The sizing guide walks through usable capacity, inverter overhead and solar recharge, so you can work out how many nights a given unit really covers.
Size a power station properly →Keeping a CPAP running in a power outage is one of the few backup power problems with a genuinely clean answer. The machine is small, the load is steady, and there is no surge to trip an inverter. The complication is the heated humidifier, which can triple the draw and turn a comfortable three-night battery into a one-night battery. Everything below is about that trade-off and the arithmetic around it.
Safety: This is a power-sizing guide, not medical advice. Never change your prescribed pressure settings, and talk to your prescribing clinician or DME supplier about your own situation. If anyone in the house is on supplemental oxygen or a ventilator, that is a higher-stakes device than a CPAP and needs a plan made in advance with their provider. Seek emergency care for chest pain, confusion, blue-tinged lips, or severe breathlessness — do not wait out an outage.
What a CPAP Actually Draws
Published figures cluster tightly, which is unusual and helpful. A CPAP running dry — blower only, no humidification — typically sits in the 15 to 30 watt range. Switch on the heated humidifier and the heated tube and the same machine commonly lands between 40 and 70 watts, sometimes higher on units with large water chambers.
| Configuration | Typical draw | Per 8-hour night |
|---|---|---|
| Blower only, DC power cord | 15–25 W | roughly 120–200 Wh |
| Blower only, through an AC inverter | 20–35 W | roughly 160–280 Wh |
| Heated humidifier, no heated tube | 35–55 W | roughly 280–440 Wh |
| Heated humidifier plus heated tube | 45–70 W | roughly 360–560 Wh |
Pressure setting matters less than most people expect. A higher prescribed pressure means the blower works harder, but the difference between a low and a high setting is a handful of watts. The humidifier is the variable that changes the answer.
The Humidifier Is the Whole Argument
A heated humidifier is a resistive heater sitting under a water chamber. Resistive heat is expensive in battery terms, and there is no clever way around the physics. Turning it off for the duration of an outage is the single biggest thing you can do, and it typically doubles or triples your runtime for free.
The cost is comfort, and for some people it is a real cost: a dry throat, nasal irritation, occasionally a poor night. A middle path that works well is to run the humidifier at its lowest setting rather than switching it off entirely, or to disable the heated tube while keeping passive pass-over humidification. Try it on an ordinary night before the outage arrives, so you know whether you can tolerate it.
Use DC If Your Machine Supports It
Most CPAPs run internally on 12 or 24 volts DC. The brick that came with the machine converts wall AC down to that. If you power the machine from a battery through an inverter, you are converting battery DC up to AC and then straight back down to DC, and you pay a conversion penalty at each step plus the inverter’s own standby draw.
Many manufacturers sell a DC cord that plugs into a 12 V socket and skips the round trip. On a power station with a 12 V output, that commonly saves something in the range of 20 to 35 percent of the total energy, which is an extra night out of the same battery. Buy the cord made for your model — a generic barbecue of adapters is a poor idea for a device you sleep attached to. Note that many DC cords will not power the heated humidifier at all, which is another nudge toward running dry.
Sizing for One Night, Three Nights, a Week
Take your per-night watt-hours from the table above, then add roughly 10 to 15 percent for battery and inverter losses. Usable capacity on a lithium power station is normally 85 to 90 percent of the nameplate figure.
- One night, dry, on DC: around 200 Wh needed. A 250 to 300 Wh unit covers it with margin.
- One night, humidifier on, through an inverter: around 500 to 600 Wh. Budget a 700 Wh unit.
- Three nights, dry, on DC: around 600 Wh. A 1,000 Wh station is comfortable and leaves room for phones.
- Three nights, humidifier on: 1,500 to 1,800 Wh. This is where you are buying a mid-size power station for one appliance.
- A week, any configuration: stop buying capacity and buy a way to recharge. A 100 to 200 W solar panel will refill a night of dry CPAP use in a few hours of decent sun.
Which Battery, and Which to Skip
A lithium iron phosphate power station in the 300 to 1,000 Wh class is the sensible default. It charges from the wall, from the car, or from panels, it has both AC and 12 V outputs, and it is useful for everything else in the house when nobody is sleeping. Look for a pure sine wave inverter — modified sine is a bad idea for medical electronics — and check whether the unit has an auto-off feature that shuts down under a low load, because a 20 W CPAP is exactly the kind of load a poorly designed power station decides is nothing and switches off at 2 a.m.
Manufacturer-branded CPAP batteries are compact, tested with the machine, and easy to travel with. They are also expensive per watt-hour and single-purpose. A car starting battery with a cheap inverter is the option to avoid: lead-acid chemistry is damaged by deep discharge, the usable capacity is far below the label, and you may find yourself with a flat car as well as no therapy.
Two Things Worth Doing This Week
First, register with your utility’s medical baseline or priority reconnection program if one exists in your area. It does not guarantee faster restoration, but it puts you on a list and sometimes gets you a phone call before a planned shutoff. Second, ask your DME supplier and your insurer whether a backup battery is covered. Coverage is inconsistent and often a no, but the question costs nothing and the answer is occasionally yes.
Then run the drill. Unplug the machine from the wall on an ordinary Tuesday, run the whole night from the battery with the humidifier at whatever setting you plan to use, and read the remaining capacity in the morning. That single number is worth more than every estimate on this page, including mine. Fold the result into your wider outage plan so the battery is charged rather than discovered flat.
What I Would Actually Buy
For most people the answer is a 500 to 1,000 Wh lithium power station with a pure sine inverter and a 12 V output, plus the DC cord for your specific machine, plus a 100 W folding panel if you live somewhere outages run long. That combination covers several nights dry, one or two nights with humidification, and it charges phones and runs a lamp besides. Two caveats I would not gloss over: check the low-load auto-shutoff behaviour before you rely on it, and remember that a battery you keep in a closet at 40 percent charge is not a backup. Put it on the same monthly reminder as testing the smoke alarms, and keep the outage checklist where you can find it in the dark.
Frequently Asked Questions
How many watts does a CPAP machine use?
Typically 15 to 30 watts running dry, and 40 to 70 watts with a heated humidifier and heated tube. Over an eight-hour night that is roughly 120 to 240 Wh dry, or 350 to 560 Wh with full humidification.
What size battery do I need for one night of CPAP?
About 250 to 300 Wh if you run dry from a 12 V DC cord, and around 700 Wh if you run the humidifier through an AC inverter. Add 10 to 15 percent on top of the raw calculation for conversion losses.
Is it safe to skip CPAP for one night during an outage?
That is a question for your clinician, not a website — the answer depends on your diagnosis and other conditions. Ask them in advance so you know your own answer before the power goes out rather than during.
Can I run a CPAP from my car?
From a running car, yes, using the 12 V socket and a proper DC cord — but never with the engine running in an attached garage, because of carbon monoxide. From a parked car with the engine off, you risk a dead starting battery by morning.
Will a solar panel keep a CPAP going indefinitely?
Paired with a battery, usually yes for dry operation. Roughly 100 to 200 W of panels in reasonable sun replaces a night’s dry CPAP use in a few hours. Running the humidifier every night on solar alone needs considerably more panel.