Free CPAP planning tool

How long will your CPAP battery last?

Estimate runtime, convert battery capacity and plan the energy for your nights away. Works with your own figures from any brand.

No account neededNo product requiredSave or print your estimate

Your power setup

Use any brand. Enter your own battery and power figures.

1. Battery capacity

Rated battery capacity Use the battery's rated energy, not its output power in watts.

Capacity unit

2. Power use and your plan

Enter a low and high average draw, or use the same value for both. Include your actual humidifier and heated-tube settings. A power adapter's maximum rating is not an average consumption measurement.

Lower average power draw (W) Use measured average consumption for your actual settings.

Higher average power draw (W) Use the same measurement point as the lower estimate.

Hours per night Your planned hours of use, up to 24.

Nights between charges Used to estimate the capacity for your whole plan.

3. Losses and reserve

The example 75–90% range and 20% reserve are editable assumptions, not manufacturer specifications. If your watt measurement already includes converter losses, do not count those losses twice.

Lower delivered-energy estimate (%) Example assumption, not a measured efficiency.

Higher delivered-energy estimate (%) Include battery availability and losses up to your measurement point.

Capacity held in reserve (%) A planning allowance you choose, from 0 to 99%.

Your estimate

Enter your figures to see a runtime range and the capacity for your plan.

Actual runtime can vary with settings, temperature, battery condition and equipment. This estimate does not confirm electrical compatibility, uninterrupted medical power or airline acceptance.

Calculation inputs stay in this page. This calculator does not save them or send them to a server.

Understand the numbers

How the calculation works

Convert capacity to energy

Watt-hours (Wh) describe stored energy. If your battery lists amp-hours, multiply by the nominal voltage associated with that rating.

Wh = Ah × V
Wh = mAh × V ÷ 1,000

Estimate runtime and capacity

The calculator applies your delivered-energy estimate and then holds back your chosen reserve.

Runtime (h) = Wh × efficiency × (1 − reserve) ÷ average W
Required Wh = total hours × average W ÷ [efficiency × (1 − reserve)]

The lower runtime uses the higher power draw and lower delivered-energy estimate; the upper runtime uses the opposite values. Required capacity uses the same bounds in reverse. Percentages are converted to fractions in these formulas. The displayed range is an arithmetic scenario range, not a statistical confidence interval or a guaranteed minimum.

A worked example

A hypothetical 300 Wh battery, 20–30 W average draw, 75–90% delivered energy and 20% reserve gives 6–10.8 hours. For one 8-hour night, the calculated rated capacity range is about 222.22–400 Wh. These figures illustrate the method and do not describe a particular device.

Before you use your estimate

Where do I find my CPAP's power draw?

Use a suitable energy meter to measure average consumption over representative use with your usual settings, or obtain consumption information for your exact setup from its manufacturer. If you measure watt-hours over a session, divide by hours to get average watts. The maximum power rating on an adapter is not the same as measured average draw.

How do I account for humidification and heated tubing?

Include them in your power measurement. Their use, your device settings and the environment can change consumption. Do not change prescribed therapy to match a calculated battery runtime; check your device instructions and discuss any treatment changes with your care team.

What does the delivered-energy percentage mean?

It estimates how much rated battery energy reaches the point where you measure power, after battery availability and conversion losses. Battery-side watts already include downstream losses; load-side watts may need an allowance for them. Use consistent measurement boundaries and avoid counting the same loss twice. The initial 75–90% values are examples you can replace with measured or manufacturer-supported figures.

Does a matching energy capacity mean my battery is compatible?

No. Confirm the exact device, voltage, output, connector, approved converter and any humidifier restrictions with the manufacturers. This calculator does not check compatibility, continuous or peak output limits, UPS operation, battery ageing or airline rules. Follow your care team's power-outage plan if treatment interruption could put you at risk.

Can I save my inputs and results?

Download the inputs, assumptions, results and calculation method as a text report, or use Print / save PDF. The calculator does not persist inputs; a reset or page reload clears them. Existing site analytics do not receive calculator inputs from this tool.

Sources and further reading

These sources explain energy capacity and device-specific battery considerations. They do not validate the example efficiency values or certify this calculator.