A Continuous Positive Airway Pressure (CPAP) machine is a medical device that uses air pressure to keep a user’s airway open during sleep. The electrical power a CPAP machine consumes, measured in watts, is not a fixed number. Power draw varies considerably based on the specific model, the pressure settings required, and whether comfort-enhancing accessories are in use. Understanding power consumption is important for users planning to use a battery backup system or portable power during travel or an outage.
Understanding CPAP Power Requirements
The baseline power requirement for a standard CPAP machine, operating without heated accessories, typically falls within a range of 30 to 60 watts (W) during continuous running. This power is needed to run the blower motor that compresses the air to the prescribed pressure setting. Newer, more energy-efficient models can sometimes operate at lower wattages, closer to 15W, when the humidifier and heated tubing are disabled.
The power drawn by a device is expressed in watts, which is calculated by multiplying the voltage by the current. The running wattage is the sustained power used during therapy. A brief peak wattage may occur when the machine is initially turned on, sometimes reaching 100W before settling back down. For calculating battery life, it is more useful to consider the total energy consumed over time, which is measured in Watt-hours (Wh).
Primary Factors Influencing Power Draw
The largest factor causing fluctuation in CPAP power consumption is the use of the heated humidifier. Heating water requires significantly more energy than simply running the air blower motor. Activating the humidifier can easily double or triple the total running wattage, pushing consumption into the 70W to 100W range.
The machine applies thermal energy to the water reservoir to create warm, moist air. Users can lower the power draw by reducing the heat setting or turning the humidifier function off entirely. Heated tubing also contributes a secondary power load, typically adding an extra 5 to 15 watts of consumption to prevent condensation.
Pressure Settings
The prescribed pressure settings also influence the motor’s power draw. The blower motor consumes slightly more power to maintain a higher pressure setting than a lower one. While this increase is measurable, it is a small fraction of the energy consumed by the humidifier or heated tubing. Users looking to conserve power for battery use should focus primarily on the temperature controls.
Converting Wattage for Battery Use
When planning for off-grid use, the most relevant metric is the Watt-hour (Wh) capacity of the battery. The Wh rating represents the total energy stored and is calculated by multiplying the wattage by the hours of use. For instance, a machine consuming 50W for an eight-hour night will require 400Wh of energy.
AC vs. DC Connection
A significant consideration for portable power is the method of connection, specifically the difference between Alternating Current (AC) and Direct Current (DC) power. CPAP machines internally run on low-voltage DC power, but they typically plug into a wall outlet using an AC power cord and a power brick that converts AC to DC. When using a battery with an AC outlet, the power undergoes two energy-wasting conversion steps: DC to AC by the battery, and then AC back to DC by the CPAP’s power brick.
Using a manufacturer-approved direct DC power cable bypasses the power brick and the battery’s AC inverter, making it substantially more efficient. Running the machine via an AC inverter can result in a power loss of 15% to 30% compared to a direct DC connection. To size a portable battery, estimate the maximum nightly Wh consumption and select a battery with enough capacity to cover the required number of nights.