I have two CPAP's and one Oxygen concentrator. I would like to power when the power goes out. The sticker on the oxygen machine says 3.4 A. The CPAP's are identical ,models and the stickers on the ac adapters claim 1 to 1.5 A apiece. That's 408 w for the oxygen and 180 apiece for the CPAP's. For a total of 768 W or 6.4 A.

My biggest concern is the surge capacity of the Oxygen machine as it has a compressor. Everflow Q at 3 LPM.

Is it better to compare by their Kw or kwh or Ah ratings. How big of a backup system do i need to power these 3 devices for a day week or month at 8 - 10 hours a night?

This is on part of this project. I am considering a generator and solar to charge the batteries. All the research i have done leads back to the batteries and size which means load and time. So this part first.

  • 1
    Hello, and welcome to Home Improvement. I'm a bit concerned that if we gave you bad advice here, someone might die. My seat-of-the-pants wild uninformed stab-in-the-dark guess is that you should at least double your worst-case calculations, but that's only a guess. Apr 27, 2019 at 19:14
  • You should find a professional qualified electrician who can do the calculations correctly, to include any starting loads etc.
    – Solar Mike
    Apr 27, 2019 at 19:26
  • @DanielGriscom if i get no advice someone might die next time power goes out. Need some help! Is their even one piece of my question anyone can give advice on? Apr 27, 2019 at 20:18

3 Answers 3


Does this equipment have DC voltage input? If so, we need the voltage and average current for it running for an hr or more.

If its AC only, then we'll want an accurate estimate of the energy consumed. Get a power meter (kill O Watt) and let the equipment run an hour or over-night. It'll tell you the kWh used.

You'll need a DC->AC inverter to power the equipment from a battery. I use a Magnum Energy MS4048 (4kw continuous inverter) to generate AC. Its a bit over kill for your project but will certainty handle the surges. Possibly the MS2000 is a better choice.

If we used your number of 768 watts, and assume a 80% efficiency of a sine wave inverter, its 960 watts from the battery. Or basically 1 kWh per hour of backup.

I use Trojan T105RE for my house backup power. Each of these is 1.24kWh @ 10 hour rate - getting 8 of them is 9.9khWh which would get you about 10 hours based on 768 watts. This is a 536 lb battery array. You can increase this array as much as you want for more time!

If you really need to go weeks - you'll need solar to maintain the array. There are two approaches. 1) Solar array -> battery charge controller. (DC coupled) 2) Grid attached solar with excess going into the utility grid. When the grid goes down, disconnect the house main, and use the MS4048 to bring the solar inverter back on line. It'll then charge the batteries with the excess solar. This is called AC coupled and will save money on the long run.

  • What are khr in 9.9khr?
    – Solar Mike
    Apr 27, 2019 at 22:05

The first rule of off-grid power is

Conservation first.

The second rule of off-grid power is

Conservation first.

You do not, do not, do not, under any circumstances, use an inefficient old piece of junk just because you already have it. Nor do you spec and acquire new equipment without thinking hard about efficiency and practicality of battery power.

As an example, suppose you have a server you want on a long term battery backup. You're using an old gaming PC with dual Radeon 980s and an 850 watt power supply (so 1200 watts practical). Do you a) install $12,000 of battery and charging apparatus, or b) buy a Mac Mini for $1200 that draws 40 watts, and fit $400 of battery and charging apparatus? $12,000 vs. $1600. Tough call.

But this is just the kind of blunder that people stumble into when they are new to the idea of battery backup/off-grid power.

So this is a frame challenge. Stop wasting your money and our time trying to power the equipment you've got. It's way cheaper to replace with efficient equipment than to power your old stuff.

The third rule of off-grid power is

Inverters suck. Get DC versions of things.

The best way to power equipment is straight off the battery. This avoids power conversion which is extremely inefficient, forcing you to triple or even quadruple battery/charging. *

I happen to know many CPAPs are produced which use 12 volts natively. They might plug into the wall, but it's via a "power brick" or "wall wart" power supply which turns mains voltage into 12 volts for the machine's actual use. And by the way, 180 watts is a ludicrous amount of power for a CPAP. There is something wrong with that spec - it may be the spec rating of the DC power brick.

Sometimes you cannot avoid running an inverter. In that case, you manually spin up the inverter only while you are actively powering a thing that needs it. That is the way to handle the oxygen pump if you can't get a DC version. So in that case you do need to account for inverter losses, but only when the unit is running.

We're not a shopping site, least of all for medical equipment, so I leave it to you to do your research.

* "But surely inverters are more efficient than that!" Not when your loads are cycled off, they're not. You have a 200W load that cycles 10% of the time, like a fridge. At that load the inverter is 80% efficient, so for 1 hour, it takes 250 watt-hours. The other 9 hours the inverter draws 60 watts standby, or 540 watt-hours. That's 790 watt-hours when you only actually use 200. That's 25% efficiency where I come from. That's why inverters suck.


Many CPAP and oxygen concentrators have battery backup options, because this is something that comes up. Your best bet would be to search for the models of your equipment, and see what supported options exist. Be sure whether you need to have the battery switch into place immediately, or if you can manually switch over to the battery. When I asked my doctor about my CPAP, they said that if the power died, I would notice the lack of air and wake up, but your conditions may be different.

In short, talk to a doctor, not an electrician.

EDIT: I found a document from RedMed regarding this exact topic.

  • I personally know a doctor who is also a qualified engineer well capable of doing those calculations, BUT most doctors won’t be able to. They can advise on the medical side...
    – Solar Mike
    Apr 27, 2019 at 19:29
  • What you need to ask a doctor is if you need automatic failover in case of power failure, not the details of the battery. Battery power sources exist already for camping and similar uses. Apr 27, 2019 at 19:33
  • Then you need to look at your location - some places have few power interruptions, others have many... multiple times a day or for many hours...
    – Solar Mike
    Apr 27, 2019 at 19:36
  • Called the home care who gave me the CPAP. Answer buy a converter cable for 80 dollars. Cool. how many batteries do i need so that willl 8 hrs? answer uMMMMMMMMMMMMMMMMMMMMMMMMMMM call the manufacturer. Apr 27, 2019 at 19:44
  • How much power a CPAP uses varies a lot by model. (And also if you use a humidifier and heated hose.) You'll need to look up your model specifically, and hope they wrote that stuff down. Apr 27, 2019 at 19:52

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