Solar Water Heater

Solar Water Heater Performance Results

Imagine being able to see how well an already installed solar water heater is performing its task of heating water right at the system owners home. Well, imagine no more!

By connecting energy monitors and temperature gauges to solar water heaters in the field it's possible to chart how much energy the electrical backup (booster) element is using to heat water at a particular home, 24 hours a day.

It also enables you to measure the temperature of the water storage tank and/or solar collectors.

And why is it important to measure the temperature and/or booster elements power consumption?

Because they are significant performance indicators of how efficient a solar water heater system is. All other things being equal, the less the booster element needs to work, the better the solar hot water heater is performing using the sun.

And the temperature measurements inform you how well the collector is converting the suns energy into heat for storage in the tank and how well the tanks insulation is working.

Choose the Most Efficient System

So if you're thinking of installing your own solar water heater the efficiency information below, collected in the field from homeowners worldwide, will be invaluable in helping you decide which system to use.

These results will also help you work out accurately (instead of a guesstimate) what the monthly operating costs are and therefore how much you can potentially save in comparison to your current water heater if you install a solar one.

You can then also determine exactly what the payback period is based on an assessment of the expected monthly savings.

A big benefit for the homeowners doing the measurements is that they can adjust their water consumption patterns to help them save as much as possible by using the temperature and electricity usage feedback to achieve that.

So if you are already the owner of a solar water heater, you can make a planet saving difference by measuring the efficiency of your system and submitting the results here. Your review will allow you to enter and possibly win the prize in this competition...

Performance Results From Installed Systems Worldwide

South Africa / New Zealand / United States / Canada

South Africa

Ave Insolation: 5.20 kW/m2 per day

Performance Indicator: Electricity Used (kwh) and Tank Temperature

Owner Name: Juan Bruwer

Tank Size: 200 L (52 gal.)

Configuration: Flat Plate, Passive, Direct

Hot Water Demand: Medium, 3 People

Cost (2009): R16 912

Juan was measuring his total household electricity consumption using a pre-pay kwh meter before the solar water heater installation in October 2009.

He continued with measurements until April 2011 and came to the conclusion that the solar geyser saves, on average, 12 kwh per day.

The temperature results were obtained by taking a reading at 16h00 every day from the Geyserwise controller.

The high insolation figures help this system to achieve good efficiency levels.

More about Juan's flat plate solar geyser.

New Zealand

Ave Insolation: 3.81 kW/m2 per day

Performance Indicator: Electricity Used (kwh)

Owner Name: Duncan Watson

Tank Size: 270 L (71 gal.)

Collector Size: 3.13 m2 (33.69 ft2), 30 tubes

Configuration: Evac Tube, Active, Direct

Hot Water Demand: Morning, Medium, 1 Person

Duncan had his solar water heater installed in April 2007 and measured its performance for several years using a PicAxe Datalogger that he built and set up himself.

The monthly Kwh power usage of the backup heating element is measured which illustrates the efficiency of the system as a whole.

The design calls for two storage tanks, one pre-heating the water before it goes into the other, which is less efficient than if only one tank was used.

But the results are still good considering the climate the system operates in...

Duncan's evacuated tube solar water heater.

United States

Ave Insolation: 3.66 kW/m2 per day

Performance Indicator: Tank and Manifold Output Temperature

Owner Name: Patti & Jerry Jennings

Tank Size: 302 L (80 gal.)

Collector Size: 5.94 m2 (64 ft2), 30 tubes

Configuration: Flat Plate & Evacuated Tube, Active, Indirect

Hot Water Demand: 2 People

Have you ever wondered which of the two different solar collector designs, flat plate or evacuated tube, are more efficient?

Well, the installation of both types of collector on the same roof of Patti and Jerry's house in April 2007 goes some way to providing a definitive answer.

Both systems are connected to the web using a CWC Datalogger so you can see live readouts of the top of tank and manifold output temperatures compared side by side.

Looking at the results it seems the flat plate collector should be crowned the winner.

Live Flat plate vs evacuated tube results.

Ave Insolation: 4.84 kW/m2 per day

Performance Indicator: Tank Temperature

Owner Name: Jeffrey Beeman

Tank Size: 450 L (119 gal.)

Collector Size: 7.43 m2 (80 ft2)

Configuration: Flat Plate, Active, Indirect

Cost (2003): $8430 US

Jeffrey is very fond of tinkering on his solar water heater and has done a number of system performance tests since it was installed in 2003.

To measure the temperature in the tank, he used a National Instruments four-thermocouple/USB interface connected to a laptop computer.

Due to the design of the system, he was able to compare the effects of using a pump versus natural thermosyphon to circulate water from a heat exchanger into the water storage tank.

Interestingly, the pump provided more overall heat than the thermosiphon effect did...

Jeffrey's thermosyphon efficiency (0.3 MB pdf) test details.

Ave Insolation: 3.89 kW/m2 per day

Performance Indicator: Tank, Collector, Roof, Boiler Temperature

Owner Name: Jerold Heidtke

Tank Size: 300 L (80 gal.)

Collector Size: 5.94 m2 (64 ft2)

Configuration: Flat Plate, Active, Indirect

Live results really do provide an excellent insight into solar water heater performance, and these are particularly useful because they measure so many different things.

Such as the temperatures of the upper and lower areas of the tank, collector panel, roof, attic, solar heat exchanger and back up boiler.

Jerold's system consists of two collectors, a water storage tank with dual heat exchangers, a differential controller and a natural gas boiler providing the backup heating when the sun doesn't shine.

Here are the performance results as they happen in realtime.

Ave Insolation: 3.55 kW/m2 per day

Performance Indicator: Tank & Collector Temperature

Owner Name: Mike

Tank Size: 300 L (80 gal.)

Collector Size: 2 x 20 tubes

Configuration: Evacuated Tube, Active, Indirect

Mike had his solar water heater system installed in November 2010 and has been taking live temperature readings ever since...

Solar Collector Maximum: 186°F (85°C).

Solar Tank Top Maximum: 164°F (73°C).

Solar Tank Bottom Maximum: 156°F (69°C).

The system is configured so that when the collector is hotter than the tank, the collector pump turns on to circulate hot glycol fluid, which in turn warms the water through a heat exchanger.

Real time efficiency measurements of this system.


Ave Insolation: 3.40 kW/m2 per day

Performance Indicators: Temperature, Solar Radiation, Water Draw, Electrical Usage

Owner: Kingston Municipality Townhouse

Tank Size: 302 L (80 gal.)

Collector Size: 5.94 m2 (64 ft2)

Configuration: Flat Plate, Passive Bubble Pump, Indirect

The purpose of this study was to determine the efficiency of a bubble pump, which is designed to use natural forces to circulate fluid between a collector situated above a storage tank, where you would normally need an active pump.

The system was installed in 1992 and monitored from September 1993 to August 1994 by the Solar Calorimetry Laboratory in Canada.

Based on the data collected the average daily system efficiency was 13.4% and a solar fraction of 32.4%. These figures are based on the average daily hot water draws of 100 to 240 litres per day.

Detailed solar bubble pump (1.79 MB pdf) research results.

How To Interpret The Performance Results

Below each performance graph you will find some key information relating specifically to those results. Here's the explanation of what each one means...

    star Installer - The installer can make a huge difference to the performance outcome, for example if they don't find the optimal collector position it can cause a loss of efficiency of up to 30%. I've added a link to the installers website so that you can learn more about them.

    star System Model - Some solar water heating systems perform better than others by virtue of their manufacture and design so take note of the model name. You can also find out what that particular system costs.

    star Location, Location, Location - Solar hot water heater performance is influenced strongly by its location in the world due to something called solar insolation, which is just a fancy term to describe the amount of solar radiation that hits a given surface area in a certain time. So for the results to make sense in your case, you need to find measurements that are taking place as close as possible to where you live.

    star Insolation - The value of this figure is that you can compare it to the insolation or solar irradiation where you stay and roughly figure out how efficient a solar water heater will be at your home. The higher the amount the better the potential performance and the more you will save. Find your solar irradiation value here by entering your location and clicking on the red marker closest to that area.

    star Tank Size and System Installation Date - Find the tank size that's relevant to your situation. The rule of thumb is normally roughly 50 litres (13 gallons) capacity for each person in a household so if for example you are a family of four, look at the 200 litre (50 gallon) tank measurement results.

    star Homeowner - The person who owns the solar water heater and measures its performance.

    star Measurement Device - Smart meters and home energy monitors don't affect performance directly but they do help you manage your electrical consumption. Most power monitors can be counted on to be reasonably accurate, enough for comparison purposes anyway.

    star Configuration - Describes the type of solar water heater...

    • Solar Collector Type: Flat Plate, Evacuated Tube, Parabolic Dish or Batch.
    • Active or Passive: Active systems make use of pumps to circulate the water or heating fluid while passive systems don't use pumps.
    • Direct or Indirect: Direct systems circulate the water between the collector and the tank itself while indirect systems use a heat transfer fluid like glycol and a heat exchanger, the water in the tank never reaches the collector.

    star Water Consumption - The amount, the timing as well as the number of people consuming hot water in a household effects performance. Match the time when you and your family predominantly shower or bath such as morning, day, or evening. Calculate roughly the amount of water you consume according to the following scale (If you bath rather than shower assume medium consumption):

    Low Medium High
    Shower time per person averages less than 10 min Shower time per person averages from 10 - 14 min Shower time per person averages more than 14 min

    star Weather - You can find out what the average weather conditions for your area are at BBC Weather. This has a big effect on solar water heater efficiency so I've added a widget displaying the current temperature, cloud cover conditions and if it's day (sun) or night (moon) wherever the system is located. It's useful to see in real time the weather conditions that cause the backup electrical element to kick in.

Top 3 Solar Water Heaters

Find out which are the top three India solar water heater companies, South Africa 150L, 200L, 300L manufacturers, recommended solar water heater installers and the best solar hot water heaters in various other areas of the world, ranked by efficiency results.

Also take a look at these South African solar geyser reviews.