I've backpacked with my portable battery and cpap in the California Sierras (insurance provided Respironics system one or older Respironics that I bought used for backup, etc) They have all worked fine for me at 10,000 and above. I just put the cpap at the level for the highest elevation.
I use an owell industries 12-266 battery and I can get 4-5 nights at level 9 per charge so its been handy and portable enough for backpacking. I would think it could be recharged with solar. A similar manufacturer sells a solar charger for its models.
A side note is that at the higher elevation levels you can keep the battery warm by putting it in a sleeping bag somewhat like putting gas containers for backpacking stoves in socks etc to keep them warm so they light easier.
Enjoy the Big Sky Country
I'm a private pilot. The same factors that impact lift on an airplane wing will impact the performance of a CPAP machine. The blades on the blower in the machine follow the same physics as an airplane wing. At lower barometric pressure in bad weather or on a hot & humid day, the density altitude will be higher than your actual altitude. In western mountain country in the summer, it's invariably dry and cool at night so the density altitude at 8,900 feet will be quite a bit lower than that.
In dry mountain air, I can't imagine not running a humidifier. The solar panel and battery need to be sized properly to run the humidifier. The key is making sure you remember to find a sunny spot at the campsite to charge the battery.
I just put a power meter on my ResMed S9 AutoSet. I'm lining up my ducks to run the machine off the house battery on my boat. I can always run the diesel to charge the battery but I'd rather let a solar panel do the work if I'm floating on the anchor or a mooring for a few days.
JScholtz and GoeffD, Thanks for the response. I'm looking for a older reasonable unit of some type for a back up and have seen a lot of respironics on CL. Since we are trailer camping albeit in a small trailer, a deep cycle battery is a likely option. I don't know much about the Solar charger issues yet. I've been looking at a 100 w panel with a charge control unit. We use almost no power when we are set up and have lasted over two weeks without a charge-up in the pre Cpap days. are there any units you'd suggest I avoid.