Is it feasible and beneficial to run an HVO oxygen generating system with photovoltaic (PV) solar panels? That is the topic I’ll be exploring in this post. I consulted with professional solar installers on the technical questions regarding modern PV solar systems.

The cost of PV solar power has come down 99% over the past 40 years. Meanwhile, “conversion efficiency“, which represents the amount of sunlight that is converted to electricity, is now as high as 22.8%. To bring the cost down even further, state and federal governments offer Renewable Energy Tax Credits.

Despite the gains in efficiency, the drop in cost, and the subsidies, solar is still expensive. Achieving break-even is a long-term prospect, especially if you’re planning to go “off–grid”, in which case battery systems and backup power generators add even more cost. But, saving money is not the only benefit of solar power.

To simplify this article, I’m going to focus on a solar configuration that is capable of supporting a 40 LPM HVO system. I’m not going to include backup power, nor will I consider additional power requirements, such as for lighting and HVAC, which you would have in a real-world setting.

## How Much Power does an HVO System Use?

HVO systems are scalable from 10 to 200 LPM, with power requirements that correlate to the maximum flow rate. Before continuing with this article, it may be worthwhile to learn more about how HVO systems work.

Every 10 LPM of output is generated by an oxygen concentrator that consumes about 600 watts. In addition, at least one central compressor is needed to store the oxygen in a tank. Each compressor can store the output of, at most, six 10 LPM concentrators. The HVO “Main” unit contains a single compressor that consumes 600 watts. If you require more than 60 LPM, you’ll have to add a “Drone” compressor for each additional six concentrators @ 600 watts each.

To compute the wattage required for an HVO configuration, use the following two calculations:

Oxygen Concentrator Watts = flow-rate / 10 * 600

For example, if you require a maximum flow rate of 40 LPM, you’ll need four 10 LPM concentrators, so that’s 2400 watts for the concentrators alone.

HVO System Watts = roundup(flow-rate / 10 / 6) * 600

For a flow rate of 40 LPM you’ll need a single HVO Main compressor. In the “HVO System Watts” calculation above, the formula in parentheses yields a fractional answer (40 / 10 / 6 = 0.67) , but you can’t have a fractional compressor, so round up to 1 and multiply by 600. Add the two answers together to arrive at total wattage. In this example, we arrive at 2400 + 600 = 3000 watts.

## Scenario: Generating 40 LPM Using Solar Power

Let’s assume that you’re building a solar array that can power a 40 LPM HVO system with a 60 gallon oxygen storage tank for eight hours a day. Further, we’ll assume that you have some backup power option available, whether it is utility power or battery storage.

The wattage figure calculated above tells you how much power you’ll need *while the HVO system is running*. However, the system only turns on when the storage tank pressure is below the low setpoint pressure and then it turns off at the high setpoint pressure. Whether or not the system is using power, oxygen that was generated and stored is able to flow to your application(s). Looking at a graph of the pressure in the tank where X is time and Y is pressure, the pattern of generating and resting should look approximately like the graph below:

In this particular graph, we see a “sawtooth” pattern that demonstrates a properly-sized HVO system, i.e. demand does not exceed supply. If the system were undersized, the blue line would either descend below the low setpoint, eventually emptying the tank, or it would never reach the high setpoint. When the system is undersized, the compressor will run continuously, but will never catch up. This might be by design, e.g. you may have planned to generate oxygen while the sun is out and use it up later in the day. You can add additional storage tanks to the HVO system, if this is your strategy.

Notice that, in the graph above, the HVO system is “on” for less than half the time. In this scenario, eight hours of power will afford you about 16 hours of oxygen. Be aware that setpoint pressures differ with each compressor model. For more information about the choice of models, read How to choose the right HVO model.

## Give Yourself a Buffer

With solar power, underestimating your requirements can be costly. Consider that you’ll have to clear a site, mount solar panels, run wires, and get an inverter that is suitable for the electrical load (or, even better, a series of micro-inverters, one per solar panel). If it turns out that you guessed low on your power requirements, modifying your system may cost thousands more, and it will certainly throw off your budget. Take the time needed to estimate your total electrical load, consider what you might do in the future that requires power, and share your estimate with experts.

To help you with that effort, you may want to create a “Load Sizing Estimate” spreadsheet that looks something like this:

Once you sum up your watt hours per day to arrive at a total, it will inform your decision about how much power you’ll need to generate.

## Batteries Required

To run an HVO system reliably after the sun goes down, you’ll need a battery system to store power. Next to the cost of solar panels, batteries are the second largest cost component of your system. You can think of them as your fuel tank. Having some amount of stored power is a practical requirement for any solar system, off-the-grid or on, but it’s certainly more critical when you’re out in the wild.

The subject of battery systems for PV solar is a rather involved topic that deserves its own post. Suffice it to say that in the year 2020, I received quotes for a 3kWh battery system (the Encharge 3 by Enphase) for about $10k, and a 10kWh battery system (the Encharge 10 by Enphase) for about $17k. The battery system alone can double the cost of your solar installation.

## Availability of Sunlight

If you live in a location that gets little sunlight, then solar is probably not a good choice for you. For example, if you live in Pittsburgh, PA (the cloudiest city in the USA), or Anchorage, AK, you will not get as good a return on your investment as in sunnier spots, like the state of Arizona.

The other factor to consider is the amount of sunlight that falls on your particular site, as trees, mountains, and man-made structures may block the sun at certain hours of the day. Ideally, you should have a south-facing location that gets at least 4 peak sun hours per day to make solar power a worthwhile investment. Money may not be your only incentive for choosing solar power, but there is a point beyond which it makes little sense.

## Cost Categories

It’s good to know in advance what you’ll have to spend money on to build a solar system. Here’s a breakdown of the cost categories:

**Equipment**, including solar panels, inverters, racking, and solar battery storage units. This will be your biggest expense.**Installation**. You might hire someone to install the system, but, even if you are a DIY gal/guy, you are investing time that you might have spent doing your real job, so you should put that in your calculations.**Ongoing maintenance**, because things break. Expect to spend an average of 2-4% of the initial cost of your system every year.- The cost of
**backup systems**, such as supplemental power (fossil fuel-based generators) and backup oxygen tanks, in case you must have oxygen when power is not available.

Even if you hire an expert, you should go through the mental exercise of figuring out your costs. The more you know, the better the end result.

## Solar Vs. Utility Power Costs

The cost of utility power varies significantly around the world, and even within the United States. The term used to represent the cost of power is “kilowatt hours” or kWh, which represents the cost per one thousand watts consumed in an hour. In the USA in 2020, the residential cost per kWh ranges from a high of 33 cents in Hawaii to a low of just over nine cents in Louisiana, with a national average of about 13 cents.

Let’s calculate kWh, which you can do by dividing the total watts consumed by your appliances by 1000. In our scenario, we’ll require 3 kWh for every hour that we run the HVO system. Let’s assume that the cost of utility power in your area is the national average of 13.08 cents. With that number, we can calculate the cost of 8 hours of HVO run-time as follows:

Electricity Cost per 8 hours = 3 kWh * 8 hours * 13.08 cents = $3.14

If you’ll be using that amount of power every day of the year, you would pay the utility company $3.14 x 365 = $1,146.10. With grid power, you pay monthly for what you use. With solar, you make a big up-front investment and then have essentially “free” power from that point forward. At some point, you may recover the cost of your investment by not having to pay the utility company for power.

For example, assume that you must invest $25,000 in a solar system that generates 3,490 watts of power. The break-even calculation assumes that you’re going to save $1,146 per year by not having to write checks to the utility company. Thus:

$25k solar cost / $1,146 per year in utility power = 21.8 years to break-even

That’s a very long time. It still might be worthwhile, but you’re clearly not going to get rich by not paying the utility company for electricity. However, let’s compare that to the cost in Hawaii, where they pay 33 cents per kWh. There, the cost for 8 hours of daily juice is $2,891 per year, in which case your break-even period would be 8.6 years. That’s quite a significant difference.

But wait, there’s more.

## State & Federal Tax Incentives

The federal government offers a Renewable Energy Tax Credit for Solar Panels (Photovoltaic Systems) that amounts to 26% of the cost of systems placed in service after 12/31/2019 and before 01/01/2021.** **Visit the link above to learn more. There may also be state, county or city tax incentive programs that you can learn more about on your local government websites. These incentives can significantly accelerate the time to break-even.

## Putting it All Together

Using the $25k figure, and taking into account the 26% federal tax credit alone, your capital cost will be reduced to $18,500. I should account for the estimated annual cost of maintenance, which is 2-4%, but I’m going to leave that as an exercise for the reader. Now we’ll divide the after-credit capital cost by the projected annual savings:

$18,500 / $1,146 = 16.1 years

You may feel that 16 years is not an unreasonable amount of time to recoup the cost of a significant capital investment. Ultimately, it’s a very subjective question. The biggest factor in determining the attractiveness of the investment may turn out to be the cost of utility power in your area.

Going back to Hawaii’s exorbitant power cost of 33 cents per kWh, the time to break-even there, after the federal tax credit, is only 6.4 years! If you stay in your lovely solar-powered Hawaiian home for 10 years, you’ll be about $10k ahead in year 10.

## Summary

This post just scratches the surface of the knowledge that’s required to purchase a PV solar power system that will meet your requirements. I hope that, at the very least, it gave you some useful hints about the amount of power you’d need to drive an HVO system and how to calculate the time to break-even.

With a set of long-term goals, some careful planning, help from an expert, and a bit of courage, solar power could pay off for you in a variety of ways. Your goal may be to save money on power over the long haul, but it could also be to reduce your carbon footprint, live off-the-grid, or to have a backup to grid power. Some or all of these objectives may be achieved with solar.

Whatever your motivation, think carefully about how much power you will require. Research the available products, as the state of the art is constantly moving forward. Lastly, let me repeat that you should find an expert to help you navigate the path of features and compromises on your journey to energy independence.

#### Acknowledgements and Sources

Thanks to Julie Biskeborn, Chris Campbell, and Katee Wood for contributing their time and knowledge to this post.