CPAP machine w/ inverter. How much power?

mena661 wrote:

Searching_Ut wrote:
With the smaller inverter you're looking at, you'll probably be looking at using at most 25 to 30 percent more than you would if you had to run straight off the battery.

Where do you get those numbers from? My 300W doesn't waste anywhere near that amount unless you're talking about the really cheap inverters. Mine is rated at 80-90% efficient.

I think you're leaving the CPAP power supply out of the equation.

To begin with, most inverters don't deliver the advertised efficiency under anything but ideal load conditions. That said, assuming your advertised 80 to 90 percent efficiency of the inverter, you lost 10 to 20 percent of your energy converting from DC to AC. The internals of the CPAP machine run on DC. Most modern units use a brick type power supply to convert AC to somewhere between 12 and 24 volts DC depending on manufacture. A few of the machines built the DC converter into the machine, so with those there isn't a practical way to bypass this portion of the equation, but you are still going to experience another energy loss converting from AC to DC. If you have a unit that can be plugged directly into the DC power supply you'll bypass the two conversion processes and realistically save somewhere between 25 to 30 percent over a comparable machine which requires putting the inverter and brick into the mix. With really good electronics, you might do a little better, but I wouldn't expect it to be all that significant of an improvement.

Note: For the technical types out there, before anyone points it out, I realize that the energy isn't "lost", but for our purposes that term works as good as any in my humble opinion....