Solar Hot Water Design and Purchase Considerations
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Solar Hot Water Design and Purchase Considerations

How much potential sun energy do you have?
Long periods of cloudy weather severely limit the amount of energy you can collect.

Do you have a southern exposure (if you live in the Northern Hemisphere)?
To find south, use an analog clock and hold it horizontal, like a compass.
Point the hour hand towards the sun.
South is about half way between the hour hand and 12:00.

What about shading?
There is an iPhone/iPod Touch app called Suntracker that shows the path of the summer and winter sun over your horizon. While this app is geared for photovoltaic information, you can find out what percentage of the total possible sun exposure you will have. 100 percent sun would have no shading, meaning you would receive the maximum energy (BTU’s) from the sun.

You can also hire someone to do a shading survey.

The amount of shading will limit the time the solar hot water collector has to generate the required BTU’s.
The more shade you have, the more collector area you will need to gain the same amount of BTU’s over a shorter period of time.
50 percent shading will require twice the collector area as 0 percent shading.

What is a BTU?
A BTU is the energy required to raise one pound of water one degree Fahrenheit.
If you are planning to heat your domestic hot water with solar energy, you need to know how many gallons of hot water per day you use. This number can vary greatly. Do you have kids taking showers a lot? Do you do the laundry often? 

There are three people living at our house and we use anywhere from 10 to 100 gallons of hot water per day.

Lets use the example of 30 gallons per person per day.
Incoming water from the well or the city water supply is about 50 degrees f.
We want the water coming out to the tap at 120 degrees f.
That is a temperature difference of 70 degrees.
30 gallons of water weighs about 240 pounds (8lbs/gal).
To raise 240 pounds of water 70 degrees takes 16,800 BTU’s. 240 x 70 = 16,800.
What does this mean? It means that you have to generate 16,800 hot water BTU’s a day for each person in your house.

How cold does it get where you live?
If year round temperatures stay above freezing you can use:

  • Batch collector system
  • Open loop drain back system

If year round temperatures go below freezing you can use:

What do you expect the solar hot water energy to do?
Heat your domestic hot water.

  • Only on sunny days?
    You can use a small storage tank (120 gallons).

  • Year round?
    You Need a large storage tank (300 gallons and up) or multiple storage tanks.
Radiant space heating?
  • At best, it will be a supplemental system to your present radiant heating system.

What collector type should I use?
Flat plate collectors are least expensive and produce the lowest temperatures (max 140 F). They also loose some of their energy potential when the weather is cold or the wind is blowing, because they are not insulated from the surrounding air.

Evacuated tube collectors are the most expensive, produce more energy earlier and later in the day (due to the tubular design) and can produce the highest temperatures (over 200 F). They are fully insulated from the surrounding air, so they produce the same energy if the surrounding air is 30 degrees above zero or 30 degrees below zero. Each individual tube puts out approximately 120-200 BTU's per hour.

How are collectors rated?
Collectors are rated at BTU’s per day. I have yet to find out how a “day” is defined. Is it a cloudy winter (short) day in the Pacific Northwest, a scorching summer (long) day in Arizona, an Alaska summer day where the sun never sets? I can not find the answer.

Ratings are biased in favor of flat panel collectors, because the standards are based on the overall size of the collector and do not account for the gaps between the tubes in evacuated collectors.

Our system (60 evacuated tubes) is putting out 90,000 BTU’s (the actual amount of BTU’s reaching our hot water faucet) on a sunny day in March. This is over a 7 hour period, so the system averages 12,857 BTU’s/hour, which is exactly what is shown by the manufacturer for “Max Btu Output Per Day Based on 1000W/m2”. 1000W/m2 is the amount of the suns energy (1000 watts) reaching 1 square meter (m2) of the earth on a "full sun" day.


If you are hiring someone to install a solar hot water system, be sure the system includes an energy monitor, so the actual output of the system can be proven. And make sure the BTU’s are measured at the hot water tank, NOT on the collector loop.

What angle should the collector be?
At my latitude (45 degrees) the collectors should favor the winter sun.
Mine are at 67.5 degrees. Use your latitude, plus 22.5 degrees.


This maximizes the short day, winter energy and minimizes the long day summer energy. You can have the largest system size, without the fear of over heating in the summer months.

Our system puts out a maximum of 90,000 BTU's in the winter and 65,000 BTU's in the summer. We use the additional BTU's in the winter to heat our attached greenhouse, through a radiant heat system (also run by photovoltaics, not the grid).

67.5 degrees also sheds snow in the winter. There is nothing I hate worse than seeing solar collectors covered in snow for weeks on end. This is especially true of evacuated tube collectors, as their insulating qualities prevent the surface of the tubes from heating up and melting the snow.

What is my latitude?
Search for your town on Google Maps. Right click on the map at your location and select “What’s Here?” The “Search” field will then show your latitude (the first number in the field).

What are the hidden costs of a solar hot water system?
AC powered pumps and controls. Up to 25 percent of a solar collectors total energy savings can be lost in the cost of electricity to run the system.

Poorly insulated or non-insulated piping. I reduced my heat loss from the collector to the storage tank by 50 percent by insulating the pipes. This is true even in warm climates.

Poorly insulated storage tanks lose heat 24 hours a day, 7 days a week.

Our system uses a photovoltaic powered pumps and controls, so there is no electricity cost reducing the energy produced by the collector. Also, when the power goes out, we don't have to worry about stagnation (heat transfer fluid boiling in our collectors because the pumps can not run). Stagnation turns the heat transfer fluid into a corrosive agent.

Batch collector systems use thermosyphoning (natural movement of hot water upwards), so you do not need pumps or controllers.


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