Spoiler Alert!! On average a 400watt inverter will work with most CPAP machines on the market today. That’s the short answer but the question is a bit more complicated than it seems, and yes, there is math involved. Don’t panic, you can do it.
Apnea – From The Greek Apnos “Without Breathing”
An estimated 18 million American adults have some degree of sleep apnea. Most still enjoy a full and active life, but like any chronic condition, accommodations must be made for proper treatment. The most common treatment for sleep apnea is a CPAP (continuous positive airway pressure) machine. Depending on the severity of your sleep apnea a CPAP may be life-saving equipment. If you camp or RV, finding a reliable way to provide power to the CPAP all night without hook ups or a generator becomes a serious concern.
CPAP – Which Type Do I Have?
There are many CPAP machines on the market and each has features and specifications unique to their respective make and model. The unique specifications of your CPAP will come into play when we do the math to determine the correct size inverter for your machine. There are travel size CPAP machines that work with a battery pack designed to power the machine for one to two nights before recharging; there are units that plug into a 110volt outlet but use a converter built into the power cord to provide the machine 12volt power; and there are units that run strictly on 110volt. If you have the option to run your machine directly on 12volts that is a more efficient use of power than asking a small box of 12volt to provide 110volt for any length of time. For this article we will be talking about the 110volt only models, mostly because they are the only units that would require an inverter. How do I know if my CPAP is a “dual voltage” or 110volt exclusive machine? It is pretty simple to identify your machine’s voltage requirements. On the back of most machines, at or near where the power cord is attached, you will find an amp rating and required voltage. If the voltage is shown as 12v your unit runs on 12volt and there should be a cord available from the CPAP manufacturer that allows the unit to be plugged directly into a 12volt power source. No inverter needed, you can stop reading now if you like. If your CPAP is 110volt exclusive we have worked our way to the math portion of the program.
The Math!
We have by now determined the amps your CPAP requires, or its amp draw, and that it runs exclusively on 110volt, but inverters are sized by watts. If only there was a simple formula like AMPS x VOLTS = WATTS that would allow us to use the information we have to determine the correct inverter size. Okay, so I tried to sneak the math past you, but you caught me. Good, this means you’re pretty sharp and should be able to follow, as we get a bit more technical. For demonstration purposes let’s say your CPAP uses 2.5 amps of 110volt.
2.5amp X 110volt = 275watts
Did you just reread the opening paragraph and think “why did they recommend a 400watt inverter if my CPAP only needs 275watts to run”? The answer is two fold. First the amp draw of each CPAP machine will vary by model and usage settings, but most will not need more than 400watts to operate. Second, you never want to run a converter, generator, or inverter at its maximum output. Running at max output will cause the component to run hotter than it was designed to withstand for extended periods. This will shorten its life and may have negative consequences on the equipment it runs, in this scenario, your expensive CPAP. I try to operate under the 80% rule, which says you should not demand more than 80% of a component’s output under normal operation. Also keep in mind you may want to run other 110volt equipment from the inverter so some “extra” power might come in handy. Whether you run only the CPAP or you incorporate other components your available amp hours or battery bank becomes part of the equation. I know, I know more math. Don’t worry we are covering that later.
Why Do I Need A Pure Sine Wave Inverter?
Inverters come in two offerings:
Pure Sine Wave
Modified Sine Wave
Pure sine wave inverters tend to be more expensive than their modified-square wave counterparts, due to the extra control and filtering stages that are necessary. Those extra controls and filtering stages provide a cleaner 110v power, much closer to the quality of the power supplied to our homes. Why is that important? It is important because the majority of the electronics in our day-to-day lives are sensitive to the quality of power provided to them. Modified sine wave can be viewed as “dirty” power by some components and prevent them from powering up. If the component will accept the modified sine 110v it may still work harder to operate compared to pure sine 110v. This can shorten the life of the component, your CPAP, and the inverter. There have even been instances where the CPAP’s data collection was compromised when the unit was powered by a modified sine wave inverter. Bottom line is a modified sine inverter may be less expensive in the beginning, but could cost you in the long run. Besides when we are talking about your life, is that really the time to go cheap?
More Math – Battery Banks and Amp Hours
Okay, now to be honest the mathematical theory here is a little more difficult, but again don’t panic, if I can do it so can you. To determine how much battery power is needed to keep your CPAP running for a specified amount of time, finding the required amps per hour is necessary. To do this, divide the CPAP machine’s max consumption (275 watts, in our case) by the voltage of your batteries (12 volts). What if I have 6volt batteries? Still divide by 12, your 6volt batteries are run in series so they provide 12volt to the RV.
275watts / 12volts = 22.9 amps per hour
So, in our example, this CPAP machine requires 22.9 amps per hour. At the risk of over simplifying, this means for every hour of operation the CPAP consumes 22.9 amps. To determine the power needed to run the CPAP all night we multiply the amps per hour by the number of hours spent sleeping. I recommend using the maximum possible hours spent sleeping versus the minimum, for safety sake. Let’s say the great outdoors and the fresh air relaxes us so much we sleep 10 hours a night.
22.9 amps per hour x 10 hours of sleep = 229 total amp hours
This seems like a large amperage demand but keep in mind our math is based on the maximum amp draw of our CPAP and we are including the amperage required for a humidifier as well. No CPAP will run at maximum demand for the entire night, so why use the maximum amperage? It actually simplifies the math. If we execute our mathematical formulas using only the maximum amperage demand we do not have to worry about underestimating and we do not have to factor in the variables that can affect the overall performance of the batteries themselves (outside temperature, battery age and condition, etc.) On average, a single 12volt group 24 deep-cycle battery has a 90amp hour rating and a 6volt battery has a 220amp hour rating. Batteries need to run at 30-50 percent of their actual capacity in order to maintain optimum efficiency and stability. For that reason, using one 400amp hour battery bank would be sufficient for this inverter application. That means four group 24 batteries or two 6volt would provide enough power to run the CPAP and the other components that require 12volt to operate for a ten hour period.
All That For One Night Camping
Remember all of our math in this example is based on maximum amp draw and we are assuming a boondocking scenario. If you have hook ups you are operating on 110v and do not need to invert your RV’s 12volt to power the CPAP, and the same is true if you have a generator. A generator can be used to recharge your batteries if you do not wish to run it all night to provide 110volt for the CPAP. Solar becomes a very attractive alternative to battery charging if you do a lot of boondock camping and you either do not have a generator or you do not want to have to worry about feeding the generator fuel regularly. Solar comes in installed and portable options and can offset 12volt consumption completely with proper sizing. YEP!! MORE MATH!!!!! But that is a topic for another day.
