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.
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…
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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