CPAP Battery Calculator: How Many Nights Will It Run?

The machine's watts: metered, not labelled

A CPAP's demand is dominated by its heaters, not its blower. ResMed publishes bench figures measured on a test lung, and they are a fraction of the nameplate: the AirSense 10 AutoSet draws 7 W as a bare machine, 22 W with the humidifier running and 34 W when a ClimateLineAir heated tube is added. The AirCurve 10 VAuto measures 41 W and a CS PaceWave 44 W. The same AirSense 10 carries a label reading 53 W typical and 104 W maximum, because a label has to cover the worst-case heater setting and the highest rated altitude.

Last column is 1,000 Wh × 0.85 usable ÷ those watts ÷ 8 h, on a direct 12 V input. Add a converter or an inverter and each figure drops — see the next table.

The battery's watts: the label is a ceiling, not a floor

Three things sit between nameplate watt-hours and energy at the mask. Depth of discharge is the share the pack is designed to give up — 90% for LiFePO4, 85% for the li-ion cells in power banks and stations, 50% for lead-acid and AGM if you want the cycles. Cold takes capacity directly: a pack at 0 C gives up roughly a fifth, and one at −20 C about half. Then the battery management system cuts off before empty, so the last few percent is never available at any voltage the machine can use.

Amp-hour batteries are the common source of error, because an Ah figure means nothing without its voltage. A "100 Ah" battery is 1,200 Wh at 12 V, 1,280 Wh at a 25.6 V LiFePO4 nominal, and 2,400 Wh at 24 V. The calculator multiplies them for you; a vendor quoting nights from amp-hours alone is guessing.

The path: where a third of the energy goes

Nothing between the battery and the machine is free. A DC-DC step-up converter for a 24 V ResMed input runs at 85–89% at light load — Victron's published figure for the Orion-Tr range — and draws a little of its own. An AC inverter is worse in both ways: 85% at best on a small load, and a no-load idle draw that a 10 W travel machine cannot outrun.

Nights = 850 Wh usable ÷ battery watts ÷ 8 h. The last two rows are the same travel machine and the same battery: the inverter's 6 W idle costs you 4.7 nights of the 10.6 available.

Sizing a battery instead of timing it

The second mode runs the formula backwards. Decide the nights, and the calculator returns the nameplate watt-hours that must be on the box:

That single comparison is the most useful number on the page. Buying into the inverter path costs you 34% more capacity for the same three nights — which is why a 12 V DC cable is the cheapest upgrade in CPAP travel, and why the "100 W CPAP needs a 1,000 Wh battery" advice floating around is usually double what the machine actually wants.

Flying: the rules that decide the purchase

The FAA divides lithium batteries by watt-hours, and the CPAP-legal sizes are small. Under 100 Wh, spare batteries and power banks travel in carry-on with no approval. Between 100 and 160 Wh, they need the airline's approval and are limited to two spares per passenger. Above 160 Wh they are forbidden on passenger aircraft — no exception for a medical device, which is why a 500 Wh power station cannot fly. Several carriers ask CPAP and oxygen users to carry enough runtime for 150% of the flight time, covering delays, so a five-hour flight wants 7.5 hours of run time: at 10 W on a DC input that is 75 Wh usable, about 88 Wh nameplate, which a 96 Wh airline pack clears.

What these figures assume

• 01A new battery at the stated temperature. Capacity fades with cycles, and LiFePO4 will not charge below 0 C at all — a frozen pack delivers its reduced figure only until the BMS stops it.
• 02Steady average watts. Pressure rises with REM and leak, ramp adds a few watts of climb, and the humidifier cycles. The metered figures already include the heater running; they are averages, not worst cases.
• 03Altitude is not modelled. Above about 2,000 m / 6,000 ft the blower works harder for the same pressure and draws more; ResMed's machines compensate automatically.
• 04No recharging between nights. Solar input extends a camping trip, and a vehicle's 12 V socket refills a pack overnight on the road. Neither is counted here.
• 05A machine in good repair. A worn humidifier chamber, a leaky mask or an old filter all raise the draw — leak compensation makes the blower work for air that never reaches you.