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. Today I discuss CPAP (continuous positive airway pressure) machines.
Hi Mike,
My wife and I enjoyed your lectures at the F.R.O.G. rally in Goshen, IN.
I have a 2017 Georgetown 335DS with a Xantrex PROwatt SW 2000 inverter to power our LG residential refrigerator. It works great. If I read the label correctly on the fridge, it uses 115v at 5.2 amps. My wife and I each need to use a CPAP [continuous positive airway pressure] machine when we sleep. Again, if I’m reading correctly, each CPAP draws 1.5 amps. The fridge is plugged into one of two outlets on the inverter. Next to the unused plug, it is labeled “120Vac 16.6 A Max.”
Could I connect a plug to outlet number two on the inverter and run a power line from it to a newly installed double outlet in the bedroom to power the two CPAP units and still power the fridge while boondocking? If this idea of mine is technically/electrically doable, do you think it would void a warranty? Thanks. —Tom L.
Dear Tom,
Yes, this should easily work. And no, it should not void any warranties. Let’s go through the math, since you sent along a bunch of pictures. (Thanks very much.)
Inverter power available
Here’s a close-up picture of the outlet on your inverter. Everyone, please note that it says 16.6 amps max. We can double-check this by multiplying 120 volts times 16.6 amps and see that it equals 1,992 watts. That’s close enough to 2,000 watts, I think.
That shows that we indeed have 16.6 amperes of current available to power everything connected to the inverter. So, as long as we don’t get near 16 amps at 120 volts, it should work.
Refrigerator power needed
Now let’s take a look at the power needed by your refrigerator. As noted on the nameplate, it does require 5.2 amperes of current at 120 volts while the compressor is running. That’s normal for a residential-style refrigerator with a 120-volt compressor.
If you’re counting watts, then 5.2 times 120 volts equals 624 watts. While in defrost mode it should lock out the compressor and draw 350 watts, which we can reverse calculate as 2.9 amps at 120 volts. Everyone with me so far?
Watt about those CPAP machines?
See (C?) how clever I am with the What/Watt juxtaposition??? I know. Don’t quit my day job.
According to the nameplate on the CPAP machines, each one uses a 90-watt adapter which is rated for a maximum 1.5 amps at 120 volts. That would be 180 watts at full power even though there’s a 90-watt rating.
But, generally, power supply adapters can supply more amperage than is needed by the appliance. However, to be on the safe side, we’ll use the 1.5 amperes at 120-volts rating for our calculations.
Let’s add it all up
- 5.2 amps for the refrigerator
- 1.5 amps for CPAP 1
- 1.5 amps for CPAP 2
—————————— - 8.2 amps Total Current Needed
- 16.6 amps Available Inverter Current
You should be able to simply run a high-quality power strip from the output of the inverter to your CPAP machine power supplies. Your total load should require less than 9 amperes of current, and your inverter is rated to be able to supply more than 16 amperes. So, all is well. Yes, there will be a bit of a compressor starting surge, but I’m pretty certain your 2,000-watt inverter should be able to handle the additional compressor starting surge.
Note that I typically don’t recommend power strips for any high-amperage loads such as a space heater. But at 1.5 amperes per outlet (3 amps total) any decent power strip should be able to power the two CPAP machines properly.
That’s a wrap…
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe 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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Definitely look into purchasing the manufacturers 12 adapter supply for your CPAP machines. More efficient than the 12 -> 120-> 24 (or whatever your machine uses internally). Both my wife and I have the 12 volt adapters for our machines and they work great. You can also save considerable power by not using the humidifier portion if that does not cause you a problem (we don’t use the humidifiers on ours when we are on 12v)
I’ve been using a 12 volt DC cigarette lighter to power my traveling CPAP for years. I don’t use a heater or humidifier while traveling. I bought the adapter at Radio Shack, yes it was yeas ago, but they are available online.
I noticed the new machine I use at home doesn’t seem to have the 12 volt option, so finding a portable CPAP or older reconditioned one might be the answer for running it straight off your 12 volt batteries. I’ve been able to run it for days, off the power grid with very little amperage draw.
I have a 12V power point installed at the head of the bed for my APAP machine. This saves the round trip from 12 V DC to 120 V AC to 18V DC my machine requires. The 12 V power supply provided by the manufacturer supplies the proper final power to the APAP machine without the AC round trip and its normal losses. I do not use the humidifier or heated hose. This lets me run the propane furnace the Dometic refrigerator, charge my computers and phones with sufficient power left over to start the generator in the morning. I have 4x6V AGM batteries and 340 W of solar on the roof. Generator for breakfast and for dinner provides all the power I need for daily life until I need the Air Conditioning.
Instead us using an inverter for you CPAP machines, buy the 12v power adapter cord for them. The power consumption will be way less. Additionally if your fridge is a 3 way, run the fridge on propane. I run a CPAP, furnace and fridge on my 290ah lithium battery for 4 nights with no issues. Hope something here helped.
Knowing if the inverter can handle the load is just the first step. The next step is to determine how much battery power is required to handle the load for a specific time period.
For example;
The average inverter is about 90% efficient. A180 watt AC load is equal to a 200 Watt DC load and at 12 volts that is equal to 16.7 amperes. If you run your CPAP for 8 hours you will consume 133 Amp-hours from your battery bank. Not all is doom and gloom though. The power supply rating is for worst case such as high air pressure and running the humidifier.
Disconnecting or turning off the humidifier will greatly reduce the power consumption.
Don’t forget to add the refrigerator power usage over the same period when determining the storage capacity of your battery bank
My APAP(Adjustable Positive Airway Pressure) machine also used a power supply that converted 120V to 12V. I used it in the sleeper of the truck by using a 12V DC cigar lighter type receptacle and compatible power cord. I could easily move the machine into the truck or home by using the power supply at home and DC power on the road, worked great except my problem was during the night I would pull the mask off my face in my sleep and find it blowing air next to my pillow. Lol
Some CPAP machines can be used with 12v powered bricks. It might be more efficient to go from 12 to 24 (what my cpap uses) than 12 to 120 to 24.
Not “might.” The standard answer to this question should be “Order the CPAP manufacturer’s DC adapter cord.” There is no reason to go 12>120>24, or, worse yet, 12>120>12.