Quick answer: CPAP watts run about 11 overnight with the machine alone. Switch on the heated humidifier and heated tubing and it becomes 59 watts. Overnight that is 0.09 to 0.47 kWh, or under nine cents in the US. The humidifier is most of the story.
CPAP watts are one of the smallest loads in our appliance wattage chart and one of the most anxiously searched, which is a bad combination: people size backup power for a CPAP as though it were a refrigerator. It is closer to a laptop.
Everything below is about electricity. Nothing here is advice about your therapy, your pressure, or whether you can run without humidification. Those are questions for the clinician or provider who set the machine up, and this article does not answer them.
CPAP watts: what ResMed actually publishes
ResMed’s own battery guide gives current draw at 12 V DC for every combination of tubing and humidifier setting. Multiply by 12 and you have watts. These are at 10 cmH₂O, a common enough pressure to be representative.
| Configuration | Amps at 12 V | Watts | An 8-hour night | Cost per night |
|---|---|---|---|---|
| Machine only, plain tubing | 0.93 | 11.2 W | 0.09 kWh | 1.6¢ |
| Heated tubing at 30 °C, no humidifier | 1.52 | 18.2 W | 0.15 kWh | 2.7¢ |
| Humidifier at 4, plain tubing | 2.30 | 27.6 W | 0.22 kWh | 4.1¢ |
| Humidifier at 8, plain tubing | 3.66 | 43.9 W | 0.35 kWh | 6.4¢ |
| Heated tubing plus humidifier on auto | 3.77 | 45.2 W | 0.36 kWh | 6.6¢ |
| Heated tubing at 30 °C plus humidifier at 8 | 4.92 | 59.0 W | 0.47 kWh | 8.7¢ |
Between the top row and the bottom, CPAP watts vary by a factor of 5.3. Nothing about the therapy changed between those two rows; only the heating did.
Pressure matters too, but less than people expect. On the machine alone, going from 6 to 20 cmH₂O takes it from 8.4 W to 19.9 W. With the humidifier at its top setting, the same pressure change moves it only from 41.3 W to 55.1 W, because the heater is already doing most of the work.
The trap: 56 watts that need a 300 watt inverter
Here is where sizing goes wrong. The AirSense 11 user guide lists typical power consumption as 56.1 W, and beside it 111.5 VA. Peak is 73.2 W and 137.6 VA.
That is a power factor of about 0.5, and it is exactly the problem apparent power causes elsewhere on this site. Your electricity meter sees 56 W. An inverter has to supply 111.5 VA. A power station whose 100 W output rating looked generous is not.
ResMed does not leave this to inference. Its own battery guide specifies, for any of its devices used with a humidifier, a pure sine wave inverter rated 300 W continuous with a 500 W peak. That is more than five times the typical wattage. Without a humidifier it allows a 150 W modified sine wave unit.
The same guide is blunt about one thing: some ResMed heated humidifiers must not be used with inverters at all, and it names them. If you are planning around an older machine, read that page rather than this one.
DC beats AC, and the maker says so
Running the machine from a 12 V battery through a DC converter, rather than through an inverter and the mains adaptor, avoids a conversion. ResMed lists four reasons of its own for preferring a converter: it is more efficient, it protects the device against reversed leads, it regulates falling battery voltage, and it shuts down at 10.5 V to stop the battery being flatly discharged.
That last point is the one people miss. A lead-acid battery run flat once may never recover, and the converter is what prevents it.
What size battery, in ResMed’s own numbers
The battery guide publishes recommended sizes for eight hours of use, already including a 50 percent safety margin, which is why they look large next to the wattages above.
- Machine only: 12 Ah at 12 V, about 144 Wh.
- Humidifier at 4: 28 Ah, about 336 Wh.
- Humidifier at 8: 44 Ah, about 528 Wh.
- Heated tubing plus humidifier at 8: 60 Ah, about 720 Wh.
Translated into modern equipment: a modest 500 Wh portable power station would run the machine alone for roughly 45 hours on paper, but only about 8.5 hours with the humidifier and heated tube going — and rather less once the losses on the way to the socket are counted. That single choice is the difference between one night and most of a week, and it matters more than which home battery you buy.
For context, the same battery would give a 1,500 W space heater twenty minutes. A CPAP is not the load that makes backup power expensive; it is one of the easiest things on this site to keep running.
This article is about electricity, not treatment. A CPAP is prescribed medical equipment. Your pressure, your humidifier setting and whether either can be changed are decisions for your clinician or equipment provider, and nothing here should be read as a reason to alter them. We have deliberately not suggested turning anything off to save power.
If you depend on a device overnight, tell your electricity utility: many run a medical baseline or priority-restoration register, and being on it costs nothing. That is the cheapest part of preparing before the power goes. Use the manufacturer’s own converter or a supply it specifies rather than improvising a connection. And a generator lives outside, well away from windows and doors, because carbon monoxide is odourless and kills quickly — a particular danger for someone asleep.
Frequently asked questions
How many watts does a CPAP use overnight?
About 11 W for the machine alone and up to 59 W with heated humidifier and heated tubing. Over an eight-hour night that is 0.09 to 0.47 kWh, or roughly 1.6 to 8.7 US cents.
Why does my CPAP need such a large inverter?
Because its power supply has a power factor near 0.5. The AirSense 11 draws 56.1 W but 111.5 VA, and an inverter must supply the VA figure. ResMed specifies a 300 W pure sine wave inverter for devices used with a humidifier.
Is it better to run a CPAP on DC or through an inverter?
ResMed prefers a DC converter: fewer conversions, protection against reversed leads, voltage regulation, and automatic shutdown at 10.5 V so the battery is not ruined. Use the converter made for your device.
What size power station runs a CPAP all night?
On the machine alone, even a 200 Wh unit covers a night comfortably, on ResMed’s own figures. With humidifier and heated tubing, ResMed’s own recommendation works out near 720 Wh for eight hours including its safety margin, which is why a 500 Wh station does not cover that night.
Does a higher pressure setting use much more electricity?
Less than the humidifier does. On the machine alone, 6 to 20 cmH₂O takes it from 8.4 W to 19.9 W. With the humidifier at its highest setting the same change moves it only from 41.3 W to 55.1 W.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
