Size CPAP backup power in watt-hours, using the energy your complete setup needs overnight—not just the wattage printed on its power adapter. Multiply average operating watts by hours of use, then account for usable battery capacity, conversion losses, and a planning reserve. Humidifier and heated-tube settings can substantially change the result, so a battery estimate should match your machine and normal treatment setup.
For example, an assumed 30 W average load uses 240 Wh over eight hours. With the explicitly assumed losses and reserve calculated below, that requires approximately 393 Wh of rated battery capacity. This is a sizing example, not a universal CPAP battery recommendation.
Find Your CPAP’s Actual Power Requirement
Do not use “40–90 W without a humidifier” as a general rule. Manufacturer data show that some configurations draw substantially less. ResMed’s August 2018 battery guide, for example, lists 0.93 A at a 12 V battery input for certain AirSense 10 models with SlimLine tubing at a treatment pressure of 10 cm H₂O—equivalent to approximately 11.2 W through the specified DC converter. Those figures apply to the listed equipment and conditions, not every CPAP.
Use one of these starting points:
- Manufacturer battery tables for your exact machine, pressure, humidifier, tubing, and power connection.
- A representative overnight energy measurement using a suitable plug-in energy meter with the machine’s normal AC adapter.
- A documented battery-runtime test that matches your equipment and settings.
For an AC measurement, record total Wh or kWh over the session rather than relying on a momentary watt reading. Calculate average power as:
[
P_{\text{avg}}=\frac{E_{\text{measured}}}{t}
]
Here, (P_{\text{avg}}) is average power in watts, (E_{\text{measured}}) is session energy in watt-hours, and (t) is session length in hours. Convert kWh to Wh by multiplying by 1,000.
A measurement at the wall includes the CPAP’s AC-adapter losses but not the battery power station’s upstream inverter losses. Keep that measurement boundary consistent when applying an efficiency allowance.
Humidification Does Not Have a Fixed Multiplier
“Humidifier doubles draw” is not a reliable sizing rule. ResMed’s battery guide, revision 198103/7, August 2018, provides the following 12 V input-current values for the listed AirSense 10 CPAP, Elite, AutoSet, and AutoSet for Her configurations at 10 cm H₂O through the appropriate ResMed DC/DC converter.
| Configuration | Listed current at 12 V | Calculated battery-input power | Calculated energy for 8 hours |
|---|---|---|---|
| SlimLine tubing, without heated humidification | 0.93 A | 11.2 W | 89 Wh |
| SlimLine tubing, humidifier setting 4 | 2.30 A | 27.6 W | 221 Wh |
| ClimateLineAir at 30°C, humidifier setting 8 | 4.92 A | 59.0 W | 472 Wh |
Power is calculated as (P=VI), and energy as (E=VIt), using 12 V and eight hours. Results are rounded after calculation. These are manufacturer-table-based estimates, not new measurements or specifications for newer machines.
The values already describe battery-side demand through the specified converter, so do not add the same converter loss again. The guide’s separate recommended battery-size entries also include a 50% safety margin; the calculated energy column above does not.
Size the battery for the humidifier and tubing settings you expect to use. Discuss any proposed comfort-setting changes with your equipment provider; do not change prescribed treatment pressure to fit a smaller battery.
Calculate Overnight Battery Capacity
First calculate the energy needed at your chosen load boundary:
[
E_{\text{load}}=P_{\text{avg}}\times t\times N
]
Where:
- (E_{\text{load}}): required load energy, Wh.
- (P_{\text{avg}}): average operating power, W.
- (t): hours per night.
- (N): nights between recharges.
Then estimate rated battery capacity:
[
C_{\text{rated}}\geq
\frac{E_{\text{load}}\times(1+r)}
{u\times\eta}
]
Where:
- (C_{\text{rated}}): labeled battery energy capacity, Wh.
- (r): additional planning reserve, expressed as a decimal.
- (u): fraction of rated battery energy available for the planned discharge.
- (\eta): effective energy-delivery efficiency from the battery to the load boundary.
Obtain (u) and (\eta) from applicable battery documentation or representative testing when possible. For an AC power station, efficiency should reflect operation at the actual CPAP load, including inverter overhead. Do not substitute an unsupported “typical efficiency” as though it were a product specification.
If the manufacturer supplies usable energy measured at the output you will use, start with that figure rather than reducing it again for losses already included.
Worked Example: One Eight-Hour Night
Assume the following solely for calculation:
- Average demand measured at the CPAP’s AC plug: 30 W, including its AC adapter.
- Required runtime: eight hours.
- Available battery fraction, (u): 0.90.
- Effective battery-to-AC delivery efficiency, (\eta): 0.85.
- Additional energy reserve, (r): 0.25.
The overnight load is:
[
E_{\text{load}}=30\times8=240\text{ Wh}
]
The rated-capacity estimate is:
[
C_{\text{rated}}\geq
\frac{240\times1.25}{0.90\times0.85}
=392.16\text{ Wh}
]
Round upward to at least 393 Wh for this arithmetic threshold, then choose a compatible product above it. The 25% reserve is an example planning choice—not a manufacturer requirement or a guarantee against every operating condition.
If average demand were 60 W instead, the same calculation would require approximately 785 Wh after rounding upward. Two eight-hour nights at 30 W without recharging would require the same amount.
Keep Battery Capacity Separate From Output Power
Battery watt-hours determine stored energy. The outlet’s watt rating determines how much power it can deliver at once. Both must be sufficient: meeting the overnight Wh calculation does not establish that an AC outlet, DC port, or converter can support the machine’s maximum demand.
For batteries labeled in amp-hours:
[
C_{\text{Wh}}\approx V_{\text{nominal}}\times C_{\text{Ah}}
]
A hypothetical 12.8 V, 40 Ah battery therefore contains approximately 512 Wh of nominal energy before usable-capacity and conversion allowances. Always use the voltage associated with the battery’s Ah rating; Ah alone is not enough to compare batteries.
A “90 W” converter label describes output capability, not a constant 90 W overnight load. For example, ResMed lists its Air10 converter as a 24 V/90 W output device for AirSense 10 or AirCurve 10 machines, accepting a suitable 12 V or 24 V DC source.
Choose a Compatible AC or DC Power Path
An AC connection uses this path:
Battery → inverter → CPAP’s AC adapter → CPAP.
A compatible DC connection uses:
Battery DC output → model-specific DC/DC converter → CPAP.
ResMed recommends its appropriate DC/DC converters over inverters for the devices addressed in its battery guide, citing better efficiency. That supports considering DC operation, but not assuming a fixed percentage improvement for every battery and machine.
Match the converter to the exact CPAP model, and check the battery port’s voltage and current limits. A connector that physically fits does not establish electrical compatibility. ResMed’s U.S. battery and converter page identifies separate converters for Air10 and AirMini equipment; they are not interchangeable universal CPAP cables.
For AC operation, confirm the machine and humidifier manufacturer’s inverter requirements. Do not apply inverter guidance for an older model to a different machine simply because its average wattage is similar.
Verify the Complete Backup Setup Before Relying on It
Before an outage, check:
- Runtime with your actual machine, humidifier, tubing, and intended sleep duration.
- Continuous and peak output capability, not just battery Wh.
- Whether low-load shutdown could disable the selected outlet.
- Whether automatic switchover and simultaneous charging/output are explicitly supported.
- Recharge time and the available charging source between nights.
Manual battery operation is not the same as uninterrupted backup. Do not assume that “pass-through charging” means the CPAP will continue operating without interruption when utility power fails; confirm the documented behavior and test the complete setup according to both manufacturers’ instructions.
Use a full charge and a representative operating test to check the estimate without deliberately running therapy to battery shutdown. If interruption would create a significant medical risk, arrange an outage plan with your clinician or equipment provider rather than relying on calculated runtime alone.