Quick answer
With the default setup, A 300 Wh battery running a 18 W CPAP load is estimated to last 13.5 hours, or about 1.69 nights at 8 hours per night. The selected target is 2 nights, so the estimated battery is classified as one-night backup.
- Runtime
- 13.5 hours
- Estimated nights
- 1.69 nights
- Usable battery
- 243 Wh
- Required for target
- 356 Wh
How to use this CPAP battery runtime calculator
Start with the battery capacity in watt-hours. If your battery label lists amp-hours instead, enter Ah and voltage so the calculator can convert it to Wh. Watt-hours are the right unit because CPAP runtime is stored energy divided by power draw.
Then enter average CPAP watts. Use a plug-in power meter or manufacturer battery guide when possible. If you do not have a measured value, start with a preset and adjust it upward when a heated humidifier or heated tube is used.
For airline battery size checks, compare the result with the Power Bank Flight Limit Calculator. For outage backup that includes internet equipment, use the Router Backup Power Runtime Calculator.
CPAP battery runtime formula
battery Wh = Ah x battery voltageusable battery Wh = battery Wh x output efficiency x usable battery shareruntime hours = usable battery Wh / average CPAP wattsruntime nights = runtime hours / hours per nightrequired battery Wh = target nights x hours per night x average watts / output efficiency / usable battery shareThe formula separates battery capacity, output efficiency, and usable battery share because a CPAP battery rarely delivers every labeled watt-hour to the machine. AC inverter output, battery age, reserve, cold temperature, and low-load behavior can all reduce usable energy.
Assumptions and safety limits
This calculator is for power planning only. It estimates battery runtime from electrical energy and load. It does not recommend therapy pressure, mask settings, humidity settings, or whether a particular CPAP setup is medically appropriate.
- Use measured watts whenever possible because CPAP pressure, mask leak, humidifier heat, and tube heat can change power draw.
- Use a manufacturer-approved or compatible DC adapter when possible. A DC adapter can reduce conversion losses compared with AC inverter output.
- Keep reserve for battery aging, colder temperatures, and uncertainty. A battery that works in a short test may run shorter after years of use.
- For air travel, check current airline rules before departure. Battery watt-hour limits, approval steps, and spare battery handling rules can vary by route and carrier.
Example calculations
CPAP battery runtime example
Suppose you have a 300 Whbattery and your CPAP setup averages 18 W with humidifier heat off. With 90% output efficiency and 90% usable battery, the calculator estimates 243 Wh usable energy.
Divide usable energy by the CPAP load: 243 Wh / 18 W = 13.5 hours. At 8 hours per night, that is about 1.69 nights.
If the same battery powers a heated humidifier and the average load rises to 55 W, runtime drops sharply. That is why the best CPAP battery estimate uses actual watts instead of only the battery label.
Common CPAP battery runtime examples
These examples use 90% output efficiency and 90% usable battery to leave room for losses and reserve. Real runtime can be shorter or longer depending on the CPAP model, heat settings, battery age, and adapter type.
| Setup | Battery | CPAP load | Runtime | Nights | Travel range |
|---|---|---|---|---|---|
| 99 Wh travel battery, CPAP only | 99 Wh | 18 W | 4.46 hours | 0.56 nights | Common carry-on range |
| 160 Wh larger travel battery, CPAP only | 160 Wh | 18 W | 7.2 hours | 0.9 nights | Airline approval range |
| 300 Wh power station, CPAP only | 300 Wh | 18 W | 13.5 hours | 1.69 nights | Above common flight range |
| 500 Wh power station, humidifier on | 500 Wh | 55 W | 7.36 hours | 0.92 nights | Above common flight range |
CPAP battery travel notes
Battery runtime and flight rules are related but separate questions. A small travel battery may be easier to fly with but may not run a CPAP for a full night. A larger power station may be useful at home or in a car-camping setup but may be above common passenger spare lithium battery ranges.
Use the Wh result as a planning number, then verify the battery label, airline approval requirements, spare battery packing rules, and CPAP manufacturer guidance before travel.
Related calculators
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Router Backup Power Runtime Calculator
Estimate how long a UPS, DC mini UPS, power bank, or power station can keep a router, modem, ONT, or mesh Wi-Fi online.
Power Bank Flight Limit Calculator
Convert power bank mAh and voltage to watt-hours, then check common carry-on, airline-approval, and over-limit flight ranges.
USB-C Charger Wattage Calculator
Check whether a USB-C charger, cable, and port can power phones, tablets, laptops, handhelds, and multiple devices.
FAQ
How long will a battery run a CPAP machine?
Runtime depends on battery watt-hours and average CPAP watts. Divide usable battery Wh by the average CPAP load in watts. A 300 Wh battery with about 243 Wh usable after efficiency and reserve can run an 18 W CPAP load for about 13.5 hours.
What size battery do I need for CPAP overnight?
For one 8-hour night, multiply average CPAP watts by 8 hours, then divide by efficiency and usable battery share. At 18 W with 90% efficiency and 90% usable battery, one night needs about 178 Wh. Heated humidification can require much more.
Does a heated humidifier change CPAP battery runtime?
Yes. Heated humidifiers and heated tubes can be the biggest power draw in a battery setup. The calculator lets you raise average watts to model humidifier use, because runtime can drop from multi-night use to less than one night.
Can I fly with a CPAP battery?
Many smaller lithium batteries are within the common 100 Wh carry-on range. Batteries from 101 Wh to 160 Wh commonly require airline approval, and batteries above 160 Wh are usually not simple passenger spare batteries. Always check the airline before travel.
Should I enter Wh or Ah for a CPAP battery?
Use Wh if the label lists watt-hours. If the label only lists Ah, multiply Ah by the battery voltage to get Wh. Use the battery pack voltage shown on the label, not necessarily the CPAP output voltage.
Is a DC adapter better than an AC inverter for CPAP backup?
A compatible DC adapter can be more efficient because it avoids converting battery DC to AC and then back to DC. The adapter must match the CPAP manufacturer's voltage, connector, and safety requirements.
Why is my real CPAP battery runtime different from the estimate?
Real runtime can change with therapy pressure, mask leak, humidifier level, tube heat, battery age, temperature, inverter efficiency, and whether the battery can deliver the required output continuously.
Is this calculator medical advice?
No. It is only a battery and power estimate. Do not change prescribed therapy pressure or required CPAP use based on this calculator. Check your CPAP manual, battery specifications, airline rules, and clinician guidance when needed.