How Solar Thermal Works          


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Flat Plate Collectors 
Flat Plate Solar CollectorFlat plate collectors have been the mainstay of solar heating for decades. They operate by using flow tubes to circulate water or antifreeze over a dark, insulated absorber plate enclosed in a glazed box.  

Direct systems circulate the actual water to be heated through the collector. Indirect systems circulate antifreeze or glycol through the collector and transfer the heat to the water with a heat exchanger. Indirect systems can operate year round in climates with freezing temperatures.

Passive solar collectors rely solely on the power of the sun to provide convective circulation.  Active collectors use pumps and controls to manage and regulate the flow of system fluids.  Kilowatt-hours used to power an active system need to be accounted for when determining net system output unless powered by a photo voltiac array.

Evacuated Tubes  
Direct Flow Evacuated Tube Solar CollectorEvacuated tubes take the flat plate concept and extend it to a higher level of efficiency.  They produce higher temperatures and can be used in both residential and commercial applications.

Inside the collector panel evacuated tubes are tied into a header pipe. Each tube contains a small copper pipe which is enclosed in a double walled glass tube.  The space between the glass tubes is evacuated to create a vacuum.  This vacuum serves as an insulation barrier, just like a Thermos bottle, minimizing heat loss and increasing efficiency.

The inner glass tube contains a black absorption layer to collect heat from the suns rays.  This heat is transferred to the fluid circulating inside the copper pipe.  Water is heated directly or antifreeze/glycol is heated and, in turn, heats water through a heat exchanger.


Since evacuated tubes are round they can capture more of the suns energy throughout the day than flat panels.  Flat panels face the sun directly at noon but are at some other angle of incidence during the rest of the day.  Round evacuated tubes expose the same amount of absorption area to the sun from early morning to late afternoon.

Solar Heating Sizing Guide

  • Domestic Hot Water: 10 tubes per person
  • Radiant Infloor Heating: 20 tubes per 200 sq. ft. area
  • Swimming Pool Heating: 2-4 tubes per 10 sq. ft. area at 30-50% Coverage
  • Hot Tub Heating: 50-75 tubes for up to 450 gallons
This is a general guide but there are several factors that affect the efficiency of a solar thermal system:
  • Insolation Level
  • Shade
  • Collector Orientation

These factors must be taken into consideration when designing the system. Our engineers are well trained to notice these points and design the system to compensate for these and other impediments that might hinder the performance of the system.

Efficiency of Evacuated Tube vs. Flat Plate Solar Collector

Low Temperature Panels 
Some solar heating applications do not need high temperature output.
Swimming pool solar heaters, for example, are designed to heat large volumes of water to no more than 90 degrees F.  They are simply  extruded, black polypropylene panels that absorb solar heat without glazing or metal piping.  Circulation is powered by the existing pool pump.

Integrated storage collector panels also limit output temperatures to less than 100 degrees F.  They are self-contained units that consist of a storage tank and glazed collector.  Output can also be used to heat swimming pools or send warm water to supplement the hot water heater. Since water is circulated directly through the collector and tank they must be drained during the months of freezing temperatures.

Measuring Kwh Output
Measuring the output of a solar thermal system can be done in one of two ways.  The first method is to determine the amount kilowatt-hours saved and the second is to measure the actual Btu's produced.   We can match  system results either way by knowing the following conversion formulas:


1 Btuh = 0.000293 Kilowatt-hour or

1 Kilowatt-hour = 3,413 Btuh

The idea with either approach is to measure the amount of electricity required to run the system without any solar thermal input.  Once you have a baseline established, activate the solar thermal system and log the energy baseline again.  Subtract the new baseline from the old baseline to determine the output of solar thermal system.

As a minimum, use at least two weeks of data to establish each baseline under similar weather conditions.  A month is even better.  Take the total energy used and divide by the number of days measurements were taken. This gives you an average daily consumption for determining the net difference.  

To verify your calculations multiply the daily net difference by 30 days and compare it with your electric bill.  Are your calculations reflected by the lower consumption on the bill?  

Apply the cost per kilowatt-hour to the net difference to determine your monthly savings.  Divide this into the cost of the solar thermal system to find out how many months it will take to recover your investment.

Measuring Btu Output
If your solar thermal system supplements a gas water heater or heats your pool, output will need to be measured in British Thermal Units or Btu's. You may recall that a Btu is the amount of heat required to raise one pound of water one degree Fahrenheit.  When expressed as Btu per hour, or Btuh, it becomes an energy unit of measure.  Btuh's can be converted to kilowatt-hours, or visa versa, as noted above.

BTU Measurement on Flat Panel Solar CollectorBtuh measurement must take into account initial temperature, final temperature, flow rate and specific heat of the liquid. Here is a diagram showing how this would be set up on a flat panel solar collector.

The initial temperature is that of the cold water entering the collector and final temperature is that of the water leaving the collector.  The flow rate through the collector is expressed is the gallons per minute and the specific heat of water is unity.  If a different fluid is used then its own specific heat would be used.

Mathematically this heat flow can be described as shown here.  First, convert the flow rate from GPM to Lbs/Hr:


Flow Rate (GPM) x 60 Min/Hr x 8.3 Lbs/Gal = Flow Rate (Lbs/Hr.)

Next, apply the change in temperature to the flow rate:

Flow Rate (Lbs/Hr.) x (Final Temp - Initial Temp) x Specific Heat =
Heat Flow (Btuh) 


For example, if our solar collector raised the temperature from 90 to 110 degrees F. and maintained a flow rate of 2 gallons per minute, how many Btuh would it produce?

  2 GPM x 60 Min/Hr x 8.3 Lbs/Gal = 996 Lbs/Hr

 996 Lbs/Hr x (110-90) x 1.0 = 19,920 Btuh 

What would be the equivalent savings on our electric bill?  Simply apply the conversion formula from above to our example:

                       19,920 Btuh   = 5.84 Kwh
3,413 Btuh/Kwh

Multiplying these kilowatt-hours times an electric rate of ten cents per kilowatt-hour would indicate the solar collector could save about 58 cents per hour when fully utilized.  However, this savings must be tempered with piping heat losses, outside air temperature, sun angle, cloud cover and the intermittent on/off cycling of the water heater element.

If you are interested in measuring the performance of your solar thermal collector,please see below for only  $100 to $125.  This meter captures inlet temperature, outlet temperature and flow rate.  Readings are taken manually and tallied in a spreadsheet for analysis.  Btu loggers are available that capture and record this information automatically but cost much more.


Build a Simple Btu Meter

A Btu meter simply measures the amount of heat a solar collector is adding to a fluid at a given point in time.  A Btu data logger, on the other hand, measures and records the amount of heat added over an extended period of time.

By taking measurements at different times of the day, under various weather conditions, a reasonable estimate of solar collector performance can be determined.  If you desire a more accurate picture of how your solar collector performs, with data you can analyze, a Btu data logger should be considered.  However, it will cost significantly more then the simple BTU meter we present here.

The information we seek with a simple Btu is the temperature of the water entering the collector, the temperature of the water leaving the collector and the flow rate of water through the collector.  Our goal is to fill in the blanks in the following equation:

Heat Flow (Btuh)=
Flow Rate (Lbs/Hr.) x (Final Temp - Initial Temp) x Specific Heat
Flow Rate in Lbs/Hr is converted from gallons per minute (GPM) as follows:

Flow Rate (GPM) x 60 Min/Hr x 8.3 Lbs/Gal = Flow Rate (Lbs/Hr.)

Flow Meters
T-Minol-130 Water MeterFlow rate in gallons per minute can be measured with a simple water meter.  Two acceptable products are the DLJ Single Jet Water Meter and the T-Minol-130 Water Meter.
The DLJ meter is available with 1/2" or 3/4" fittings while the Minol meter is only available in 3/4". Simple adapters can be used to fit it to 1/2" or 1" pipes.  

Although the basic DLJ meter is less expensive than the Minol meter, optional upgrades for the DLJ meter bring its final price in line with the Minol unit.

If you are considering a future upgrade to a Btu logger, plan to buy a flow meter with a pulse output.  The pulse output is standard on the Minol Meter but a $20 option on the DLJ meter.

It is best to locate the flow meter on the output (hot) side of the solar collector so any flow losses due to pressure drop through the flow tubes will be taken into account.  The hot water feature is another $20 option on the DLJ meter but comes standard on the Minol meter.

Temperature Sensors
Temperature Sensor InstallatiionThe most economical way to capture temperature is to buy a couple of indoor/outdoor digital temperature gauges at your local home center or you can order them on line.  Be sure the gauges have a wired remote sensor to collect outside air temperature. Prices are typically $10 to $15 each.

The remote temperature sensor is strapped directly to the side of a 4" to 6" copper or brass nipple using a hose clamp, tie wrap or black electrical tape. Since copper is an excellent conductor of heat, a probe into the fluid is not required. You may want to add a piece of pipe insulation around the assembly for more accurate readings.

This nipple assembly is spliced into the PVC plumbing on either side of the solar collector. Be sure to use female metal fittings when mating with PVC to prevent splitting. If you want to capture the heat loss of the piping between your solar collector and the point of use, locate the nipples close to your hot water heater.

If the temperature gauges can be located in a dry area near the solar collector, the indoor temperature reading can be used to give an ambient outside air temperature (OAT) while the outside sensor connects to one of the pipe nipple assemblies.  Tracking OAT data will give you a better idea of how your solar collector performs in different weather conditions.

Locate the flow meter and each of the temperature sensors in a common spot so you can take readings from all of them at the same time.  The flow meter will need to be mounted horizontally, face-up, to operate properly. If your temperature sensors are located outside, build a small enclosure that keeps them dry but does not insulate them from the ambient air.

Using the Btu Meter
Build a table for collecting Btu meter data with the following information:

# Date   Time   O.A.T. T-inlet T-outlet Flow Rate Remarks

Record date, time and temperature readings.  Monitor the flow meter closely counting the number of revolutions the small red indicator makes in a minute.  Each revolution corresponds to one tenth of a gallon.

Take three or four readings during each session to ensure the Btu meter is reading consistently.  If you record data each hour the solar collector is active, you will get a more realistic picture of total daily Btu output. Record data on sunny days, cloudy days, warm days and cool days noting weather conditions in the remarks column.

Transfer your figures to a spreadsheet and use the equations above to calculate Btu output.  Graph the output measured by day and compare performance across different weather conditions.  The area under the curve of the graph will be energy in Btu output per day.  

Average the daily readings using a pattern that reflects your typical weather conditions.  In other words, if you typically have 3 cloudy days and 4 sunny days per week, prorate accordingly.  Depending upon where you live, this will vary between wet and dry seasons of the year.

Multiply the average daily Btu's times the number of days in the month. Knowing the monthly Btu output of your solar collector you can determine equivalent savings on your electric bill through the following conversion:

1 Kilowatt-hour = 3,413 Btuh

or your gas bill:

1 Therm = 100,000 Btuh

Simply multiply the converted kilowatt-hours or therms by the current rate on your utility bill to find out your monthly savings.  Divide this into the cost of your system, including the cost of the Btu meter, and you will know how many months it will take to recover your investment.


You Tube Video Published on Mar 21, 2013

Hey it's 7 pm here in Canada on March 20 th, 2013 and it's  -30 c outside, but we had a sunny day here in Manitoba so it's 48 c in my hot water tank. Produced by my 30 tube solar evacuated tube system on my roof!

(Lorena Mitchell - President of Evolve Green)

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