Call it 450 watt-hours a night for a CPAP, and closer to 530 out of the battery if you run it through an inverter. That is the number to size around, and it is bigger than most people expect from a machine that fits in a shoebox.
Here is where it comes from. ResMed’s user guide for the AirSense 11 lists typical power consumption at 56.1W and peak at 73.2W, on a 65W power supply putting out 24V at 2.71A. Eight hours at 56.1W is 449 watt-hours. Eight hours at the peak figure is 586.
Where the extra comes from
Your battery does not deliver 449Wh to the machine. It delivers 449Wh plus whatever the conversion costs.
If the CPAP is plugged into a household outlet on a power station, the station is turning DC into AC and the machine’s brick is turning it back into 24V DC. Inverters in this class typically give up 10 to 20 percent. Round it to 15 and you are pulling roughly 530Wh from the battery for the night. Then there is the station’s own usable capacity, which is always less than the number on the box.
So a nominally 500Wh power station is a one-night unit at best, and you should expect to see the low-battery warning before morning rather than after.
Skip the inverter
ResMed sells an Air11 DC/DC converter that plugs into a 12V or 24V vehicle outlet and feeds the machine directly. Its listed input is 12V/24V and output 24V. One conversion instead of two, and no inverter sitting on all night drawing its own idle current.
If your machine has a DC option, use it. This is the single biggest change most people can make, and it costs less than a bigger battery.
The number we are not giving you
Every CPAP guide on the internet has a table breaking draw down by humidifier level and pressure setting. We are not publishing one, because ResMed does not publish one. The user guide gives a typical figure and a peak figure for the system, and that is it. The tables you see elsewhere are measurements someone took, or estimates someone made, and they are presented as manufacturer specs.
What is safe to say: the humidifier’s heater plate and the heated tube are the large loads, and turning them off will drop your draw a lot. If you want your own number, put an inline watt-hour meter on the machine for one night at your actual settings. That reading beats every table, including any we could write.
Two other things worth knowing before you plan around this. A CPAP is medical equipment and running it flat at 3 a.m. is a different kind of problem from losing your fridge, so size with margin rather than to the exact number. And if you are recharging off solar, 530Wh a night is real: at 4 usable sun hours that is a 150W panel doing nothing but the CPAP, before the rest of your loads. Run it through the RV solar calculator with your fridge in the same total, because the fridge is usually the bigger draw and the two together are what actually sizes your bank. More on system sizing in our RV power section.