Off-Grid PV Power for an O2 Concentrator or CPAP, by Tunnel Rabbit

With the advent of new variants of Coronavirus in mind, and other diseases that will be present in a collapse when no doctor can be found, we may be faced with the long-term care of loved ones. The production of oxygen, and/or running a Continuous Positive Airway Pressure (CPAP) machine could be necessary. What follows is my ‘notes on the back of an envelope’ attempt — a starting point that goes toward a solution. I am merely offering the reader my estimation of what it would take to get this done. This is the minimum PV power generation and storage requirement that I would recommend for operation for each of these life-saving machines.

Disclaimers and Provisos: I have no relationship with any of the companies mentioned. I am not a medical professional, nor am I an electrical engineer, but as most preppers, I strive to do the best I can and offer this educated estimate to others as a starting point for their own research. This is intended to be only a feasibility study for those who would install it themselves.

My estimate is based upon my experience living off-grid using only a small off-grid PV system. Simple math can only be a guide, as there are many variables that cannot be factored in. Therefore, it is better to over-engineer to compensate for an electrically inefficient design and hidden losses within a system. I am not recommending an inverter, or calculating in the loss of power due to the  use of an inverter. It is far better to use a DC appliance that used 12 VDC directly, if it can be found.

If one cannot find a suitable 12VDC appliance, then the purchase of an expensive and quality inverter is necessary. The low-cost inverters are not recommended as their reliability is questionable, and the dirty power they produced might harm the appliance. If the use of an inverter is required, then add approximately 20 percent to the recommended PV array wattage and battery storage capacity, and at least $2,000 to the overall cost. This estimate is based upon an oxygen concentrator on the market that requires the least amount of power, and that can be operated directly with 12 VDC power, should that be found. It is the most efficient, the smallest, and least expensive, most reliable system for the money.

Here is a link to a useful article that details the power consumption of several models of oxygen concentrators.

Overall, it is less expensive to purchase a more efficient appliance that requires the least amount of power to function, rather than purchase the equipment need to generate more power. Most patients need only 2 liters per minute, and that means a machine will use less power than if run at its maximum rate of 5 liters if it is rated to do so. However, some machines may not put out a maximum of 5 liters, and therefore their power consumption rating could be less. Determine the level of oxygen required, and then shop for a unit that uses less power. Reducing demand on a battery bank is less expensive than buying a larger battery bank and more panels. I am basing my estimate on a consumption of 120 watts (i.e.: 10 amps at 12 VDC), for 24/7 operation.

For three years of continuous operation (the limit is due to the battery type used, flooded lead-acid batteries and a 50 percent discharge per day), the PV system required, at a minimum, needs to be around 1,600 watts. More is always, more better… The battery bank should be no smaller than 1AH per watt, or at least 1,600 AH if flooded lead-acid batteries are used. This system could produce enough power for about three years. This is the usual service life of batteries when they are discharged daily to 50 percent of their capacity.

Outdoor monocrystaline photovoltaic (PV) panels will last for decades. Doubling the size of the battery bank would greatly increase the life of the battery bank, because the depth of discharge would be reduced. I plan to limit the depth of discharge of my battery bank to no more than 10 percent. This will give me the longest battery life possible, up to 8 years at the extreme, but more likely at least 6 years. If running a CPAP only, the minimum I can recommend is about 500 watts and two 6 VDC 210 amp-hour golf cart batteries. It would be much better to have four golf cart batteries and get five-to-six years of use out of them as a set instead of only three years. Generated sources of power will be needed during the winter months if you do not live in the southernmost states. There will undoubtedly be other electrical demands as well, so size your system appropriately.

One can lower the cost of the panels by buying them by the pallet. Not including the freight charge, large grid-tied panels can cost as little as 60 cents per watt. However, a higher cost MPPT charge controller needed for this type of panel will increase the cost of the system. If using a PWM type charge controller, extended runs of heavy gauge wire will increase the cost of the components. I have chosen the most reliable system possible and purchased Renogy brand 100-watt panels at a cost of $100 each, shipping and handling included. I used discarded well pump 6 gauge wiring to contain the cost of a 800-watt system. The good Lord provided 4,500 feet of used copper cable at no charge, and exactly when I needed it! Truly amazing.

I would also use the most reliable and lowest cost battery type that are flooded lead-acid deep cycle batteries. These are also the lowest cost battery type. However, there may still be a shortage of these batteries on the market. Marine batteries are not deep cycle batteries if their rating is expressed in CCA (Cold Cranking Amps). Marine batteries are actually a hybrid or a compromise and some are better than others. I would use them only as a last resort. My rough estimate for the components of this DYI system might be $5,000 for a 1,600 watt system, and around $1,500 for a 500 watt system. An inverter is not included in this rough cost estimate. The use of lithium or AGM batteries would drive the cost much higher.

I recommend the use of Morningstar charge controllers. A lower-cost charge controller for a 500 watt system intended for a CPAP could be the Morningstar Sun Saver Duo (25 amp). This is a PWM type charge controller, and it can charge two sets of batteries independently of each other. This is a huge advantage. For a 1600 watt system, I would recommend two Morningstar TS60 charge controllers. These are among the most RF quiet, lowest cost, and the most reliable in the industry, with a 15-year service life.

The TS45 and TS60 are also a PWM type that can handle up to 48 volt systems, and could also be used to manage power from a wind turbine, or micro-hydroelectric dam, that I personally plan to install one day. MPPT charge controllers have their problems, and can be RF noisy. Perhaps RF noisy enough that one’s location could be DF’d. Some MPPT charge controllers can be so RF noisy that the operation of a transceiver could be problematic.

Yes, MPPT charge controllers are popular, and they allow the use of less costly wiring, and offer up to a 20 to 30 percent increase power output, but they are not near as reliable as the Morningstar charge controllers that I mentioned. If an additional 25 percent is needed, the cost per watt is now so low that it is less expensive to simply buy another panel or two than step up to a costly MPPT charge controller. I should also add that at 12 VDC system is much easier for the homeowner to install themselves, to operate, and to maintain. And even if a MPPT charge controller could be justified, I’d rather pay a bit more for the simplicity and reliability. If I am planning a trip to the moon, then I’d like to come back.