TL;DR: Plug it in. It will be fine. CPAPs have their own power supply that regulates the power. Even at 50W, that's significantly less than most laptops use while charging.
As far as the battery suggestions go, TSA limits flyers to 100Wh, or 160Wh with airline approval (unclear how you get that). If you use a CPAP with a humidifier, it's going to pull between 30-50W. With that draw, simple math shows that 100Wh may not even last 2 hours. The TSA guideline goes on to say that you could also carry two spare batteries with the same restrictions as above. But who wants to change batteries every ~2 hours...
As far as real power draws go, I bought a backup battery before the hurricanes this year, and I wanted to make sure I had enough to last all night, so I ran some tests to see how many Watts my ResMed 10 pulls. It pulls in the mid to high 30s with the humidifier. If I turn off the heater, it drops to like 16W. Ended up getting a 240Wh power bank. Which is enough for a good night sleep (for me).
Lithium ion batteries (a.k.a.: rechargeable lithium, lithium polymer, LIPO, secondary lithium). Passengers may carry all consumer-sized lithium ion batteries (up to 100 watt hours per battery). This size covers AA, AAA, cell phone, PDA, camera, camcorder, handheld game, tablet, portable drill, and standard laptop computer batteries. The watt hours (Wh) rating is marked on newer lithium ion batteries and is explained in #3 below. External chargers are also considered to be a battery. With airline approval, devices can contain larger lithium ion batteries (101-160 watt hours per battery), but spares of this size are limited to two batteries in carry-on baggage only. This size covers the largest aftermarket extended-life laptop batteries and most lithium ion batteries for professional-grade audio/visual equipment.