How long will a battery or power station run a CPAP?
Multiply your CPAP's average watts by the hours you sleep, then allow for conversion losses. That gives the watt-hours one night uses. Divide the battery's watt-hours by that number to get the nights it will last. The biggest unknown is your machine's real draw, which depends mostly on whether the heated humidifier and heated tube are on.
The formula
Nights = battery capacity (Wh) ÷ energy per night
Efficiency accounts for the energy lost converting battery power into what the machine needs. Plugging the CPAP's normal power brick into a power station's AC outlet goes through two conversions, DC to AC in the inverter and then AC back to DC in the brick, so expect to lose around 15%. Running it from a DC outlet with the manufacturer's DC cable or converter skips the inverter and wastes less. The calculator uses 85% for AC and 90% for DC, which are cautious planning figures.
Calculate your nights
Nights from this battery at other average draws
| Average draw | Energy per night | Nights |
|---|
The "battery size to aim for" adds a 20% reserve, because a power station's usable energy is usually a bit less than its rated capacity and batteries lose capacity as they age. If your capacity is in amp-hours, convert it with the Ah to Wh converter.
How many watts does a CPAP use?
It varies a lot by machine, pressure and settings, so treat any single number with caution. Published estimates for a CPAP with the humidifier off mostly fall somewhere around 20–60 W on average. Turning on the heated humidifier and heated tube can roughly double or triple that, because heating water takes far more energy than running the blower. A higher pressure, mask leaks and a cold bedroom all push the draw up.
Two common mistakes make estimates wrong:
- Using the power brick's rating. The label on the power supply, often 65 W or 90 W, is the most it can deliver, not what the machine draws through the night. Using it overstates the energy needed, which is safe but can push you towards a bigger battery than you need.
- Using a figure for a different machine or setting. A number from someone else's machine at a different pressure, or with the humidifier off when yours is on, can be off by a factor of two or more.
Measure it instead
The most reliable figure is your own. Plug the CPAP into a plug-in energy meter (a "kill-a-watt" style meter) for a normal night, with your usual settings. In the morning, read the watt-hours or kWh used and divide by the hours you slept to get the average watts. For example, 0.26 kWh over 8 hours is 260 ÷ 8 = 32.5 W. That number goes straight into the calculator above. A reading taken at a wall outlet already includes the power brick's losses, but not the power station inverter's, so keeping the 85% setting for AC use stays on the cautious side.
Worked examples
Humidifier off, 500 Wh power station
Say the machine averages 30 W and you sleep 8 hours: 30 × 8 = 240 Wh. Through the AC outlet at 85%: 240 ÷ 0.85 ≈ 282 Wh per night. A 500 Wh power station gives 500 ÷ 282 ≈ 1.8 nights. With a DC cable at 90%, the night costs about 267 Wh and the same unit covers about 1.9 nights.
Humidifier and heated tube on
If the average rises to 60 W: 60 × 8 = 480 Wh, ÷ 0.85 ≈ 565 Wh per night. Now the 500 Wh unit doesn't make it through a single night. For two nights with a 20% reserve you'd want about 565 × 2 ÷ 0.8 ≈ 1,412 Wh. This is why many CPAP makers suggest switching the humidifier off when running on battery, if that's comfortable and fine with your provider.
Ways to make a battery last longer
- Use DC power if your machine supports it. Many CPAPs have an optional DC cable or converter made by the manufacturer. It avoids the inverter's losses, and a power station's DC outlets often stay on without the inverter running at all. Use the manufacturer's own accessory, since the wrong voltage can damage the machine.
- Turn off or lower the heated humidifier and heated tube, if that suits you and your provider agrees. This usually makes the biggest difference.
- Fix mask leaks. A leaking mask makes the blower work harder for the whole night.
- Keep the battery warm. Lithium batteries deliver less in the cold. Keep the power station inside the tent, van or house rather than outdoors.
- Turn off what you don't need. A power station's AC inverter uses some power just by being on. Switch off unused outlets, screens and other devices on the same battery.
Choosing a battery for CPAP use
Look for a pure sine wave AC output if you plan to use the normal power brick. Check that the battery's watt-hours cover your nights with a reserve, using the calculator above. If you want to recharge from the sun on a camping trip, the solar charging time calculator shows how long a panel takes to put a night's worth back. For outages, a unit that can charge from a wall outlet while powering the CPAP (often called pass-through charging) lets it act as a simple backup. Check the maker's documentation for how it behaves when grid power drops. Our power station size guide walks through sizing for everything else you might want to run.
Frequently asked questions
Will a 300 Wh power station run a CPAP all night?
Often, with the humidifier off. At 30 W for 8 hours through the AC outlet, a night uses about 282 Wh, which is just under 300 Wh and leaves almost no reserve. With the humidifier on, almost certainly not. Measure your machine's real draw before relying on a small unit.
Can I run a CPAP from a car battery?
A typical car starting battery isn't designed for deep discharge, and running it flat can leave you unable to start the car and shorten its life. A deep-cycle or lithium battery, or a portable power station, is a better choice. If you do use a 12 V battery, the 12 V battery runtime guide shows how much of its capacity you can actually use.
How many watt-hours do I need for a three-night camping trip?
Multiply your energy per night by three and add a reserve. At 282 Wh a night (30 W, 8 hours, AC), that's about 846 Wh, or around 1,060 Wh with a 20% reserve. Enter 3 nights in the calculator above with your own draw. Solar recharging during the day can let a smaller battery cover a longer trip.