
    • Solar Systems are based on your “Need”… What do you what to power up? How long will they run? How much Wattage or Amperage do they use?

      We are full-timers…

      We have a 42′ rig complete with Washer/Dryer, (1) 15,000 A/C Heat Pump, (1) 15,000 A/C, Dishwasher, a Convectional Microwave, LED lights throughout, a Crock Pot, a Coffee Maker, (2) TV sets, (1) DVD Player, a satellite system, a water pump, a 40,000 BTU furnace, an electric fireplace, a 12 Cu Ft Norcold Refrigerator/Freezer with Ice Maker (electric/propane), a 12 Gallon Suburban Water Heater (electric/propane), a ceiling fan, a cell phone booster, a security alarm system and a box fan that could all run on electric. It’s a glamping paradise for sure. So this is kind of a mid to extreme look at solar. We live this lifestyle. If your a weekender, your “needs” will be different…

      What do we routinely use while boondocking? Only the LED lights, an occasional TV, a coffee maker, the microwave, the water pump, and the security system. Oh…and we charge devices like IPads, Cell Phones, and Laptops. And we sometimes try to use the booster (see cell phone booster article for comments there). Of course, there are those “ghost” loads like smoke/CO2 detectors, alarm clocks, night lights, etc.

      Everything else is propane operated. (i.e. the electric/propane water heater when needed then turned off, the furnace when needed, the stove/oven, the propane/electric refrigerator in propane mode. We bought this way on purpose knowing we would boondock a lot). If you have a residential refrigerator or electric cooktop, you’re going to need a lot more battery storage…

      Our solar array is 1350 watts. We have (4) Battleborne 100AH Lithium Batteries and (6) 225 W solar panels. We have a 2800W Magnum Inverter. 2021 is the last year for any kind of tax credit. I believe it is down to 20% or so. We bought when the Government was giving 30% rebates in 2019(?).

      When considering the total size of your system, take into account “where” you will camp. Upper Michigan is one of the worst areas for solar (primarily due to clouds). Also take into account inefficiencies like the inverter itself. It will drain 30W per hour from your system when in use. The inverter is also inefficient to power equipment taking another 10%-20% to convert the power. There are also cloudy days, rainy days, partly cloudy days, mountains, trees, your roof-top A/C unit Cowlings and actual daylight hours available (winter is shorter than summer). These are just a few reasons we decided bigger is better. There are also 12V, 24V, and 48V systems. (We opted for 12V. In hindsight, maybe 24V might have been better but more expensive). We have “tilt” panels but it is a pain to crawl onto the roof and manually adjust them. We just leave them in the down position 99% of the time.

      So far, in the two to three years or so that we have been using the solar system, we have had to resort to the backup 6500W generator just a few times. Mostly due to clouds, rain and trees at times. Solar is great!

      We run a box fan all night (80W x 8 to 10 hours). We rarely use the heater unless temps are below 50F (T-Stat set at 60F. The coffee is early in the AM (runs 850W for about 15-20 minutes). We start the propane water heater, take our showers and turn it back off. Do a few dishes while the water is hot. Use the microwave for food preparations during the day (we also use the propane stove or oven at times). We might use the crock pot for 3-4 hours at a time (electric). The propane/electric stove, refrigerator and water heater are perfect for boondocking. The residential refrigerators and cooktops are battery hogs. We NEVER run an A/C unit on solar. Even if you have soft starts, the A/C units would drain a battery in nothing flat.

      So, if you are anything like us (see above), you’ll also want a similar sized unit AND a 6500W backup generator (remember tat there are de-rating factors such as elevation and outdoor temperatures that affect performance. At 8,000 feet, your 6500W generator can quickly become 4500W). I used a wattmeter to take actual readings on everything we use. We really could not go much smaller. On the best days, we can get 1200W out of 1350W. Many days, it’s more like 800W out of 1350W. It just depends on the day and location. Of course, if all you need is to recharge cellphones or laptops, you can get away with much smaller and less expensive systems. Your needs won’t be as great.

      As for cost…Are you sitting down?? Be ready for $11,ooo. Even after the rebates, it still cost us $7,700. Is it worth it? If you are full-timers and like to take advantage of free locations, then yes it is. But will you ever actually get your money back? Doubtfully…At $50 a night, it would take 154 nights to pay it off. Most might not have an RV that long or travel the way we do. But if you can afford it and really want it for the convenience, its definitely worth it. Oh…The 6500 Onan generator was another $6,000…We don’t plan to stop doing this for at least 10-15 years. We boondock 2 weeks. We go into town and re-charge for a couple of nights (at a cost). then we’re out for two more weeks of free parking. We’ve probably come close to paying off our system by now and will take advantage of the features for years to come which should save a lot of money in the end.

      Best wishes for your system and future travels.

      Hope this helps.

      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
  3. I take 120 and divide it by 2 and end up with 60. This battery has 60ah total, it’s wired in parallel with another one of the same kind so that means the RV has 120ah of battery power but only 60ah of that is technically safe to use.

    Duh, so having two 12v batteries in parallel does not give you more AH?

    Reply
  4. Great informative site, retired heavy construction contractor, builder, I am in my finishing , building a custom camper trailer, I have a never used Atwood 8535 furnace to install. Knowing two lead acid batteries might last ? One night? .I also have a generator, 12vdc converter to ac 120 vac. Then I have a very expensive 120 to 12vdc power supply 20 amp 16 amp continuous, used for ham/ cb radios it’s big, heavy, huge heat sinks, I suppose this running off the generator will suffice the amperage for my furnace?? Thanks for any further information beyond Atwoods manuals, J.R. new England usa

    Reply
    • Hi James,

      Without knowing the size of the batteries I can’t give you an exact estimate on how long they can run the Atwood RV furnace but normally it is just a few hours.

      RV furnaces only need power to run the fan so I believe your setup with the generator should be able to power it.

      Reply
  5. Primarily I am concerned with running the furnace while sleeping.
    2 Battle Born Lithium iron batteries 50 ah ea. = 100 ah × 0.8 = 80 ah. Atwood 7916-11 16000 btu 3.4 amps 40.8 watts.
    According to my calculations I can run my furnace for 23.5 hrs
    under ideal conditions (lol). I figure to divide that by 2 = 11.75 hrs. Am I being realistic ? This is in a Northstar 850c truck camper. Thank you in advance for any info.

    Reply
    • Hi Charles,

      Yes, that looks correct to me. I think halfing the total run time is a good way to plan for other things that could potentially take some battery as well.

      Reply

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