RVelectricity – Just Ask Mike (J.A.M.): Feel the heat – Staying warm in your RV


By Mike Sokol 

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.

Dear Mike,
I’m trying to boondock without a generator running at night, and am constantly running out of enough battery power overnight to run my RV furnace. I’ve been thinking about getting an electric blanket to stay warm. How does that compare to running an electric space heater from battery power? Is any of this practical without adding a bunch of batteries? —Sue in Sioux City

Dear Sue-Sioux (sorry),
Let’s explore just how much power a space heater takes compared to an electric blanket in order to stay warm in an RV. And to throw in one more variable, what about using a 12-volt DC blanket vs. a 120-volt AC blanket that needs an inverter running.

You can take that to the bank…

Battery banks are like, well, banks. That is, you make deposits and withdrawals. So if you take out more than you put in, at some point you’re going to bounce a check (or end up with a dead battery).

Let’s first consider how much currency you might have in your RV battery. I’m going to assume a 100 amp-hr, lead acid, deep-cycle battery. In order to find out how much energy is available we just need to multiply the amp-hr capacity (100 amp-hrs) by the voltage (12 volts) to come up with 1,200 watt-hrs. While that seems simple enough, you really don’t want to discharge your lead-acid below 50% or it will soon stop working at all. That suggests you have around 600 watt-hrs of energy to use overnight.

How much energy does a portable space heater use?

Okay, let’s consider a 1,200-watt space heater. To find out how long a 100 amp-hr/lead-acid battery can power it though an inverter, you just have to divide 600 watts by 1,200 watt-hrs to get 0.5 hrs. That’s right… in only 30 minutes of use, that 1,200-watt space heater will suck all the available capacity out of a 100 amp-hr lead-acid battery. So that’s not an option at all.

What about an electric blanket to stay warm?

My crude testing has shown that an electric blanket draws between 15 watts (on low) to 75 watts (on high). So let’s pick a middle number of 40 watts for comparison. To find out how long this would take to use 600 watt-hrs from the battery, all we have to do is divide 600 watt-hrs by 40 watts to come up with around 15 hours of blanket run-time. That’s much better, isn’t it? You could stay warm all night.

Should you use a 12-volt DC or 120-volt AC electric blanket?

Well, if you use a home-style 120-volt electric blanket, you’ll need to leave your inverter on to power it. And I’ve found that many inverters use anywhere from 10 to 20 watts of power at idle. That suggests simply leaving the inverter (with 20 watts parasitic drain) on with no 120-volt power being used would drain the battery in 30 hours, all by itself. As a side note, make sure you only use a pure-sine wave inverter to power any 120-volt electric blanket. This is because some blanket controllers will be destroyed (burn up) by modified sine wave AC power.

Go 12-volt DC when you can to stay warm and for other uses

In general, if you want to use an electric blanket, a CPAP machine, or an oxygen concentrator overnight on batteries, then you want to get the 12-volt DC option when possible. Stepping up your battery’s 12-volt DC to 120-volt AC just so your appliance can step it back down to 12 volts DC internally is a waste of battery power.

Your mileage may vary…

Finally, remember that you’ll likely have other things in your RV that use 12-volt DC on at the same time, so my numbers above are approximate. And if you have a lithium battery that can be drained down to 0% SOC (State of Charge), you can double the usable times compared to a lead-acid battery. That means that a 100 amp-hr lithium battery can really provide 1,200 watt-hrs of power without wrecking it.

In all cases, make sure you try out your 12-volt things while boondocking in the driveway before you count on it all working in the middle of the forest or desert or wherever your boondocking adventure takes you.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Happy boondocking and let’s play safe (and stay warm) out there….

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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And you don’t want to miss Mike’s webcasts on his YouTube channel.

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1 month ago

So, my first question would be why did the battery not last overnight with the propane furnace on? Unless it was set really high, or the fan was on on rather than on auto, or the insulation in the RV is really bad, or it was very windy, or some combination thereof, the battery capacity should be adequate to last through the night with some reserve. I’d check battery water, connections, charging, and look for what else might have been on.

1 month ago

I installed a cigarette socket in the bedroom to run my c-pap directly from batteries.
however when you do it this way the heater does not work in the humidifier.
We also use a Big Buddy heater to heat up the trailer at times and this draws no power from your battery’s.
But we never sleep with it on.

1 month ago

Consider trapping your body heat and then setting the furnace thermostat to a much lower temperature overnight: down blanket, sleeping bag, long underwear, socks, o a sleeping cap.

What about a battery-powered heated-vest–about $100 on Amazon? You may or may not find it comfortable to sleep in. The enclosed battery should last overnight and you can recharge it the next day.

People tent camp with overnight temps in the 30’s. There’s no need to keep an RV above 60° overnight.

Mitzi Agnew Giles and Ed Giles
1 month ago
Reply to  Irv

Preach it, Irv! As an ultralite former backpacker, I have kept nice and warm with polyester knit caps (when your feet get cold put on a hat) double layers of longjohns and longsleeve shirts-silk or polyester skin layer, wool or polyester or fleece- NOT flannel- outer layer, sox without a cotton content, fleece not flannel sheets and blanket, and a Celiant ™ mattress cover. I grease with petroleum jelly when we’re looking at under 32*F- just like swimmers do to avoid hypothermia. DH refuses the grease. As a younger woman when backpacking I used a silk sleeping bag liner for my 2.5 lb polyester fill sleeping bag and it took me down to 32*F with the doubled jammie layer. Remember that Cotton Kills!

Mike Sokol
1 month ago

Update: I’ve just discovered that my favorite warm sock maker (Heat Holder) now has an oversized Heat Holder throw. Yup, I’ve asked them to send me a review sample so I can test it in my sometimes cold office. I’ll let you know how it works. https://www.heatholders.com/collections/throws-blankets/products/oversized-throws-blankets

1 month ago

We just took an 8 day trip in our 2006 Winnebago Aspect 26A Motorhome. On the trip, we boondocked 4 nights at Walmart. Due to the cold, we heated using our propane (which ran the fan off the battery). 3 nights were fine, battery drained a little, but recovered quickly while driving. But the last night the battery drained much more, only 1 light lit on indicator (which has a total of 4 lights). In the morning, we could not start the generator and the lights were noticeably dim. On the drive home (about 325 miles), the battery hardly recovered, at one point it had 2 lights, but I tried to start the generator, it wouldn’t start and drained a bit off the batteries again. At home, I plugged in the RV and the charge showed full and I could start the generator. On the road, I tried to read the voltage at the Parallax Charger/Power Convertor, it said 10.33, then 10.38. Now the battery seems fine and reads 13 something. A few questions. (in replies)

1 month ago
Reply to  John

1. Should the engine alternator be charging the house batteries?
2. How can I tell if they are getting a charge from the alternator?
3. Is the charge much slower from the alternator than when the RV is plugged in?
I have (2) AGM Optima batteries in parallel. In July 2013 I upgraded my charger/power convertor to a BEST PD4645V Parallax Upgrade Convertor.

1 month ago
Reply to  John

One other thing, our MH is built on an E-450 Chassis. A tech at Best just talked with me and said it might be a fuse or relay in the Engine Compartment Power Distribution Center, preventing the Solenoid from engaging. Have you come across anything like this? Have a most likely culprit I could check? How do I check a relay?

1 month ago
Reply to  Mike Sokol

I do have a voltmeter and when driving home that last day (while my wife was driving) at the Convertor I got the 2 voltages I listed. Yes I do have a Battery Boost rocker switch. I know there is a “Battery Mode Solenoid” located in a compartment near my steps (you have to access that from outside and remove a cover plate, then remove the breaker panel (these are the push style breakers). At one point, the charge light was not lit (early on the drive home). I toggled the Battery Boost and that light lit up, so I thought all was well, but apparently not. Very puzzled about this.

1 month ago
Reply to  John

So I just did some testing.

1. Engine off, press Battery Boost, heard click.

2. Engine off, had wife turn key to last pos before starting, heard click.

3. Started Engine, heard click, the first time the battery voltage at the Convertor went from 12.8 to 13.2, so I thought maybe it was working now. Then we retried this and every additional attempt, even with engine running, voltage at test point stayed at 12.8. So it seems to be something intermittent (possibly the contacts in the solenoid itself). But also, it seems everything that allows that solenoid to work (Battery Boost button or any fuses/relays that control the solenoid), are probably good, since it is clicking when turning the key/starting the engine. Just not consistently closing the contacts to pass the current.

What is you opinion on this? Is this a fairly common component failure?

Bob Eastman
1 month ago

Hi Mike, what do plug your 12V blanket or 12V CPAP machine into ? Would it be similar to a car 12V receptacle? Do RVs come with them installed or are they installed by a tech or dealer?

if so, what is the best practice for their installation?

Much Thankage

RV Staff (@rvstaff)
1 month ago
Reply to  Bob Eastman

Hi, Bob. I’m sure Mike will respond when he sees your question. But I just had to tell you I got a chuckle out of your “Much Thankage”! Love it! 😆 —Diane at RVtravel.com

Mike Sokol
1 month ago
Reply to  Bob Eastman

Bob, thanks for your thankage…
Yes, these are typically cigarette lighter style outlets. There’s a number of companies that make combo USB/Lighter outlets for boats and RVs. Here’s one example: [2021 Upgraded] Cigarette Lighter Socket Outlet Splitter, Quick Charge 3.0 USB Charger Power Outlet with LED, 12V Marine Waterproof Power Panel Adapter DIY Kit for Car Boat Marine RV, etc. Here’s the link for it on Amazon.

1 month ago

Good info, this is a problem of many of us dealing with freezing nights in the southwest right now. The inverter doesn’t seem like a solution, mine is crazy noisy. I did get a 12V outlet installed in the middle of the rig. But I suppose an extension cord is not a good idea if running a blanket 10′ farther away?

Mike Sokol
1 month ago
Reply to  wanderer

An 12 volt DC electric blanket might draw 4 or 5 amperes of current, so you should easily be able to use a heavy-duty extension like this one on Amazon: CUZEC 12V/24V 12ft / 3.66m 14 AWG Heavy Duty Extension Cord with Cigarette Lighter Plug Socket.

Dennis Sprague
1 month ago

Good lesson. I like the button fashioned after the “Ike” button of long ago. Enjoyed your RV electricity book. Ready to read it again.

1 month ago

Hi Mike, I didn’t actually see an answer to Sue’s question, it was about the RV furnace, not a space heater. Though you gave us some good information!

1 month ago

Sorry you did answer her question after reading it again, though that would be good information to add to options!

Mike Sokol
1 month ago

She asked about using space heaters and electric blankets instead of the furnace. The furnace power requirements are a separate issue.

1 month ago

I had a similar problem and was tired of having to run the portable generator first thing in the morning. I finally realized that I was continuing to charge our phones and tablets overnight just like at home. After doing the math I realized how much power all those negligible(?) usb chargers were actually using. I now wait for sun-up before plugging all those in so the solar panel can take over. Problem solved.

1 month ago

Hi Mike, you mentioned 12 volt electric blankets but I’ve only been able to find small 12 volt throws made for in car use and not the variety for a bed. 45” x 60” won’t quite work for a Queen size bed let alone a king size. Also many have an automatic shutoff control after only 3 hours which would be a rude awakening.
I would say you’re probably going to have to get the 110 volt blanket and live with an inverter’s gluttony.
I know that you really like Lithium batteries but if Sue can find the space for a second battery she can buy two 265 amp 6 volts for less than the cost of 1 100 amp Lithium and have 265+ amps available. Yes they will need replacement sooner than the Lithium but if you take PROPER care of them I have got them to last 10 years. Per amp cost wise you can’t beat lead acid yet.

Wayne c
1 month ago
Reply to  Cal

Hello Cal,
Two 265 amp hour, 6 volt batteries wired in series to produce 12 volts will yield only 132 amp hours (50% of 265) of useable power before degrading the battery. When connected in series, voltage adds, amp hours remain the same as each single battery.
I somewhat agree that “Per amp cost wise you can’t beat lead acid yet” for the short term but once I experienced :
1) Constant 13 volts until battery is nearly depleted
2) The reduced weight, less than half of lead acid
3) No corrosion from battery acid
4) No venting required
5) Much faster charge time
There’s no going back to lead acid RV batteries for me.
Since you know how to treat your lead acid carefully enough to last 10 years, it’s likely a lithium battery could last you 40 years. Of course in my case that’s longer than I will last.

Mike Sokol
1 month ago
Reply to  Cal

Interestingly, Flowrite is sending me one of their single-point water filling systems for flooded cell batteries along with some lead-acid batteries to try it out on. While Lithium batteries are great, flooded cell batteries can still be an affordable solution, but only if the electrolyte (water) level is properly maintained. But in some RVs the batteries are placed in places that make them difficult to check and refill them. https://www.flow-rite.com/battery-care/