My wife has a CPAP and uses a humidifier on hers when we are at home but she does ok without it when we are on the road. She has many machines from over the years and the one she travels with is a 12VDC unit. When we took our trip to Alaska in 2013 we decided to break down and buy a battery just for it. So we purchased the Lithium Ion battery. Expensive but small and compact. This little battery would run my wife's CPAP for 3 nights without the humidifier. Then when driving during the day I would plug it into the truck to charger it back up. After 3 days of discharge one day driving charges it back up.