To size a portable power station for CPAP camping, estimate the energy your exact setup uses each night, multiply by the nights you need without recharging, and account for available battery energy and reserve. You need both a compatible power connection and enough watt-hours; a large AC wattage rating answers neither question by itself.
Correction, October 22, 2026: Earlier versions included unsupported firsthand testing and experience claims. Those claims have been removed. The calculations below are worked examples, not measurements of a CPAP model or promises of overnight runtime.
Record the inputs for CPAP camping battery size
- Exact CPAP model, tubing and accessories, used at your usual prescribed settings.
- Supported AC power supply or device-specific DC converter.
- Hours of use each night and nights between dependable recharges.
- Energy use measured or documented at a clearly identified connection point.
- Other loads sharing the station, such as a phone or light.
- Battery reserve and an available-energy assumption that fits the connection.
A power adapter label such as 90W describes a rating. Multiplying that number by eight hours can overstate or otherwise misrepresent actual energy consumption because the device does not necessarily draw its maximum rating continuously. A momentary reading on the station display has the opposite problem: it can miss demand changes during the night.
Where your equipment provider supports doing so, record total watt-hours over a representative period with the exact setup. Follow the device instructions and keep treatment settings consistent. Record the meter location as carefully as the number; it determines which conversion losses are already included.
Use one of two calculation methods
Method 1: energy needed at the load
If your documented or measured average demand at the load is P watts and intended use is H hours, load energy is P × H watt-hours. For N nights, add any other load energy and then calculate nominal battery capacity:
Nominal capacity = total load Wh ÷ [available-energy fraction × (1 − reserve fraction)].
In this method, the available-energy fraction accounts for conversion losses and the amount of nominal battery energy that reaches the load. The reserve fraction is the share of that available energy you intend to leave unused. Neither fraction is universal. Manufacturer test conditions, output type, battery condition and temperature can change available energy; treatment backup needs affect the reserve.
Method 2: an observed complete setup
If you have a reliable test of the same station, converter and device showing how much energy or battery charge the whole arrangement uses, base the plan on that complete setup. Do not divide by a generic efficiency again when the observation already includes the station and converter losses.
Battery percentage is a less precise planning input than measured energy and a full verified run. A percentage change over a brief period may not scale linearly to an entire discharge. Confirm the result with your equipment provider before relying on it away from mains power.
Worked examples for one and two nights
The following examples assume eight hours per night, 80% of nominal capacity reaching the load, and 20% of available energy held in reserve. These assumptions illustrate the calculation and are not clinical recommendations. They do not identify which CPAP draws each wattage.
| Assumed load and nights | Load energy | Nominal capacity estimate |
|---|---|---|
| 20W, one night | 20 × 8 = 160Wh | 160 ÷ 0.64 = 250Wh |
| 20W, two nights | 20 × 8 × 2 = 320Wh | 320 ÷ 0.64 = 500Wh |
| 40W, one night | 40 × 8 = 320Wh | 320 ÷ 0.64 = 500Wh |
| 60W, two nights | 60 × 8 × 2 = 960Wh | 960 ÷ 0.64 = 1,500Wh |
If the same one-night 20W example includes 40Wh for other devices, the load budget becomes 200Wh and the nominal estimate becomes 200 ÷ 0.64 = 312.5Wh. That is why charging a phone from the same station should be recorded rather than ignored.
Round up for shopping, then verify the actual combination. A calculation landing exactly on a product’s published capacity leaves little room for an inaccurate input. Increasing capacity alone does not resolve low-load shutdown, the wrong output voltage or an unsupported converter.
Estimate runtime from a battery you already own
Rearranging the same method gives runtime hours = nominal Wh × available-energy fraction × (1 − reserve fraction) ÷ average load W. With a hypothetical 500Wh station, the example assumptions leave 500 × 0.80 × 0.80 = 320Wh for planned use. At an assumed 20W average demand, that is 16 hours; at 40W it is eight hours; at 60W it is about 5.3 hours.
Those results depend on the chosen input demand and energy fraction. Heated accessories, device behavior and the converter can alter them. Treat the result as a budget to verify, not a guarantee that a particular machine will complete treatment.
Understand what the ResMed battery tables mean
The ResMed battery guide, revision 198103/7 from August 2018, specifies particular devices, converters and deep-cycle battery arrangements. It assumes eight hours of use and daily recharge. Its converter and inverter battery sizing tables include a 50% safety margin.
Read the row for the exact device and accessory arrangement rather than borrowing another device’s current draw. The document predates devices absent from its tables, so use current manufacturer guidance when your model is not included. A table for a separate deep-cycle battery does not certify a particular portable power station.
When converting an amp-hour rating to nominal energy, use the voltage at which that rating applies: Wh = Ah × V. For example, 10Ah at 12V is 120Wh nominal, while 10Ah at 24V is 240Wh nominal. Neither figure states how much energy reaches a CPAP. A lead-acid battery sizing recommendation also should not be treated as a measured lithium station runtime.
Account for recharging without spending the energy twice
For two nights with no recharge, count two full night budgets. If you can recharge, count only energy that can realistically be stored before the next night. A panel rating, a car charger’s maximum input or a brief sunny reading is insufficient evidence that the overnight energy will be replaced.
Suppose 320Wh of battery-side energy must be restored and observed charging input is a steady 80W. If your consumption figure was measured at the load, account for battery-to-load losses before using it as the amount to restore. Dividing gives four hours before charging losses or tapering. Two hours of such charging offers at most 160Wh at the input, so it does not replace the 320Wh budget. Use the CPAP battery recharge planning guide to check the available source and time.
Verify the electrical behavior as well as the arithmetic
Check the manufacturer’s supported adapter, current limits, inverter requirements and output shutdown behavior. Confirm the station keeps the required output active at the device’s actual demand. Ask your clinician or equipment provider about treatment continuity and a suitable backup arrangement. Do not alter prescribed treatment settings to reduce the calculated capacity.
For product comparisons, use the CPAP camping power station buying guide. For the connection itself, use the DC converter compatibility guide. The general power station runtime formula can help organize other loads sharing the battery.