A battery or UPS must work with your exact PAP device and intended setup. A large capacity on the box is not enough: voltage, connector, approved power adapter, output rating and how the system changes over during an outage all matter.
Before buying, give the supplier the complete device model and ask for written compatibility information. Confirm whether the proposed solution supports the humidifier and heated tubing, and whether it is approved for charging while the device runs.
Estimate runtime carefully
Battery capacity is usually stated in watt-hours, while the machine draws power in watts. Dividing usable watt-hours by average watts gives a rough runtime estimate. For illustration, 400 usable Wh divided by 40 W gives 10 hours before allowing for further losses or reserve. Those numbers describe arithmetic, not a recommended battery or a promised night's treatment.
Actual demand changes with pressure, accessories and operating conditions. AC conversion can waste energy; battery age and temperature can reduce capacity. Do not confuse a machine's maximum power rating with its average overnight draw.
Ask for runtime evidence using a comparable setup, including humidity settings. Disabling prescribed or needed features just to match a sales claim may make treatment uncomfortable or unsuitable.
Check the changeover
Some batteries supply continuous power; some UPS arrangements have a transfer delay or may not restart the device as expected. Ask what happens when mains power fails and returns, which alarms sound and whether treatment data reveal an interruption.
Have the supplier demonstrate the setup safely. A person who depends on ventilation needs a clinician-approved contingency plan; do not interrupt essential support to test a battery yourself.
Keep the plan usable
Record charging and storage instructions, replacement expectations and who to call if the system fails. Keep the approved cables together and protect them from damage. Do not assemble a direct battery connection from an online wiring suggestion.
The backup plan may need more than electricity: a spare device, another prescribed support option or urgent assistance can be necessary for some users. Agree on that plan before the next power cut.
Related reading: Using CPAP throughout sleep: why four hours is not the goal