How long will a battery actually run your CPAP? Put your numbers in and get hours and full nights — not a guess. The short answer for the question people ask most: a regular 12-volt car battery runs a CPAP about 5 to 6 hours with the heated humidifier on — not quite one full night — and just over one night with the humidifier off and a DC converter. A 1,000 watt-hour power station does about 2 nights humidified, and a 2,000 watt-hour station about 4. This calculator shows the math for your own battery and your own machine, because the humidifier switch matters more than the brand on the box.
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Amp-hours are not watt-hours. At 12 volts, watt-hours = amp-hours × 12, so a 100 amp-hour battery holds 1,200 watt-hours on paper — before the usable-capacity cut below.
Where do these numbers come from?
Every figure in this calculator is a published number or a range we state as a range — nothing here is a guess dressed up as a fact. The watt draws are the planning figures from our own runtime guide, and they trace back to the manufacturers: measured draw on a ResMed AirSense 11 runs about 30–40 watts with the humidifier off and 50–65 watts with it on; ResMed's own specification for the AirSense 10 lists 53 watts as typical consumption; add a heated tube turned up and draw climbs toward 80 watts. So this calculator plans 40 watts dry, 55 watts humidified, and 80 watts for humidifier plus heated tube — and lets you type your own machine's exact figure over any of them.
The two cuts between the battery's label and your machine are just as plain. Conversion: a CPAP runs on DC power inside. Through a wall outlet, the energy converts twice — battery DC to outlet AC, then the machine's brick back to DC — and published inverter testing puts that path at roughly 85% delivered, which is the same 0.85 factor our home power sizer uses for every load. A DC converter made for your machine skips the inverter step; we plan it at 95%, which works out to the 10–15% saving our runtime guide reports. Usable capacity: a lead-acid battery should only be drained to about half if you want it to live — the standard published planning figure — while LiFePO4 batteries are routinely rated for 80–90% usable, so we plan a loose LiFePO4 battery at 85%, the middle of that range. A power station's electronics already manage its battery's depth, so for a station we apply no second depth cut — the conversion factor is the derating, exactly as in our guide.
Worked example: the 12-volt car battery, step by step
Take the battery in an ordinary car: 12 volts, about 60 amp-hours. Step one, turn amp-hours into watt-hours: 60 × 12 = 720 watt-hours on paper. Step two, the lead-acid cut: only about half is safely usable, so 360 watt-hours. Step three, the conversion cut: through an AC inverter at 85%, about 306 watt-hours reach the machine. Step four, divide by the draw. With the humidifier on at 55 watts, that is about 5.6 hours — the machine stops before the night does. With the humidifier off at 40 watts, about 7.7 hours — still short of a full 8-hour night. Swap the inverter for a DC converter at 95% and the dry setup reaches about 8.6 hours: one night, with almost nothing to spare. That is the honest answer to "how long will a 12-volt car battery run a CPAP?" — it is an emergency bridge for one dry night, not a plan.
A 100 amp-hour deep-cycle battery is the stronger version of the same idea: 1,200 watt-hours on paper, 600 usable, about 510 delivered through an inverter — roughly 9.3 hours humidified, so one full night with the humidifier on. And the power-station numbers check against our published guide: a 2,000 watt-hour station, humidifier on, through the wall outlet gives 2,000 × 0.85 ÷ 55 = about 31 hours, just under 4 nights — the same "about 4 nights humidified" our CPAP, fridge, and freezer runtime guide prints.
| Battery | Humidifier on, AC | Humidifier off, DC |
|---|---|---|
| Car battery, 60 Ah | ~5.6 hours | ~8.6 hours |
| Deep-cycle, 100 Ah | ~9.3 hours | ~14.3 hours |
| Station, 1,000 Wh | ~15.5 hours | ~23.8 hours |
| Station, 2,000 Wh | ~31 hours | ~47.5 hours |
Read the table the plain way: the humidifier column is the honest one for most people, and the right-hand column is what the two cheapest changes — humidifier off, DC instead of AC — buy you on the same battery.
Should you run a CPAP on DC or AC from a battery?
DC, when your machine has a proper converter for it. It is the cheapest runtime you will ever buy: skipping the inverter step keeps roughly 10–15% more of the charge, which on a dry setup stretches a 1,000 watt-hour station from about 2.7 nights to about 3, and a 2,000 watt-hour station from about 5.4 dry nights to about 6. Two cautions, both firm. Use the converter made for your machine — most home CPAPs are 24-volt devices, and a generic cable is a bad experiment on medical equipment. And if you run through AC instead, the outlet must be pure sine wave; that is what the machine's power brick expects, and it is standard on the stations we review. Whichever path you choose, run the full setup for one ordinary night at home before an outage asks the question at 2 a.m.
Frequently asked questions
How long will a 12-volt car battery run a CPAP machine?
About 5.6 hours with the heated humidifier on, and about 8.6 hours with the humidifier off through a DC converter — the worked example above shows every step. In plain terms: one dry night at best, and only if the battery starts full and healthy.
Can I use a car battery to power my CPAP machine?
You can, with an inverter or converter between the battery and the machine, and it works as a one-night emergency bridge. It is a poor standing plan: a starting battery drained halfway night after night wears out fast, and the setup has no low-battery protection unless you add it. A deep-cycle battery or a power station is the right tool for anything recurring.
How do I calculate the exact runtime for my specific CPAP model?
Find your machine's real draw — the watt figure on its power brick is the ceiling, and a plug-in watt meter gives you the true overnight average at your pressure and humidifier settings. Type that number into the watts field above, set your battery's size and kind, and the calculator does the rest. The formula it runs is the one printed with your result: watt-hours × usable share × conversion efficiency ÷ watts = hours.
Does the heated tube matter as much as the humidifier?
No — the humidifier is the big one. In our planning figures the humidifier takes a typical machine from about 40 watts to about 55, and the heated tube on top takes it toward 80. If you need to stretch a battery, the order is: humidifier down or off first, tube second, and know before the outage whether you can sleep comfortably that way.
What if the battery runs out in the middle of the night?
Then the machine stops, and for many people that means waking up — unpleasant, and for some conditions worse than unpleasant. That is why this page plans with both cuts applied and a margin left over, and why we say to test at home first. If a full night of therapy is non-negotiable, size for the humidified number, not the dry one, and keep a second night of capacity in reserve where you can.
Stations that fit this math
These are the picks from our CPAP runtime guide, scored there — the calculator above is how their nights were counted. We do not print prices on tool pages; the guide carries today's checked prices.
Anker SOLIX C1000 — 84/100
The CPAP-first pick: 1,056 watt-hours, about 2 nights humidified or close to 3 dry, at a weight one person moves easily.
Bluetti Elite 200 V2 — 86/100
The longest runner: 2,074 watt-hours, about 4 nights humidified, and a 2,600 W inverter that also starts a fridge.
EcoFlow DELTA 2 Max — 85/100
2,048 watt-hours with the fastest wall refill in the class and expansion batteries later — the rolling-outage pick.
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Ordinary Prepper is a research desk, not a test lab and not a medical practice. This calculator works from manufacturer specifications and published battery guidance; it does not measure your machine. Your equipment provider is the authority on your therapy and your outage plan — this page just makes the arithmetic honest. More medical-backup questions live on the medical backup power hub, and the whole-house version of this math is the home power sizer.