There are ten performance factors that influence a commercial and homemade solar water heaters ability to provide hot water efficiently and cost effectively (see what they are below). I kept them all in mind when I designed and built my own DIY solar geyser, and was able to obtain performance that compares well to commercial systems at a reduced cost.

Here's how I did it...

My Homemade Thermosyphon Solar Geyser

The Flat Plate Solar Collector

I designed and built a traditional flat plate solar collector with an aluminium absorber but it has two pretty significant differences...

The first is that it contains a 1700 mm (66.9 inch) long, 90 mm (3.5 inch) diameter copper tank instead of the more conventional grid design with small bore copper pipes comprised of risers and top and bottom manifolds.

The second is that the collector has internal air circulation inside the box itself by using double glazing, thereby minimizing convection and conduction loses from the lower lying, polycarbonate encased copper tank and aluminium absorber.

Solar Water Heater Tank

The tank is vented to the atmosphere (non-pressurised) and constructed from stainless steel with a capacity of 150 litres (39.6 gallons).

It's a lot cheaper to manufacture than a pressurised tank and because it's designed to stand vertically it stratifies hot water more efficiently than systems that use a horizontal tank configuration.

It stands outside exposed to the elements so I designed an EPDM tank cover for it to weatherproof the insulation that should last for 12 - 15 years.

The solar tank insulation is 100 mm thick which helps to keep the temperature of the water inside from dropping by more than 2 degrees centigrade overnight.

Ten Solar Water Heater Performance Factors

The factors are based on the efficiency results obtained from in the field measurements of already installed homemade solar water heater and commercial solar water heater systems worldwide.

These characteristics will help if you are planning to build your own homemade solar water heater system, but also if you are buying a commercially designed one, because you will know what to look out for to choose the most efficient setup.

1. The Size of the Storage Tank - Bigger is not always better, but in this case it certainly is. A large hot water storage tank keeps its contents warmer for a longer period of time due to a property called thermal inertia.

   So the bigger the tank is the more you will have in reserve to cover several cloudy days and incoming cold water will have a lower overall effect when you draw off hot water.

   This is a big advantage that a homemade solar water heater has over a commercial system because you can upsize the tank without an astronomical increase in cost.

   Commercial tanks are roughly sized by using the formula of 50 litres (13 gallons) per day capacity for each person in a household plus another 50 litres (13 gallons) for general household use but for a diy system you can safely and cost effectively double that estimation.

2. The Surface Area of the Solar Collectors - This factor is pretty obvious as the bigger the collector area the more water you will be exposing to the heat of the sun.

   Here again, the home made solar water heater builder has an advantage over commercial units because you can increase the size with only a small increase in material cost and effort.

   The size of the storage tank will determine the size of the collectors with the rule of thumb in moderate climates being 1.5 gallons (5.6 litres) per square foot (0.09 m²).

   In hot climates increase the storage to 2 gallons (7.5 litres) per square foot (0.09 m²). If you don't get this ratio right, there's a possibility that the system will overheat during summer or not heat the water enough during winter.

3. Collector Materials Used - Traditional (commercial) solar collectors have copper tubing attached to copper heat absorbing fins because this provides the best thermal performance, but at a premium price.

   Homemade solar water heater collectors can be made at a much lower cost using copper tubes mated to aluminium fins and obtain almost the same performance.

   Pex tubing/aluminium fins cost about ¹/₆th the price of commercial ones with approximately 15% loss in performance (source: Builditsolar).

4. Tilt Angle and Orientation of Collectors - Generally speaking, homemade solar water heater collectors in the Southern hemisphere should face due north and in the Northern hemisphere, due south.

   The optimal tilt angle you mount the collectors at should be equal to the latitude of your location but the performance difference only becomes noticeable if the angle is more than 10% out.

5. Coil Heat Exchanger Material and Dimensions - Copper heat exchangers are the best but there comes that old bugbear again, high cost. To make things more affordable a pex/copper hybrid or pure pex heat exchanger is acceptable but they will be less efficient than the copper equivalent.

   The longer the coil, the more water can be heated at a time, so a 1 inch (2.54 cm) diameter 300 foot (91.44 meters) coil of pipe can hold 12 gallons (45.42 litres) of water (Source: Builditsolar) which will be pretty efficient because many hot water draws are less than that volume.

6. Insulation on Tanks and Piping - It's important to insulate properly because the less standby heat lost the longer the water will stay warm and the less the homemade solar water heater system has to work to heat it up again. It also helps to extend the time hot water will be available under cloudy conditions.
7. Hot Water Demand - The more hot water you use, the worse your system will perform due to the fact that it has to heat up a larger volume of incoming cold water to the desired temperature.

   So the more you consume, the higher your operational costs are going to be because the backup booster (electrical or gas) will come into play more often.

8. Site Obstructions - Your commercial and homemade solar hot water collectors should be situated so that they get as much exposure to the sun as possible and that means making sure that obstructions such as trees are taken into consideration before deciding on where to locate them.
9. Geographic Location - The closer you get to the equator, the more energy the sun will provide to heat water, due to the fact that the surface of the earth faces the sun directly there.

   As you travel north or south the curvature of the earth means that the same amount of the suns rays covers a larger surface area and therefore the energy supplied diminishes.

   The fancy term for this is called solar insolation which is a measure of the amount of solar energy received at a given surface area over a given time.

10. Climate - Nothing much you can do to alter the weather except move somewhere else but it will have a major performance impact on a commercial and homemade solar water heater due to temperature variations and clouds.

    NASA estimates that on average worldwide only 57% of the sunlight that strikes the upper atmosphere reaches the earth's surface due to cloud cover and pollution.

Top 3 Commercial Solar Water Heaters

These are the top three solar hot water systems manufactured and installed commercially in various areas of the world, ranked by efficiency results.