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Solar Hot Water Is NOT the Solution!

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Two friends and colleagues of mine (Carl McDaniel and David Borton) recently worked together to figure out why a solar water heating system required so much electricity to provide the modest amount of hot water used in one of their homes. After conducting a careful study that involved the solar water heating system’s installer, it was concluded that using just an on-demand water heating system was far more cost effective.

The installation and study were done at McDaniel's home (Trail Magic) in Oberlin, OH (pictured to left) with a climate very similar to ours (Both in Zone 5A per 2009 IECC, 6497 HDD vs. 6680 HDD for Albany, NY, and percent of sunshine 49% vs. 53%). Electric costs are more varied, with electricity costing $0.10/kWh in Oberlin and about $0.165/kWh in our area. While the authors were very careful not to make general assertions, it seems to me that we should consider this information carefully before investing in solar domestic hot water in our Region. Well insulated tank

Below in Resources, you can download a full copy of the study, “Evacuated Tube Solar Hot Water Systems Are Not Energy Efficient or Cost Effective for Domestic Hot Water Heating in the Northeastern U.S. Climate.”

Here is a summary of the article:

Domestic hot water was preheated with an AP-30 evacuated tube system and preheated water was stored in 80 gallon tank before being warmed to 111˚ F, if needed, by a Bosch RP17PT on demand electric heater. Over a 6 month period the following were measured: 1) incoming water temperature, 2) the kWh to pump fluids in evacuated tube system, 3) the kWh to heat water by on demand heater, and 4) the gallons of hot water used. These data were analyzed to establish that sunshine provided ~8% of the energy to heat the hot water used annually.

Less than 1% of the annual solar heat energy harvested by evacuated tube system was in hot water used: ~9,000,000 BTUs harvested; ~81,000 BTUs used. This 2 person household annually uses ~3,000 gallons hot water while the average 2 person-household uses 9,000 gallons. Even at this higher hot water consumption only a small fraction of the harvested solar BTUs would be used.

The evacuated tube system cost, $6,589; the on demand heater cost, $499, not including cost of wiring. With the evacuated tube system saving 30 kWh or ~$3.00/year, it was a very poor in-vestment economically and equally ineffective for reducing carbon emissions.

The evacuated tube system was removed after two years. Data collected over a 3 month period indicated that heating water to 111˚ F with the on demand heater used 0.12 kWh/gallon. With an annual use of 3,000 gallons, the electrical energy required to warm water with the on demand heater is estimated to be 360 kWh annually or ~$36.00.

Tubes being removedHoward Stoner is a retired math professor, gardener and lives in a home retrofitted for energy efficiency. He is interested in doing things by hand, the use and care of hand tools: crosscut saws, harvesting scythes, grain threshers, grinders, rollers, etc.

Editors Note: The authors are careful not to generalize the specific study to all installations; however, even so this is contrary to my way of thinking. If the article had not be written by two well respected scientists I would not have published it, without much further investigation. We need to make good decisions, based on science and specific reasons. The McDaniel-Borton study is a step in that direction. Please comment and provide your input on this interesting and important subject. Dan Gibson. 


PDF Document 290 KB:   Evacuated Tube Study

Comments on "Solar Hot Water Is NOT the Solution!"

  1. OEIC default avatar smorton October 24, 2011 at 6:03 pm

    Coincidentally, I installed (West Sand Lake, NY) a 2 panel solar hot water system back in the early 1980’s that in spite of my best efforts I considered a failure. I want to provide some details, so I posted it in the Renewable Energy Forum - Solar Domestic Hot Water. http://www.oeic.us/community_forums/viewthread/9/

  2. OEIC default avatar pcpc21 October 25, 2011 at 10:51 pm

    Given the annual usage of 360 kWh to heat water, the most you could save would be $36, which has a very long pay back period. I estimate my hot water usage to be 4220 kWh based on 12 therms of natural gas usage in the hottest summer month.  At an average cost of $1.59/therm, my potential savings is $228/year.  A 10 year pay back time requires the system to cost $2280 or less.  All this assumes that there is enough heat collected at the right times to offset all natural gas heating of hot water.  From the original post and smorton’s story, it seems like that’s not the case.

  3. OEIC default avatar Bill Lasher October 27, 2011 at 4:27 pm

    I use about 2/3 of the electricty produced by my 4 yearl old PV system.  I researched solar hot water options because my hot water was heated by an oil-fired furnace year round.  To use more of my surplus electricty and save fuel oil, I installed a “hybrid” electric hot water heater and since December, 2010 have used it only in the eheat pump mode.  I still send surplus electricity going to the grid and the fuel oil saved should pay for the hybrid heater in two years.

  4. OEIC default avatar Michael Cellini October 31, 2011 at 10:07 am

    As a voluntary simplicity practitioner and someone who produces 100% of my energy (PV, solar thermal and growing trees for wood heat) I’d like to correct:

    1) A SHW system for 2 people in NY costs less than $3,000 not $6,500

    2) The AVERAGE 2-person HH will use over 14,000 gallons of hot water per year not 3,000.

    Folks (the authors) who VS or live a conscious lifestyle tend to forget we are a deep minority. In a Utopian society we would all be using 1/10 of the energy we use.

    Both David and Howard (authors) are friends of mine but they are not mainstream and their study is not geared to mainstream lifestyles.

    Solar thermal is a totally viable and economical solution to hot water needs.

    I ask that readers actually read the entire study and think about it. A SHW system lasts 30 years and produces FREE energy.

    Do you know what the rest of the world is doing? I’ve posted more to (http://allurasolar.com/?p=378) showing world usage for SHW.

  5. OEIC default avatar moagman October 31, 2011 at 10:19 am

    I believe coal is the least expensive way to heat a home and we should roll back fuel economy standards to 1950 or so. I think we should certainly advocate the use of electricity in greater qauntity because there are several lakes that have not been ruined by acid rain yet-Seriously?
    Solar water heating removes about 2 tons of carbon annually from our atmosphere. On that basis alone it deserves to be an option for customers who want to make a difference. We know carbon once combusted never leaves our atmosphere and interferes with long wave radiation getting back out to space. The less fossil fuels we consume the better. I am sure the oil & coal lobby would love to get a copy of this article. I think we should promote sustainable fuel choices—My 2 cents….

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Ratings for this Entry:

  1. Howard Stoner's avatar smorton October 24, 2011 at 6:03 pm


    Howard, thank you for sharing this article. I feel a lot of people invest in technology without a good understanding of what the real benefits will be. We need more real data. Thanks.

  1. Howard Stoner's avatar jjhixon October 23, 2011 at 9:16 pm


    Very interesting. I admire the editor for including an article "contrary to [his] way of thinking."