By Russell Lowes, Rincon Group Energy Subcommittee Chair, April 2, 2017
Have you ever wanted to get off the electricity grid? You might have a number of reasons to do so. What about saving money? The economic breakeven may be here sooner than you think. There’s an interesting and eye-opening thing you can do with energy usage and cost numbers (step 4, below) to make your own cost estimates.
Let’s say that you have decided there are four things you want to do at your house. One, you want to reduce your energy use. Two, you want to buy solar. Three, you want to buy a battery system to back up your solar when the sun is not shining. Four, you want to go off the electricity grid.
This is how the process of battery-backed solar might work in the near future. However, you can get started with step 1 & 2 right now, and later with steps 3 & 4.
1) Reducing Energy Consumption
Let’s say you use 575 kilowatt hours (kWhe) of energy per month, a typical usage rate in southern Arizona;
200 kWhe is a typical reduction per month by using energy efficiency techniques like insulating shades for your windows, weatherization, insulation for your attic, or getting a an evaporative cooler “piggyback system” added to your air conditioning system.
This translates into:
Your usage has been 575 kWhe X 11¢/kWhe, a typical energy cost in So. AZ, which equals $63.25 plus basic service charge, and other charges per month, going down to:
Your new usage, with a 200 kWhe reduction, would be 375 X 11¢/kWhe, or $41.25/month + other base utility charges.
If you were to leave it at that and not do the next steps, you savings would be $22.00/month, $264/year, $5280/20 years.
2) Adding Solar to Your House
Now that you have reduced your energy consumption, when you add solar, you won’t have to buy as many panels. Instead of paying for maybe 5.6 kilowatts of capacity (the average used by the National Renewable Energy Lab, at https://www.nrel.gov/news/press/2016/37745.html), you now would buy around 3.9 kWe.
Your new solar panel array would deliver energy at about 7.0¢ per kilowatt-hour to your home, plus financing, so maybe 8.5¢.
Solar Prices Continuing to Fall
3) Adding Battery Backup to Your Rooftop Solar
Batteries are the big unknown in this process. Costs are falling quickly, and there is a goal by the industry to bring them down to 14¢/kWhe, when combined with solar. This is a bit more costly, when compared to the roughly 11¢ average cost of electricity by the utilities of southern Arizona. However, you only have to get a portion of your energy from batteries, and with lower solar costs here in the Southwest, the deal gets sweeter. For example, you can get 35% of your energy needs met with energy efficiency, from step 1 above, and 45% from solar, from step 2, and 20% from battery energy, from step 3, well that leads us to that point I opened with. . .
4) Going Off-Grid . . . “There’s an interesting and eye-opening thing you can do with energy usage and cost numbers.”
First, you have to boost the number of solar panels a bit to power the batteries, so your cost of solar would go up from 8.5¢ to roughly 10¢/solar kWhe, fully financed. Let’s project that future battery costs are 20¢/kWhe, fully financed.
Take a look at the following table and if you copy these values and formulas onto a spreadsheet (or ask me for a copy at firstname.lastname@example.org), you can change the percentages in column D, and as long as the total equals 100% at the bottom of that column, all the figures will automatically and accurately update! Likewise, if you change any of the projected costs/kWhe in column E, the spreadsheet will auto-self-adjust. But, you math wizards out there already knew that!
This has been about the process of going off the grid, but there are reasons to stay on the grid. The main one is so you can share your electrons with others so they don’t have to use coal, gas or nuclear energy from the grid. However, if the utilities resist the solar revolution, we may not have much choice. If the utilities keep fighting solar rooftop and keep putting onerous charges on our bills, the best choice for you and your family, and for you and your business, might be to go off-grid.
*A side note about the above NREL chart: One interesting thing about the residential-size solar (rooftop solar) versus centralized utility scale is that with rooftop there is much less non-power-generation cost. With centralized solar there are new transmission requirements, more distribution costs, land acquisition costs, switch yard and substation and a myriad of other costs that are not required, as much, as with rooftop solar. Right now, rooftop solar is cheaper when you consider these non-generation costs. I believe that rooftop solar will widen the gap of cost benefit over large utility-scale centralized solar in coming years.
Two utilities, Tucson Electric Power and its sister subsidiary UNS Electric, are applying for rate hikes with the Arizona Corporation Commission. Included in these rate cases is a troubling and unprecedented restructuring of how rates are applied. These proposed rate reshufflings are bad for the families and businesses in these monopoly areas. Additionally, these proposals are assaults on family and business-owned rooftop solar energy installations.
TEP and UNS have engaged in a public relations campaign to promote the inaccurate idea that rooftop solar energy is costing non-solar customers more than if there was no additional rooftop solar installed.
Tucson Electric Power has recently made a number of erroneous statements about rooftop solar costs. However, we will focus here on the most glaring blunder, in what has NOT been said. The utility company does not consider the “opportunity lost cost” for not going with rooftop solar. TEP again made this error of omission in a recent exchange with our County Board of Supervisors, who are opposing the proposed rate shuffle. That is, what happens if families and business owners, schools and local governments in the TEP service area do not install solar panels? TEP is installing centralized utility-owned solar energy plants, and this solar is costing non-solar customers much more than the customer-owned rooftop solar. See the table below, which further explains this.
Examples of Typical Un-Subsidized Energy Costs for New Power Capacity in Southern Arizona, in Cost Per Kilowatt-Hour
Energy Production & Efficiency Options
Initial Un-Subsidized Cost
Trans-mission & Distribution Component
Cost Covered by Rooftop Solar Families & Business-Owners
Maximum Cost Borne by Ratepayers
Homeowner Rooftop Solar Financed with Homeowners Equity Line of Credit, 5%
Homeowner Rooftop Solar Financed with Lease
Medium-Size Business Rooftop Solar Financed with Commercial Loan, 6%
Utility-Owned Rooftop Solar, Financed with Blend of 50/50 Rate of Return and Corporate Bonds, 9% (per IRP)*
Utility-Owned Centralized Solar, Financed with Blend of 50/50 Rate of Return and Corporate Bonds, 9%
Utility Solar via Power Purchase Agreement (Subsidized Fixed Contract)
Utility-Owned Centralized Gas Plant Financed with Same Finance Mix
* The vast majority of this cost will be borne by the ratepayer directly benefitting from this installation.
**Energy efficiency comes in many forms and at many different costs and benefits. The ratepayer-
borne portion of this, on average is likely under 1¢ per kilowatt-hour saved.
Recently TEP just secured more fossil fuel power capacity. This will cost much more for non-solar customers in total dollars, and in cents per kilowatt-hour.
TEP claims that family-owned solar energy increases costs for its non-solar ratepayers. In this claim TEP is probably really talking about what the utility company losses will be. The company financial losses to customer energy efficiency and solar investments are real, if you do not count the gains to the company in terms of grid diversification, performance fees TEP earns on customer energy efficiency investments, etc. However, these gross costs (before these other offsetting benefits) are very minor, at this point of grid penetration, well under 5 percent.
What TEP and UNS Electric ignore, in this “solar costs non-solar customers argument,” is that all the other options of electricity generation expansion are more expensive than customer-installed rooftop solar. Centralized solar built by the utilities costs non-solar customers far more than rooftop solar. Fossil-fuel generation is even more expensive, as well as polluting and climate-changing. In addition, the 0.5¢/kilowatt-hour cost that is purported to be shifted to non-solar customers, is actually returned to customers numerous times, by diversification of the grid, reduction in peak gas-generated electricity, and by many other benefits that solar provides to all families and businesses.
Consequently, it is in the best interest of our families and business-owners that customer-owned rooftop solar continues to be installed, under the current net-metering system. This is not best for the utilities only under the current business models that are now outdated. These models need to change. The Commission needs to require that TEP and UNS update their business models to mesh with the new technologies, the new ways in which people are living, and the improving costs of options customers did not have until recently. Additionally, the business models need to be changed to reflect the far lower impact the newer technologies have on the environment and on human health.
When a rooftop-solar customer invests in solar, that family or business pays all of the construction cost, all of the interest and all of the maintenance costs. These costs add up to about 11¢ per kilowatt-hour if financed through a home equity loan, or a business loan. When a utility builds solar, it pays for these three categories and more (land acquisition, transmission lines, etc.), but then passes it on to the ratepayers. Similarly, when TEP acquires more natural & fracked gas capacity, it pays for these components of overall cost and passes them on to the ratepayers.
TEP and UNS should not be allowed to ignore the fact that if solar rooftop is not invested in by families and businesses, the utilities will have to invest in other more expensive power-generation options and pass those costs on to their customers. To ignore this is deceitful and only works to further undermine the trust of ratepayers in the TEP and UNS Electric monopolies.
>>> Action to take! For anyone wanting to comment before these cases close, you could address your comment as follows. Nobody knows when these two rate cases will close, but it will probably be open through July or August of 2016.
Re: Rate Cases E-04204A-15, E01933A-15-0322 and E-00000J-14-0023
Dear Commissioners Little, Burns, Stump, Forese and Tobin,
Methodology and References
a) This is calculated based on typical sale price of $3000/kilowatt of D/C electrical capacity, .8431 conversion rate to A/C electricity, a lifetime average degradation rate of 13.2% over the 30 year minimum life span, with a capacity factor (average output, compared to A/C rating) of 20.85% with 5% APR financing for a home equity line of credit (HELOC).
b) Based on reviews of leases for solar homes in Tucson, Arizona, by one of the authors, Russell Lowes.
c) Based on lower cost per kilowatt installed but higher loan rate, 6% APR.
d) Based on $2800/kW D/C, 0.8431 conversion rate to A/C, a 13.2% average degradation rate for a 30 years, with a capacity factor of 20.85%, with 9% average financing, per Tucson Electric Power Integrated Resource Plan, which lists 8% as the average corporate bond rate, 10% as the average rate of return on equity and a typical 50/50% blend of the two financing options.
e) Based on $2200/kW D/C, 0.86 conversion rate to A/C, a lower 9.5% average degradation rate for a 30 years, with a lower capacity factor of 18.3%, with X% average financing, based on the Tucson Electric Power Integrated Resource Plan, which lists X% as the average corporate bond rate, X% as the average rate of return on equity and a typical 50/50% blend of the two financing options.
f) Based on what TEP is typically getting for Power Purchase Agreements and what it uses as the basis for its proposal to reimburse solar rooftop owners.
g) Gas-produced power from Lazard’s Levelized Cost of Energy Analysis—Version 9.0, at: https://www.lazard.com/perspective/levelized-cost-of-energy-analysis-90/, p. 2 (click on “View the Study”). This is at the lower end of the two combined Gas Peaking and IGCC (more toward baseload) options. The average of these two is 16.6¢/kWhe. Additionally, see table below for similar approach to gas-generated electricity costs. This has to take into consideration more peaking energy costs for electricity that rooftop solar would displace. These costs can be as high as 21.8¢/kWh, according to Lazard, p. 2.
h) , p. 2, energy efficiency is taken from the top of the range from Lazard’s (see g).
Cost for Conventional Combustion Turbine Gas Electrical Generating Plant
Using O&M & Fuel Costs from Table 8.4*, 2012 Dollars
kWe capacity scenario
cost per kWe**
Capitalization Rate (including principal, interest, taxes and fees)
By Russell Lowes, Sierra Club Rincon Group Energy Chair, August 4, 2014
A year and a half ago my wife, Lhasha, and I took the leap! After 17 years since buying our house, we finally installed solar panels.
Some of the crew members that installed our solar panels, picture by R. Lowes
This article shows how solar is affordable now. Prices have come down even more, since we installed our solar panels. The costs of owning a solar array to power your home are now far cheaper than buying power from your utilities.
We had done a number of things to get ready for solar. We thought it would be best to first reduce our energy needs, so we engaged in a number of energy and water-saving techniques:
Added an evaporative cooler onto our air conditioner, so that we could switch back and forth with this piggyback system – we also put in a barometric damper between the AC and cooler so we would not have to do anything but turn one off and the other on (no getting up on the roof or putting metal sheets in place);
“Piggyback, or dual, evaporative cooler/HVAC system” picture by R. Lowes
Had insulation blown in to our attic;
Installed insulating blinds;
Installed double-pane windows (the noise reduction alone was worth it);
Replaced our lawn with desert landscaping and put in a grey-water system on our clothes-washer (with a Watershed Management workshop more water than energy-saving);
Replaced our A/C system with a much more efficient HVAC system; and
Insulated behind the cabinets on our kitchen cabinets.
We did all these efficiency things first, to save energy and to reduce the panels we would need to buy. After saving up for solar by late last year, we decided to get three or four quotes. We received quotes from Sungevity, Technicians for Sustainability and Net Zero Solar, and a ballpark quote from Geo Innovation. These quotes were for similar products, and had similar contracts.
I was hoping to go with Sungevity, because they linked with the Sierra Club in donating $750 to the Club per installation. However, Net Zero Solar in this instance provided the best bid. Their cost, pre-tax reduction and rebate, was $8925 (without the utility rebate, which we signed over to them). Technicians for Sustainability gave a $11,029 quote, and Sungevity gave a $16,910 quote (gross cost, pre-tax benefit reductions). So, you might ask, how quickly does solar pay for itself? How good of an investment is it? Here is a breakdown of how I would answer this question. First, it largely depends upon how you pay for the system. If you are buying the system and are comparing the cost of the system to what you would pay in electric bills, that would require a projected interest rate for a loan, and an electric price prediction. If you are borrowing to buy the system, and are borrowing money at say 6%, it will be different than buying with cash. For my personal approach (after all, it is a personal approach), I do not think that investments will yield very much in the future, as the stock market is very high, so here I focused instead on the electric grid comparison. I also believe our U.S. economic foundation is weak and that we are likely to go into hyper-inflation in several years, similar to the early 1980s. I believe this will increase electricity bills substantially. It is important for me to emphasize to you: once you invest in solar, there is a good chance that your investment will be good for well over 25 years. Some solar panels are now still in use after 40 years. Your solar investment is not likely decrease in value like stocks or bonds during an economic downturn. It will keep its value and maybe increase in value if the cost of electricity does what I think it will do. This is how I personally approach it. If you are borrowing or leasing, you could come up with a different approach, or you could modify the table below. If you have positive equity in our house, home equity loans are a good way to go. The positive impacts on the environment are matched by the positive impacts on your wallet. Solar energy is economical now.
Energy Chair for the Sierra Club Rincon Group, August 9, 2012
Some states in this fine nation export goods in such a way as to benefit all or many within the state. Let’s take the examples of maple syrup from Vermont, fish catch from Alaska, honey from Utah, or high-technology solutions from California. All of these examples incur some handsome benefits for many or all of the state population in export revenue. That revenue can come in the form of tax revenue or in the form of business income, and perhaps high numbers of jobs provided or even more intangible benefits, like crop pollination.
Not so with energy exports of Arizona. With more than a third of our electricity being exported, there is very little benefit to any significant population of this state. Sure there are some construction jobs that actually don’t go to out-of-state construction workers, and really do go to in-state residents. Sure there are some maintenance jobs for running these plants that also go to in-state residents of Arizona.
However, there are a scant number of jobs in coal, gas or nuclear power production. For every million invested in coal production, only 6 jobs are produced. Fossil-fuel and nuclear plants are capital intensive industries, where the money goes largely for capital-intensive power plant and construction components, many of which are produced overseas.
In contrast to 6.9 jobs for coal and 4.2 jobs per million dollars spent on nuclear energy, solar energy installation produces about 13 jobs per million dollars spent. Whenever you put money toward low job-producing options, you deplete funds for higher jobs-producing options. To put money into coal and nukes reduces overall employment, because that money would have gone to other projects, or perhaps even just into more discretionary spending, which has a much higher jobs output than 4.2 or 6.9 jobs per million dollars spent.
Energy exports from Arizona are not taxed in any significant way that would bring further benefits to the state, except for property taxes that benefit the local areas a bit. We do not tax the payroll that goes for power plant components from out-of-state -– and mostly out-of-country -– workers who create these parts and machinery for the coal, nuclear and natural gas plants. We do not put a sales tax on the exported energy. We do not tax the income of the out-of-state corporations like Bechtel, GE-Hitashi, Toshiba-Westinghouse or others who build these plants.
Then comes SunZia, which some think of as Sunzilla, a monster transmission facility. This system would transport electricity from coal and natural gas producing plants right through Arizona. The company behind SunZia, SouthWestern Power Group, would have you believe that the 16-story high transmission lines would primarily transmit renewable energy. However, every one of their many options for routing their transmission lines goes by a planned fossil-fuel plant in southeastern Arizona and other potential gas plants in New Mexico.
The owners of the Bowie, AZ fossil-fuel plant and SunZia apparently own no renewable energy facilities to speak of. This is a good example of green-washing, where they promise renewables and then you actually deliver dirty energy. Explicitly put, they are using renewables as a cover to deliver their dirty fossil fuel plant.
It is SouthWestern Power Group that wants to build a large natural gas plant north of the Chiracauhua Mountains, near Bowie. It would pollute the air of Chiracauhua National Monument, the Coronado National Forest lands, the Wilcox Playa and the Wilcox area. This plant is east of Tucson, toward the New Mexico boundary line.
The wind from this facility would blow pollutants to Tucson during our hot summer months. This fossil-fuel plant would pollute a large region including parts of Arizona, New Mexico and Mexico. Of course, winds don’t stop at boundary lines, so the pollution, like all pollution of fossil and nuclear plants, would thin out and spread globally.
There is no need for this huge transmission line. Instead, there is a large precedent for energy efficiency improvement in the U.S., in the Southwest and in Arizona. The Arizona Corporation Commission, which is a top regulator for electricity and its transmission in Arizona, has established a requirement for Arizona of 22% reduction in power production in Arizona by 2020. This large electricity reduction is going to make new transmission lines much less viable. On the other hand, to build transmission lines essentially refocuses attention on production, rather than reaching our energy efficiency potential.
All the while, if Arizona were to use its energy as efficiently as California, which has focused on EE programs for a long time, it would reduce its overall electricity production by 52%!
With all this energy reduction going on, why would it be beneficial to build SunZia? It is highly beneficial for out-of-state and overseas corporations. For typical Arizona residents, it is the opposite of beneficial.
Arizona stands to lose environmental quality, and the economic negatives that go along with these environmental quality reductions. The towers and lines themselves contribute to visual blight of the beautiful natural settings of Arizona, and New Mexico. The lines will contribute to transporting more electricity from natural gas – an absolute certainty, with the tie-in to the natural gas plant near Bowie.
Economically, this is not the way to go. Many studies have been done on the average cost of natural gas electricity, on coal electricity, on wind and on the cost of energy efficiency. Here are rough cost estimates for each of these delivered electricity options, or in the case of energy efficiency, saved electricity costs:
Costs Per Kilowatt-Hour of Newly Constructed Power Plant Electricity Delivered or Electricity Saved
Coal 13 cents per kilowatt-hour Natural Gas 11 cents
Nuclear 24 cents
Solar PV 6-12 cents, depending upon solar gain for each area
Wind 11 cents
Energy Saved/Efficiency 3 cents (yes, as in one eighth the cost of nuclear energy or one fourth of coal)
We have enough base load electricity generators for our current use in Arizona, regionally and nationwide, on average, already. We will have even more than enough base load electricity generation with the reduction in load that will occur with nation-wide and state-wide energy efficiency portfolios.
The least-cost approach is energy efficiency. The next least-cost approach is EE mixed with renewables that are distributed generation, in other words, renewables that are generated and distributed locally.
The federal Bureau of Land Management is the agency that is controlling this environmental impact statement (EIS) process. The Draft EIS for SunZia has been done now. It is very biased. For example it makes the claim that this line is for renewable energy transmission, without any significant justification for this claim. The BLM is clearly in cahoots with the company promoting this highly profitable but destructive energy system.
I ask the BLM to clarify what the cost is of the “no-build” option for Arizona and New Mexico, compared to the cost of the SunZia project. I want the BLM to go back to the drawing board and get perspectives on what a no-build option would ultimately do to the total energy cost outlay from the citizens of Arizona and the region. The BLM should contract with reputable firms that do not have a hand in perpetuation of the 20th Century technologies of coal, nuclear and natural gas electricity production. They should consider companies like Synapse, the New Rules Project and others that are not enmeshed in the technologies of the past.
The BLM knows that this system has variable boundaries, as electricity marries electricity, once it gets on the western grid system. However, the BLM also knows that it can reasonably quantify what electricity will cost with a system that is unneeded versus what it will cost with a grid system that is not unnecessarily expanded. The BLM knows that if we put the energy dollars into energy efficiency and distributed generation renewables, the overall cost of energy to citizens in the West will be lower.
So, is Arizona headed to becoming a resource-depleted slave state, a third-world country-like state? Is this beautiful state going to be beholden to outside interests that profit from this potential deterioration? Or is Arizona going to start taking the reins in hand and steer away from this outside domination?
Do we want to go down the tired path of fossil and nuclear energy, or do we want to ramp up our energy efficiency and blend it with renewables, cleaning our environment and reaping economic benefits of cheaper energy costs and more jobs?
A deadline of August 22nd has been set for this important phase of opposition to this project.
Saving Energy Comes in Many Forms “Saving Energy Series, Part I”
by Russell Lowes, April 2, 2011
In 1973, at the height of the OPEC Oil Embargo, America was coming to grips with the concept of limited oil reserves. During that year, all companies, citizens and governments in the U.S. used a total of 77 quads of energy—that is, 77 quadrillion British thermal Units (Btu).(1)
Thirty-eight years later, the country’s annual consumption is 98 quads,(2) only 27% more than in 1973.
“Wait a minute,” you might ask, “our economy has expanded much more than that, right”? You would be right. Our economy expanded from $4.93 trillion to about $13.19 trillion. These figures are in 2000 dollars with the inflation adjusted out.(3) Yet, all of the energy that we use as Americans — living in houses, driving everywhere, producing goods and services, governing our nation, states, counties and cities — adds up to just 96 quads, just 27% more than almost 4 decades ago.
That means that we had a 267% increase in economic output, an increase that is radically more than the 27% energy growth. When you factor in our conversion from a medium manufacturing country in 1973 to a lighter manufacturing country today (manufacturing uses more energy than services) the energy equivalency needs to be adjusted downward. However, still, our improvement in energy consumed per dollar of economic output since 1973 is undeniably impressive.
This is illustrated by the table below.
So how did we do that? How did we increase our economic activity with so little energy expansion? We did so by saving energy. Saving energy falls into two categories: energy conservation through cutbacks in the use of energy, and what I will call energy efficiency, through improving the way goods and services are produced. This article and the table above, address only energy efficiency.
Energy efficiency includes producing more services like delivering packages around the country for less energy. It also includes producing more goods for the same buck, like reducing the plastic and metal in a radio that performs the same function.
How Are YOU Saving Energy Through Energy Efficiency?
In all likelihood, you are contributing to this increased energy efficiency. You may not even know that you are buying something that has been manufactured in a way that has improved in efficiency.
Take the clothes you are wearing. Since 1973, that first year of increased energy awareness in the U.S., clothing has been dyed using more effective technologies, like using electrostatic adherence techniques. That has allowed manufacturers to use less dye, which means producing less dye and reducing all the energy that used to go into manufacturing. You may not have even known it.
On the other hand, if you have changed the type of light bulbs you use, you probably do know that compact florescent lights save about 75% of the energy that old-fashioned incandescent bulbs use. These CFLs have improved in recent years to give better lighting. For example, the U.S. Government Energy Star-rated CFLs now start out with the same amount of light almost the instant you turn them on, the amount of mercury has been reduced, the light spectrum has improved, and the annoying hum has been eliminated.
Even some power plants have contributed to our energy efficiency gains. These power plants have increased their thermal efficiency, which means that for every 100 units of heat they produce, they now convert more of that heat to electricity. That reduces the need to produce so much heat (raw energy production) and pump so much water to cool these plants, which uses a tremendous amount of energy.
With that in mind, below is a graphic of the energy efficiency categories that will be helping America reduce its energy use per dollar of economic activity, or per average item bought. This is a projection of what might happen between now and 2020. The point of presenting this is to show the vast array of efficiency techniques that we both have been using and are still improving upon.
The improvement in energy efficiency since 1973 has saved more energy than all the additional energy expansion since that year. This will continue on into the future, and negate the need for additional power plants and oil consumption for transportation and more.
Above table: McKinsey Report finds that U.S. could save $1.2 trillion through 2020, by investing $520 billion in improvements. Kate Galbraith, “McKinsey Report Cites $1.2 Trillion in Potential Savings from Energy Efficiency,” New York Times, July 29, 2009,
An earlier version of this article appeared in the April-June 2010 Sierra Club Rincon Group Newsletter.
Which cooling system is best for energy use? Which is best for water use? Which is best for reducing CO2 output of electrical plants?
For several years, a business columnist at the Arizona Daily Star regularly berated evaporative coolers as water wasters and outmoded technology. He said refrigeration was the way to go in the modern world. Many readers disagreed with him but they gave only qualitative arguments. We decided to see if we could find some quantitative data to compare the two systems. We put together our data on our own rooftop systems. One of us (Roy) has had only evaporative coolers since he came to Tucson in 1960. The second author (Russell) has a combined evap/air conditioner/heat pump unit.
Russell’s combo “piggyback” evap cooler/A/C Heatpump system Photo by Russell Lowes
Although evaporative coolers used to be the standard cooling device for Tucson homes, they are less common today, so a brief description of how they work is in order. You’ve probably noticed that even on a very hot summer day, when you come out of swimming pool you find yourself shivering. This is because it takes energy to evaporate water (or any liquid for that matter). This energy, called the latent heat of evaporation, comes from your body and cools it. The evap cooler uses the same principle. It is a box with a tank of water, pads of aspen fiber, corrugated paper, or composite (MasterCool), a pump to distribute water to wet the pads, and a blower fan to pull air in through the pads and force it into your house. The air is cooled as it flows through the pads by the evaporating water. On a hot, dry summer day, this method of cooling is very effective; however, because less water evaporates when the air is more humid, these coolers are admittedly not as effective during the humid monsoon season.
Also, as you probably know, Tucson’s water contains lots of dissolved minerals. These minerals precipitate out on the cooler pads eventually making them useless. To combat this problem, the more modern coolers have pumps that empty out the water tank every eight or twelve hours of operation, thereby purging the salty water. This is good for cooler pad life but uses more water. Because this latter type of cooler is more common today, we included the use of this pump in our experiment.
Refrigeration or “air conditioning” systems are based on the Joule-Thomson effect: a gas cools when it expands. For example, when you let air out of a tire, it is cool. Here a mechanical pump compresses a gas (usually Freon), which warms it. It then goes through a copper coil where air cools it until it condenses. The resulting liquid then flows through a small opening and expands, causing it to cool, and chill your house.
In the table above, we summarize the energy and water consumption of the two types of coolers. Since our electric bills are usually the first concern, we start there. Our data in column 2 are taken from a number of research papers. There is an amazing spread of water usage, almost a factor of ten, in usage for similar houses, so we have used mid-range values that would apply to Tucson. The $0.113/kWh (kilowatt hours)used in Column 3 for calculating the energy cost comes from dividing Roy’s last July bill of $42.91 by the 380 kWh used.
Next we determined the cost of the water used by the evap cooler. Tucson water has a lower rate ($1.39/ccf) for less than 15 ccf (hundred cubic feet – 748 gallons) and much more ($5.14/ccf) for over 15 ccf. We assumed that folks would use some amount of water that fell into the higher category, so estimated $3/ccf as a reasonable average. This results in the total cost for the two systems in Column 9.
The trickier part was figuring the total water usage, Columns 4, 6 and 9. It may come as a surprise, but air conditioning or heat pump refrigeration is not a water-free process. Water—lots of it— is used in the generation of electricity. You may have noted clouds of steam coming from the cooling towers at power plants. Much of the cooling water is recycled, but even so about 0.5 gallon of water is used to generate one kWh of electrical energy at the Tucson Electric power plants.
Hydropower is even more water consumptive, as a huge amount of water evaporates from the reservoir behind the power dam. Lakes Meade and Powell lose almost a million acre feet per year and although some of this must be budgeted to irrigation, recreation, and flood control, at least 4 gallons/kWh could be attributed to hydropower. Nuclear power is even more water intensive than coal plants. Since we are on the Western Power Grid, it is difficult to say what fraction of our local power comes from which source. Once again, we used an average, and calculated 0.8gal/kWh as a reasonable estimate.
There are also indirect water consumption and environmental factors associated with electricity that must be taken into account. Electricity production uses water in the coal and uranium mining process. Extraction of water at these mines often devastates the local environment around the mines. Another area of environmental impact is that of CO2 production. We address this in the last column of the Table. Here you can see that the evap uses so much less electricity that the CO2 impact is 75% lower than refrigeration.
The Table reflects these assumptions on energy and water consumption. It also compares the total energy and water consumption for a typical home in the Southwestern deserts. Depending on the assumptions, the results are quite variable. For example, if you predict that the energy costs per kilowatt-hour in this area are going to increase, which many energy analysts project, then the evap cooler gains favor. If you plan to buy a super-efficient A/C, then this option gains favor. We did assume a high efficiency A/C, but there are even higher efficiency units becoming available.
There are also other factors not considered in this analysis. For example, some people do better, health-wise, with an evaporative cooler, while others do better with A/C. All air contains bacteria, mold and fungi. These microorganisms can even be beneficial for your health, but some people have problems with the very dry air an A/C produces, while others have problems with the moister air an evap produces. To most people it does not seem to make that much difference, except that in the driest conditions, many people say they like the moisture of the evap for their skin, hair and overall health.
Ultimately, the data seem to suggest that environmentally evaporative is the better choice, but using A/C during the most humid times, and using the evap the rest of the time is still a responsible option. Perhaps the most important lesson is not to use either unnecessarily – turn down the thermostat. That didn’t used to be an option for the old evaportative coolers—they were either on or off—with a high or low option. The modern evaps, however, offer affordable thermostats which pre-wet your pads, turn the system on and off like an A/C thermostat, and allow you to program the hours of startup and shutdown. These thermostats let you further reduce your water and energy consumption.
As for initial cost of system, and of repairs, refrigeration systems are much higher in cost than evaps. Evaps take more maintenance, but the routine maintenance is significantly lower in cost than the infrequent maintenance needs for refrigeration units.
What Can Homeownders Do to Reduce Energy and Water Consumption in Cooling Their Homes and Businesses?
Homeowners have several options if they want to reduce energy and water consumption and still cool their
homes during our hot summer months. If you are willing, like Roy, to weather the humidity, then the lowest cost option is the good ol’ evaporative cooler. If you aren’t quite that tough, you can do what Russell has done and install a “piggyback” unit, or cooler/heat-pump-A/C combo. This allows you to use the evaporative cooler during the drier months of April through June and September through October. It also allows you to use the evap during the drier parts of the days July through August. However, when the humidity increases and evap is no longer cooling efficiently, you can turn it off and the A/C on. If you do get a piggyback,
it is important to get a “barometric damper” which swings freely to open to whichever system you turn on. These allow you to not do anything but shut one system off and the other on. If you have a piggyback, you never want to run both systems at once (see picture of piggyback).
Home insulation is also important, especially with refrigeration. Some of the wide variations in experimental results for cooler energy use are no doubt due to the quality of the insulation of the house. Finally, note that in this article we are discussing retrofitting existing buildings. If you are building new, there are many ways to reduce your heating costs to nearly zero and greatly lower your refrigeration or evap consumption. But, that is another story—or at least another article!