How To Calculate RV Battery Furnace Runtime
When you’re camping in cold weather, your RV furnace keeps you warm through the night, but how long can an RV furnace run on battery before you’re left in the cold?
Figuring out your RV furnace battery runtime involves more than checking your battery’s amp-hour rating. You need to understand how much power does an RV furnace use, what your setup can handle, and how real-world conditions affect performance. Temperature drops, battery age, and battery type all impact your actual RV battery run time furnace.
In this guide, we’ll show you how to calculate realistic runtime using straightforward math. We’ll cover what drains your RV battery faster than expected, how to get accurate estimates for your situation, and why having a backup heating plan is essential for safe cold weather camping.
RV Furnace Runtime Formula
(Battery Amp Hours × Usable Percentage) ÷ Furnace Amps = Estimated Run Time
How Cold Affects RV Batteries and Usable Power
Cold weather will seriously impacts how well your RV battery performs. When temperatures drop, batteries drain faster and risk permanent damage if discharged too low.
Lead-Acid Batteries (Flooded, AGM, or Gel)
Don’t let lead-acid based RV batteries drop below 50% charge, especially in freezing conditions. A low charge can cause the battery to freeze at 32°F, making it tough to recharge and cutting its lifespan short. Deep discharges also cause sulfation, where crystals form inside and reduce the battery’s ability to hold a charge.
Lithium Batteries
Lithium RV batteries handle cold better and can safely discharge to around 20% without damage. The catch? Never charge them below freezing. You can use them in cold weather just fine, just warm them up before plugging in.
Calculating Your Usable Power
To figure out your actual RV battery capacity calculator results, use these formulas:
- Lead-acid: Divide total amp-hours by 2.
- Lithium: Multiply total amp-hours by 0.8.
Example: A 100Ah AGM battery gives you 50 usable amp-hours, while a 100Ah lithium delivers 80.
Understanding your RV battery discharge rate and usable capacity helps you estimate how many hours will an RV battery last running the furnace and prevents you from damaging your battery on those freezing nights.
Other RV Appliances Using Battery Power
Your RV furnace isn’t working alone, other appliances are quietly draining your battery too, which affects how long will an RV battery run the furnace.
Lights and your water pump don’t use much, especially if you’re only flipping them on when needed. The RV fridge is usually the main culprit when RVers are trying to solve the mystery of the dead RV battery. Even when it’s running on propane, it still pulls battery power for sensors, the control board, and electronic ignition. A typical 6-cubic-foot RV fridge uses about 0.8 amps (10Wh) per hour.
That might not seem like much, but over the 24 hours in a day, it consumes roughly 19 amp-hours (228Wh) from your RV battery. That’s just from powering the fridges electronics! It’s just enough to cut into your furnace runtime. It’s not consuming enough power to turn off your fridge, but when you’re trying to figure out how to keep RV furnace running while boondocking for several days, every amp counts.
How To Estimate The Amp Hours Of An RV Battery
Not sure about your RV battery amp hours? There’s an easy way to estimate if you can’t find the specs online.
Most RV batteries have a “reserve capacity” (RC) number stamped on top. Just divide that number by 2 to get approximate amp hours. If your battery shows RC 120, you’ve got roughly 60 amp hours.
Got two batteries wired in parallel? Add them together, two 60Ah batteries give you 120Ah total. Just remember the 50% rule for lead-acid batteries means only about 60Ah is actually usable from that 120Ah setup.
This quick calculation using an RV battery capacity calculator approach isn’t perfect, but it gets you in the ballpark.
How Many Watts/Amps Does The RV Furnace Use?
Not every RV furnace has the same power usage, the size of your RV, number of vents, and furnace model all matter.
In our 32-foot travel trailer, the furnace has three ducts and draws about 7.6 amps per hour (around 91 watts in RV furnace wattage). Smaller furnaces in truck campers might use less, while bigger units in fifth-wheels can pull more.
Want to know your exact RV furnace draw amps? Check your owner’s manual or search your model number online.
Atwood, Dometic, and Suburban all list amp and watt ratings for their furnaces. Knowing your specific RV furnace power consumption numbers is essential for calculating can you run an RV furnace on battery only and for how long.
You can see some of the most popular RV furnace models and their power usage in the tables below.
Atwood RV Furnace Power Usage
| Model # | 8516-IV or LD | 8520-IV or LD | 8525-IV or LD | 8531-IV or LD | 8535-IV or LD | 1522 Low | 1522 High | 2334 Low | 2334 High |
| BTU | 16,000 | 20,000 | 25,000 | 30,000 | 34,000 | 15,000 | 22,000 | 23,000 | 34,000 |
| AMPS | 4.6 | 4.6 | 7.6 | 7.6 | 9.8 | 4.8 | 10.2 | 6.5 | 13.2 |
| Watts | 55 | 55 | 91 | 91 | 118 | 58 | 122 | 79 | 158 |
Dometic RV Furnace Power Usage
| Model # | AFMD16 | AFMD20 | AFMD25 | AFMD30 | AFMD35 |
| BTU | 16,000 | 20,000 | 25,000 | 30,000 | 34,000 |
| AMPS | 4.2 | 4.2 | 7.5 | 7.5 | 11.1 |
| Watts | 50 | 50 | 90 | 90 | 132 |
Suburban SF-Q & SF-FQ Series RV Furnaces
| Model # | SF-20Q/FQ | SF-25Q/FQ | SF-30Q/FQ | SF-35Q/FQ | SF-42Q/FQ |
| BTU | 20,000 | 25,000 | 30,000 | 35,000 | 40,000 |
| AMPS | 7.0 | 7.0 | 7.0 | 8.0 | 9.0 |
| Watts | 84 | 84 | 84 | 96 | 108 |
Suburban NT-SQ & NT-SEQ
| Model # | NT-16SQ | NT-20SQ | NT-16SEQ | NT-20SEQ |
| BTU | 16,000 | 19,000 | 16,000 | 19,000 |
| AMPS | 3.1 | 3.1 | 2.7 | 2.7 |
| Watts | 37.2 | 37.2 | 32.4 | 32.4 |
Suburban NT Series
| Model # | NT-30SP | NT-34SP | NT-40 |
| BTU | 30,000 | 34,000 | 40,000 |
| AMPS | 4.5 | 7.0 | 8.5 |
| Watts | 54 | 84 | 102 |
Suburban Park (P) Model
| Model # | P-40 |
| BTU | 40,000 |
| AMPS | 1.5 |
| Watts | 18 |
Suburban SH Series
| Model # | SH-35 | SH-42 |
| BTU | 35,000 | 40,000 |
| AMPS | 8.2 | 10.6 |
| Watts | 98.4 | 127.2 |
If the model of RV furnace in your RV 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 Calculations
Once you know your battery’s amp hours and your furnace’s power draw, calculating runtime is pretty straightforward.
Remember: lead-acid batteries are only safe to drain to 50%, while lithium can go to about 80%.
The Simple Formula:
(Battery Amp Hours × Usable Percentage) ÷ Furnace Amps = Estimated Run Time
Example: Let’s say you have a 100Ah lead-acid battery, giving you 50 usable amp hours. Your furnace pulls 7.6 amps.
50 ÷ 7.6 = roughly 6.5 hours of runtime
That 6.5 hours assumes perfect conditions and camping rarely gives you perfect. Cold weather tanks your RV battery discharge rate, older batteries don’t hold what they used to, and other appliances are stealing power too (remember that fridge?).
For real-world cold weather camping, it’s smart to cut that calculated time in half. So that 6.5 hours? Plan for maybe 3-4 hours of actual furnace time on a freezing night.
This conservative estimate using how to calculate RV battery run time methods keeps you from waking up cold at 3 AM wondering why your math didn’t work out. When you’re figuring out how many hours will an RV battery last running the furnace, always give yourself a cushion.
How to Keep RV Warm Without Power: Our Boondocking Strategy
When we’re boondocking in freezing temps, keeping warm without killing our batteries takes some planning. Lots of RVers fire up a generator for those brutal overnight hours or cold mornings. It works, but generators are loud and guzzle fuel.
We run a portable power station instead. It lives inside our travel trailer and plugs into the RV’s 30-amp cord using a simple 30-amp to 15-amp adapter with an extension cord, basically like plugging into a house outlet.
This setup powers our furnace fan for about 6 hours while simultaneously topping off the RV batteries (which we have two of wired in parallel). The built-in display shows exactly how much juice we’ve got left, so there’s no guessing. Come morning, we hook up solar panels to recharge the station for the next night.
Frequently Asked Questions
How long can my RV battery run the furnace?
It depends on your battery’s size and what your furnace pulls. A 100Ah lead-acid battery gives you about 50 usable amp-hours, which runs a furnace drawing 7.6 amps for roughly 6 to 7 hours. But cold weather, an aging battery, and other appliances running at the same time will cut into that number.
What happens if my RV battery runs out of power in cold weather?
Your furnace shuts off, and things get sketchy fast. Your RV gets dangerously cold, and you risk frozen water lines, which means expensive repairs come spring. Always watch your battery level and keep a backup heat source handy, like a propane heater or generator.
Can I charge RV battery while running furnace?
Absolutely. Whether you’re on shore power, running solar panels, or firing up a generator, your battery charges while the furnace runs. Just make sure whatever’s charging it can keep up with the power drain.
Is it bad to let my RV battery drop below 50%?
For lead-acid batteries? Yes, it kills their lifespan and can cause permanent damage. Lithium batteries are more forgiving and handle discharge down to about 20%, but never charge them when they’re below freezing.
How does cold affect RV batteries and performance?
Cold weather hammers battery capacity. Lead-acid batteries can actually freeze solid if they’re too discharged. Lithium handles the cold better but still loses efficiency and needs warming up before you charge them.
Can solar panels run my RV furnace?
Not directly, they recharge your batteries, which then power the furnace. You’ll need a decent-sized solar setup with a good charge controller to keep those batteries topped off throughout the day.
What’s the best battery for running RV furnace?
Lithium batteries win hands down. They deliver more usable power, charge faster, and last way longer. They’re pricier upfront, but if you’re serious about cold-weather camping, they’re worth every penny.
Lead-acid batteries work too, but they’re bigger, heavier and can’t discharge as deep.
Do other appliances affect furnace runtime?
Definitely. Your fridge, lights, and water pump all nibble away at the same battery. Even when your fridge runs on propane, it still needs electricity for the control board, enough to trim your furnace runtime.
How can I make my RV furnace more efficient?
Beef up your insulation. Use window covers, throw down some rugs, add weatherstripping, and close off areas you’re not using. Small electric or propane heaters can also help take pressure off the main furnace.
Can you run an RV furnace on battery only all night?
Sometimes. It depends on your battery capacity, how hard your furnace works, and how cold it gets outside. Most campers make it through a good chunk of the night but need to recharge come morning.
What’s the best way to keep RV batteries warm?
Battery heaters or thermal wraps work great. If you can stash batteries in a heated compartment, even better. Warm batteries hold charges longer and perform way better than cold ones.
Should I upgrade to a larger battery bank?
If you camp off-grid regularly or love winter camping, absolutely. More battery capacity means longer runtime and less stress about running out of juice. Just remember, you’ll probably need beefier charging to match.
Can I switch from lead-acid to lithium batteries?
You can, but do your homework first. Lithium batteries need a charger and converter specifically designed for lithium chemistry. Once you’ve got the right setup, though, they’ll outperform lead-acid in almost every situation and last longer.
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
I’m asking about the furnace specifically, but if you know anything about the water pump wattage also, great lol!
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.
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.
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?
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.
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?
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
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.
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.
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.
Would you please share which Power Station you like and use to power your RV heater? Thanks
Hi,
We mostly use our Jackery Explorer 1500 (click to view on Amazon) since it can power the microwave in our RV as well.
Hello. We have a Suburban NT-16SE. Spec on furnace says Input BTU/ hr =16,000 and Output BTU/ hr = 12,160. Wondering how I could calculate the amperage draw. Thanks
Hi Gary,
According to Suburban, the NT-16SE has a 2.8 amp draw. That’s 34 watts.
If you have two 60 amp he batteries you have 120 ah divided by 2 is 60 amp hr why are you cutting that 50 percent again you should have 60ah