Solar heat collector efficiency

In the Plumbing and Heating industry, hydronic “hot water” boiler systems can be easily combined with solar thermal hydronic technology. We commonly find that by adding solar thermal collectors to a well-designed thermal hydronic system, we can easily eliminate at least half (and usually more) of the annual heating fuel consumption (depending on the building and the climate). This not only represents a substantial long-term savings in fuel cost, but also results in an even more substantial reduction in carbon dioxide emissions and other pollution so common in existing buildings. 

The two most common types of solar thermal collectors are flat plate and evacuated tube. The decision to use one or the other must include a fair comparison of thermal performance, often characterized by efficiency ratings. Here are two different ways of comparing collector performance first looking at efficiency and secondly looking at heat output.

Efficiency defined

Efficiency is really a simple relationship between the total energy available (the heating ‘fuel’) and the useful part of it that is put to good use. You just divide the ‘useful energy delivered’ by the ‘energy available’ and you get Efficiency expressed as a fraction or as a percent. It is often abbreviated using the Greek letter Nu (Nv). 

The Thermal Efficiency of a Solar Heat Collector is not static. It changes as the operational conditions change. This can make a fair comparison of one collector to another rather difficult, since panels come in different sizes, are made of different materials, and can be used in innumerable different climates and temperature applications.  Clearly there is a need for a standard way of testing and comparing solar heat collectors, and in the United States, that standard is maintained by the Solar Rating and Certification Corporation (SRCC). 


The SRCC provides our most widely used national solar heating test standards. It was founded in 1980 as a non-profit organization whose primary purpose is development and implementation of certification programs and national rating standards for solar energy equipment. They administer a certification, rating and labeling program for solar collectors and a similar program for complete solar water heating systems. The rating and labeling has become more important to installers and owners in recent years, since this is required for the solar equipment to qualify for the government-provided solar credits in the U.S. That is why nearly every solar heat collector sold in the U.S. these days has an SRCC performance certification label attached to it.

The labels themselves can be useful when making an energy performance comparison, since they show a standard energy performance rating similar in concept to those found on refrigerators and automobiles. The SRCC database is the one place where all these ratings can be found side by side for an easy and useful comparison. This information is free on the SRCC web site at   

Solar collector efficiency

Efficiency, as stated above, is calculated by dividing the “Useful Energy Out” by the “Energy Available.” In the case of the solar heat collector, the Energy Available is the solar radiation that arrives at the collector aperture surface. This can change from moment to moment with passing clouds and other local conditions. The Useful Energy Out is the net thermal energy embodied in the hot fluid (liquid coolant) leaving the collector outlet pipe. A colder outdoor air temperature surrounding the collector tends to cause more immediate heat loss, so cold ambient temperatures can lower the useful energy delivered.
When this situation is boiled down mathematically, it turns out that there are only three things you need to know to evaluate the collector efficiency for any heating application:


  • How hot is the fluid you want to heat (Ti)?
  • How cold is it outdoors (Ta)?
  • How sunny is it (I)? 
  • So, the collector efficiency (η) is directly linked to these three values which can be combined as follows.
  • (Ti – Ta) / I   [This is also called the “Inlet Fluid Parameter” (p)] where
  • Ti is Inlet Fluid Temperature, 
  • Ta is Ambient Temperature, and 
  • I is Solar Radiation at the Collector Surface. [I is for solar Insolation.]

The SRCC provides collector test results which include the slope and the intercept data for each collector tested. The slope and the intercept allow you to draw a straight line on a graph that defines the Collector Efficiency for any conditions of (Ti – Ta) / I. I have done this in Figure 90-1 for three collectors that are listed in the SRCC ratings; a Flat Plate Glazed, a Flat Plate Unglazed and a Glass Vacuum Tube collector. (The intercept is the point where the data crosses the vertical axis and the slope is the, negative, ‘Rise over Run’ of the line as it tilts downhill to the right.)

Please note that this only describes the Collector Thermal Efficiency, which is the solar collector by itself. This is not to be confused with the System Thermal Efficiency, which is complicated by “parasitic” energy consumption from pumps and controls, heat loss from piping, heat exchanger efficiencies, heat storage losses, etc. For now, we are focused on comparing the collectors only.

The SRCC data includes not only the Slope and Intercept of the Collector Efficiency graph, but also the heat output of the collector under five different standard temperature conditions. These ratings represent solar heating jobs that range from very easy (low temperature pools) to very difficult (high temperature process heat) and are presented as Category A, B, C, D and E respectively. 


  • Category A-Pool Heating (Warm Climate) Ti-Ta=(-9)°F
  • Category B-Pool Heating (Cool Climate) Ti-Ta=9°F
  • Category C-Water Heating (Warm Climate) Ti-Ta=36°F
  • Category D-Water Heating (Cool Climate) Ti-Ta=90°F
  • Category E-Very Hot Water (Cold Climate) Ti-Ta=144°F

In Figure 90-1, you will notice I have added rectangular gray boxes on the graph that represent where four of the different solar/temperature Categories are located. The SRCC lists the solar availability in over 50 major US cities, and they all fit within each of the grey boxes on Figure 90-1. For example, if you have a Category C heating job, the collectors on this graph will perform to the left side of the Category C box in Albuquerque or Los Angeles and to the right side of the box in Seattle or Boston.

The examples shown in Figure 90-1 show an interesting result. For many of the common solar heating categories, the Flat Plate Glazed collector performs better than the Glass Vacuum Tube collector, with a higher Collector Efficiency for these models. (Both of these collectors are from the same manufacturer.) So, if the price of the vacuum tube collector is much higher than the flat plate of the same size, the higher cost may not be worth it unless you are to right side of Category D or into the Category E area, where the Vacuum Tube collector clearly dominates.

Temperature, efficiency and energy output

Solar heat collectors are only effective if they can produce a useful temperature to meet the needs of any attached heating job at any given moment during the daylight hours.  When operated at higher temperatures, the efficiency of a solar collector tends to drop.

In practical terms that means that the heat output (BTUs/Hour) of the collectors may drop and the energy savings drops with it, even when the solar temperature delivered may be remarkably hot. It is important to strike a balance between temperature and energy output when designing solar heating systems. It’s true that a, “happy collector is a cool collector.” 

So it is always preferable to design solar/hydronic heating systems so that they can operate effectively at lower temperatures whenever possible. This usually involves choosing heat exchangers and heat distribution methods that are compatible with lower hydronic supply fluid temperatures.

Solar collector heat output

Solar heat collectors are designed to raise the temperature of the incoming fluid whenever there is solar radiation available. Or, as I like to say, “When there is daylight, a collector collects.” A collector will respond to a rising input fluid temperature by raising the output temperature as well. This phenomenon has its limits, of course, which can be seen in Figure 90-2 where the Heat Output (in kBTU’s) is compared to Temperature (F).  

Efficiency graphs (like the one in Figure 90-1) are often used to illustrate how collectors perform, but in this graph I am using SRCC test data to show the BTU heat energy output from two different collectors, rather than the efficiency. This is a direct measurement of the potential fuel savings from a collector. And, fuel savings is the whole point behind collector installations.  

The graphs in Figure 90-2 show the heat output available from two different kinds of collectors based on the standard SRCC OG-100 test results. The collectors, taken for this example, are Viessmann Vitosol collectors, one Flat Plate and one Vacuum Tube, of similar aperture surface areas (~40 feet2). For simplicity, the graph in Figure 90-2 shows a single collector using Clear Day data and temperature-performance examples for a day when the average outdoor air temperature is just below freezing (30°F). Using the SRCC collector rating data, anyone can take the solar conditions of interest and plot them on a graph like this using just five data points (one from each Category rating).

The graph in Figure 90-2 shows how the Heat Output from a collector varies with temperature conditions. The temperature of interest is really the Temperature Difference calculated by subtracting the outdoor ambient air temperature from the inlet temperature entering the collector. The colder it is outdoors, the more heat is lost from a hot collector.  It is obvious that the bigger the temperature difference, the less heat is produced by the panel. A large temperature difference can be caused by extremely hot fluid entering the panel, or extremely cold outdoor air or both.


The collector performance graphs presented here demonstrate that it is a mistake to assume that one kind of collector is fundamentally better than another. When comparing thermal performance, the proper choice of solar collector depends upon the required operating temperature, the intensity of the solar radiation and the severity of the outdoor temperature. Once this has been evaluated, the final choice may depend on other factors beyond thermal performance. Cost, Reliability, Compatibility, Operation and Maintenance issues often prove to be equally important.

Final notes

These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement. 

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the Chief Technical Officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit

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