All CPAPs will operate on DC power with the use of an inverter to change the current from DC to AC before it reaches the machine. However, many will operate on DC power without the need for an inverter, just a power cord that plugs into the CPAP and ends in a cigarette lighter plug.
Inverters do require a lot of power. If you need to use an inverter to run your machine on battery power, the inverter will probably take about half the life off the battery.
To determine if your CPAP will operate without an inverter, look for a DC outlet on the machine, either at the back or on the side, with a single opening directly in the center. It should be labeled “DC Power” or similar term.