Inverter capacity

[Bill0828]

I'm posting a question for some friends who want to power their (2) C-PAP machines using inverter power. They have a total of four batteries. Two engine/starter and two house type. All four are the 31 series. Their present inverter is a 175 and they plan to upgrade to a 1000 pure sign wave inverter. The question is; will this size inverter operate the two C-PAP machines for up to eight hours during the overnight? Using the generatror all night poses a problem when in close proximity to other RVer's even when using a genturi for the exhaust. Using a smaller portable gen set poses other logistical problems as well.
Any techs out there who might be able to shed some light on this? Looking forward to an informed response.

[RocketDork]

I use a CPAP.

I have 2 Exide 6V batteries for 245AH capacity. I run my generator daily.

I have run my CPAP on a 400 Watt inverter on a number of occasions. I currently run it directly off the 12 house batteries.

I ran it for a night on the inverter, with a Kill-a-Watt, about 200 Watt Hours consumption.

It depends on the model of the CPAP as to whether or not the inverter needs to be true sine, or if a modified sine will work. There are a number of different kinds of CPAP machines too...some operate at Higher pressures, heated humidifiers, heated hoses, etc. Depending on the make and model it may or may not require more energy to run.

The easy way to evaluate the Maximum energy the system will require is to look on the power supply or the bottom of the CPAP. Look for the rating for watts, if it doesn't have that, the current at 120V will get you there too. Multiply the current times the Voltage and you'll have watts. If they have other devices that they use that aren't attached and powered by the CPAP, total all the watts. This will be the maximum the CPAP system will draw. The inverter must equal or exceed this amount. Since there are two of them, the inverter must equal or exceed the rating of both machines.

The next concern is the amount of battery they have. Take the number of watts they'll be consuming (max from above) multiply by the number of hours they will be using them. This will be the Watt/hour demand from the battery. Divide the W/H calculation by 12, this will be the Amp hour required in the batteries. Inverters aren't 100% efficient, so add in another 20% or so for safety margin. Keep in mind this is what has to remain in the batteries independent of any other loads like the heater, lights etc.

They could also get a kill-a-watt and measure how much their machines use during the night and go from there.

If you'd like me to advise them directly you can contact me via PM and I'll give you my email address to send to them.

[Clay L]

The heated humidifiers are real power hogs. If they can do without the heat it would be better from a power consumption standpoint.

[wa8yxm]

IF you can do w/o the heat.. You can get CPAP machines that run on battery power and forget the inverter upgrade

Note at the 20 hour rate each pair of G-31's has just over 1KWH of power. so you can draw a bit more than 50 watts constant per pair for 20 hours.. Likely 100 watts for about 10 hours

That is total, PER PAIR (The math is non-linear)

[RocketDork]

The ratings I see on most group 31 batteries are in the 100AH range...that would be around 1kWH each...

You should only use about 1/2 of the battery capacity if you want them to last.

My CPAP is rated at 12V@4.15A or ~50W, at 8 hours of run time at its maximum draw that's ~0.4kWH. This is approximately the same as running two of the 12V lights for the night...

My experience with my machine is that it uses about 0.2kWH per night. That is WITH the humidifier running. YMMV

The best thing you could do is use a kill-a-watt and measure what is needed on a per night basis. Size the inverter and batteries to ensure there is energy to keep you breathing during the night.