What Size Battery Do I Need to Run a CPAP Overnight?

What size portable power station will run my CPAP for eight hours?

The right size is the measured energy your complete CPAP setup uses across a normal night, plus margin; the watt-hour number printed on a power station is only the starting point. ResMed tells users to identify the machine model, therapy pressure, humidifier and tubing configuration, and power path before selecting battery capacity. That is the useful buying sequence. Reverse it, and you are guessing.

Run the machine as you actually plan to run it, connected through the same adapter or converter you would use in an outage. If its draw changes during the night, a single watt reading is not enough. The U.S. Department of Energy's guidance on measuring power says to measure energy over a period and divide by the measurement duration when consumption fluctuates. That captures the load that matters: blower, accessories, and conversion path together.

Use this simple decision rule: compare your measured overnight watt-hours with the station's rated watt-hours, then leave room for uncertainty rather than treating those figures as interchangeable. ResMed's battery-sizing tables are based on eight hours of use and include a 50% safety margin. The margin is not decoration. It is the buffer for a colder night, an older battery, or a configuration that draws more than expected.

For context, the ResMed battery guide lists AirMini DC-to-DC examples at 12 V: 5 Ah at 10 cm H2O, 6 Ah at 12 cm H2O, and 9 Ah at 20 cm H2O. Those are device-specific examples, not a universal shopping chart. Your measurement decides the purchase.

Does a CPAP humidifier or heated hose dramatically reduce battery runtime?

Yes. Heated humidification and heated tubing can dramatically reduce runtime, so treat them as the first settings to evaluate when backup capacity is limited.

The Philips Respironics PAP battery instructions state that its 90 Wh battery delivers more than 14 hours only in CPAP mode at 10 cm H2O with no heated humidification and no heated tubing. The same document states directly that use of either heated feature significantly reduces runtime. That is not a small footnote. It is the dividing line between a published best-case runtime and your real configuration.

ResMed makes the same boundary clear for its 97 Wh Power Station II: more than eight hours at specified average settings, but not when heated humidification and heated tubing are used. Do not compare a heat-off runtime claim with a plan that leaves both accessories on. That comparison creates false confidence.

Use a before-and-after check. Weak question: “Will this battery run my CPAP?” Strong question: “Will this battery run my CPAP for the full night with heat on, and what changes if heat is off?” Record both configurations if you may change settings during an outage. Any treatment-setting change should follow the direction provided by your clinician or device instructions; the battery guide cannot decide therapy settings for you.

Should I power a CPAP through AC or DC for better efficiency?

Use an approved DC-to-DC converter when your CPAP supports it; it generally avoids the extra conversion stage of an AC inverter and preserves more of the stored battery energy.

ResMed explicitly recommends a DC-to-DC converter whenever possible, explaining that converters are more efficient than inverters and reduce the potential for unexpected results from the wrong inverter type. If an inverter is necessary for a supported device, ResMed recommends pure sine wave output with 300 W continuous and 500 W peak or surge capacity. Match the connection method to the manufacturer guidance for your model, not to a generic cable listing.

The mechanism matters. An AC route asks stored DC power to become AC, then asks the CPAP power supply to convert it back again. Each stage can consume energy. The Department of Energy notes that conversion efficiency tends to fall below 25% of rated output because fixed losses make up a larger share of input power. A modest CPAP load can land precisely where that overhead matters.

Think of this as the Power-Path Check: buy capacity only after selecting the path. The same practical comparison of routes before committing applies when weighing DDP or DAP for commercial shipments to Spain; the cheapest-looking option can become costly once the handoffs are counted. Here, the handoff is electrical conversion, and the cost is lost runtime.

Build an outage and travel plan before the lights go out

Capacity is only one part of a workable backup plan. Confirm that the station can be recharged using the options available to you, verify the CPAP alarms and settings while on backup power, and rehearse the cable connection before an outage. The first use should not happen in the dark.

For flights, battery size becomes a compliance question as well as a runtime question. The Federal Aviation Administration states that rechargeable batteries from 0 to 100 Wh are allowed on passenger aircraft; batteries from 101 to 160 Wh require air-carrier approval; batteries above 160 Wh are forbidden. Spare lithium batteries and power banks belong in carry-on baggage, and airlines can impose stricter limits.

Verify watt-hours rather than relying on amp-hours alone. The FAA formula is watt-hours equals volts multiplied by amp-hours; its example is a 12 V, 8 Ah battery equaling 96 Wh. Ask the airline about its current rules before travel, then carry documentation for the battery if the airline requests it.

Run this week's drill: measure one ordinary night, write down the exact power path and heated-accessory settings, and compare the result against the backup capacity with a safety margin. Then run one brief setup check with the equipment you would actually pack. That is the difference between a battery purchase and a backup plan.