We3 wise man! :) :)
UL prohibits an AC only unit. I am not familiar with every model on the market but something is wrong withthe idea the device is AC only. My Respironics BiPAP draws 53 watts with humidifier. With a Delta T of 20 degrees F the humidifier heats about 30% of the time. With a 50 degree room it's working 100% of the time.
Be careful to NOT have the unit connected to 12-volts when starting any engine on that battery circuit. The warning comes direct from Respironics. They saud they do not wish to see more than 13.5 volts enter their machines.