Solar Collector Efficiency Calculator

Solar heating collectors are offered in several types, including unglazed plastic collectors, traditional glazed flat plate collectors, and evacuated tube collectors. 


The efficiency of these collectors vary depending on the solar radiation, outside temperature, and collector fluid temperature.  This simple calculator will give you an idea of the efficiency and output of a collector for the particular conditions you want to use it in.  It may be able to help you to make a decision on which type of collector is best for your situation, or to change your system design for better efficiency.


This calculator uses performance data from the SRCC collector certification testing to calculate efficiency and output.  The SRCC is an independent test organization that certifies and measures the performance of collectors that manufactures submit for testing.  The SRCC publishes performance data for all the the collectors it has certified here ...   Since the SRCC tests all collectors in the same way their data are a good base for comparisons.

Of course, efficiency is not the only important characteristic of a collector.  Its durability and price are also of great importance.  A collector that is 5% more efficient, but 30% more expensive than another is not a good deal.  You can divide the price per square foot by the heat output that the calculator gives to get an idea of the best BTU for the buck.

 For a better understanding of how collector performance varies for the various types of collectors, and why, have a look here...
For some help with understanding how to use the material on the SRCC site, have a look here ...

The Calculator Inputs:

These are the numbers you need to enter to get an efficiency calculation:

Intercept -- This is efficiency value where the efficiency curve cross the vertical axis -- that is, when there is no difference between the collector temperature and ambient temperature.

Slope -- This is the downward slope of the efficiency curve.  It is the rate of drop in efficiency as outside temperature and solar radiation drop.

These two parameters define the efficiency curve for the collector.  You can get these from the SRCC certification report for each collector.  I have also provided these values for several common example collectors of each type in the calculator.  Just click on the one you want.

For more information on the efficiency curve, what it depends on, and how its determined, look here...

Ambient air temperature -- This is just the temperature of the air around the collector -- i.e. the outside air temperature.

Collector Temperature -- This is the average temperature of the fluid going through the collector.  This would ideally be the average of the collector inlet and outlet temperatures, but a good approximation would be to use what you believe the storage tank temperature will be plus about 5F.  You might want to do the calculation for several storage tank temperatures, and see what the effect is on efficiency.  The difference between the collector temperature and the ambient temperature is a major factor in determining the heat lost from the collector to the outside, and therefore the efficiency of the collector.

Solar Intensity -- This is the intensity of the solar radiation striking the collector measured in BTU per square foot of collector area per hour. 
A typical sunny day  value with the collector pointed at the sun would be about 300 BTU/ft^2-hr.   Cloudy weather values vary considerably, but the SRCC uses about 75% of the full sun value for "Mildly Cloudy", and about 50% of the full sun value for "Cloudy Day". 
During the early morning and late afternoon, the solar intensity that the collector sees is less, because of the angle that the sun makes with the collector face.  For example, if the sun makes a 45 degree angle with the collector face, the effective solar intensity is cut by 30% from the full sun value.

The Calculator Outputs:

After filling in the inputs, click the "Calculate" button, and calculator will calculate the two output values.

Efficiency -- The efficiency of the collector.  That is, (Collector Heat Output / Total Solar Input).  The efficiency is calculated for the specific conditions you entered.  You will find that the efficiency varies greatly for different conditions.  The idea of the calculator is to give a feel for how the efficiency and output vary as the collector and the system design are changed.

Output -- This is the heat output of the collector per square foot of collector area per hour.  This is the the output for the specific conditions you entered.

Note that this is not at all rocket science -- the actual calculation of efficiency is one very short line -- this calculator is 99% fluff and 1% calculation.  But, I feel that a lot of people struggle with deciding which collector is going to do best under a particular set of circumstances, and what effect a change in the system design might have on efficiency.  My hope is that this calculator helps a little in making a good decision.

Please read this...

If you have any comments, corrections, or suggestions on the calculator, please email me: Gary

Enter Values Below
Efficiency curve intercept no units I will input the efficiency curve Intercept and Slope values
Fafco Sunsaver unglazed, plastic pool collector |0.8210, -2.7271
Suntrek Sun Trek unglazed, plastic pool collector |0.8600, -3.1154
Heliodyne Gobi Flat Plate, single glazed, copper, selective absorber |0.737, -0.805
Heliodyne Gobi Flat Plate, single glazed, copper, black paint absorber |0.726, -1.071
Stiebel Eltron Flat Plate, single glazed, copper, selective absorber |0.660, -0.755
Sun Earth Imperial Flat Plate, single glazed, copper, selective absorber |0.714, -0.727
Apricus AP-30 Evacuated Tube, evacuated glass tube, copper selective absorber |0.418, -0.206
Seido 1-16 Evacuated Tube, evacuated glass tube, copper selective absorber |0.529, -0.299
Solamax AST Evacuated Tube, evacuated glass tube, copper selective absorber |0.574, -0.537
Sunsiaray air collector, black nickel selective absorber |0.508, -0.853
Efficiency curve slope BTU/hr-ft^2-F
Ambient air temperature degrees F The outside air temperature around the collector
Average collector temperature degrees F The average fluid temperature within the collector
(storage tank temperature +5F is usually about right)
Sun Intensity BTU/ft^2 -hr The intensity of the sun (in SI,  317 BTU/sqft-hr = 1000 watt/m^2 )
Click Calculate button to update efficiency calculation
Collector Efficiency % The efficiency of the collector for the temperatures, sun intensity, intercept, and slope  you input.
Collector Output BTU/sqft-hr This is the collector heat output per sqft of area per hour for steady operation with the temperatures and sun intensity you input (to convert to watts/M^2, multiply by 3.15)