The SRCC rates solar air and water heating collectors. The rating includes a number of durability related tests as well as a test of the panel heat output and efficiency. Since the test is done the same way on every collector, its provides a good way to compare panels on an apples to apples basis. You can also compare different types of collectors -- for example, flat plate collectors compared to evacuated tube collectors.
This page includes a sample page from the SRCC Directory of SRCC Certified Solar Collector Ratings, along with some thoughts on how to use these rating sheets to compare the performance of collectors. The download below has these ratings for dozens of collectors, including plastic pool collectors, flat plate collectors and evacuated tube collectors. There are (unfortunately) very few solar air heating collectors.
These ratings take a lot of the supplier BS out of comparing collectors from different makers. You may be very surprised to see how some of these collectors compare.
Remember that these ratings give the heat output for collectors, but say nothing about the cost of the collector. If brand A's collector puts out a few more BTU per day than brand B's collector, but it costs twice as much as brand B, then brand B is still the better buy even though it puts out a bit less heat. The notes below tell you how to estimate the heat output per dollar spent, which is what most people are interested in.
You can download the full directory of SRCC ratings here:
The actual directory is this file:
Directory of SRCC Certified Solar Collector Ratings
The performance and characteristics of each collector is summarized on a one page report. The sample below is for a water heating collector.
The table in the top part of the page provides the heat output for the collector for the standard SRCC tests.
The left half of the table gives test results in metric units, and the right have in US units.
Since various collector designs perform differently under different sun conditions, the SRCC gives the heat output for clear sky, mildly cloudy and cloudy days. And, because collectors perform differently depending on how much temperature difference there is between the collector temperature and the outside ambient temperature, the heat output is given for various various types of service that require heating water or air to different temperatures.
Sun Condition Category:
Clear Day -- The panel receives 2000 BTU per sqft over the full day
Mildly Cloudy -- The panel receives 1500 BTU per sqft over the full day
Cloudy Day -- The panel receives 1000 BTU per sqft over the full day
Note that these definitions will not correspond exactly to your location. For example, a properly tilted panel in a location in the mid US might receive anywhere from 1600 to 2300 BTU per sqft on a clear day depending on the time of year.
The SRCC rates the panels for 5 kinds of service:
Category A - Pool heating in a warm climate -- Tinlet - Tambient = -9F
This service covers pool heaters in warm climates where typically the ambient temperature is actually warmer than the pool water going through the collector. These collectors are often unglazed and made from plastic or EPDM rubber.
Category B -- Pool Heating in a cool climate -- Tinlet - Tambient = +9F
Similar to category A, but with a lower ambient temperature.
Category C -- Water Heating in a warm climate -- Tinlet - Tambient = +36F
This service would apply to water heating in a warm climate.
This might be the most generally applicable category for water and space heating for people living in southern US climates.
Category D -- Water Heating in a cool climate -- Tinlet - Tambient = +90F
This service would apply to water heating in a cooler climate that Category C
This might be the most generally applicable category for water and space heating for people living in mid to northern US climates.
E -- Air conditioning or industrial uses -- Tinlet - Tambient = +144F
This is for applications where very high temperature output is needed.
This category does not apply to most consumer applications.
You will note that the panel output drops as you progress from category A to B to C... E. This is because each category requires the panel to put out hotter water relative to the outside temperature -- this increases the heat loss from the panel and decreases efficiency and output.
So, for most people, the right choice will be category A or B for pool heating, or category C or D for water heating and space heating.
The panel output is given in thousands of BTU for the whole day for the whole panel. For this panel, under Mildly Cloudy conditions, and in Service D (domestic water heating), the panel produces 13,000 BTU over a full sunny day.
When comparing panels of different sizes, its important to remember that the output is given for the full panel, so, if you want to compare panels on an equal basis, divide the panel output by the panel area -- so this panel has an output per sqft of (13000/32.25 sqft) = 403 BTU per square foot of collector area.
The bottom half of the SRCC rating page gives the physical description of the panels, and also provides the efficiency curve parameters.
So, for this panel, the gross area is 32.25 sqft, of which 29.83 sqft actually collects sun.
It has an aluminum frame, and is single glazed with tempered glass. The glass is "low iron" -- this makes it more transparent than common glass, which has iron in it. It is tempered to make it resistant to breaking under thermal and mechanical stress.
The absorber plate is made from copper tubes and fins, and the fins are coated with a Black Chrome selective coating (which makes it more efficient than black paint, but adds to the cost).
The back and sides of the panel are insulated with Polyisocyanurate insulation -- a high performance urethane based rigid insulating foam board.
You can compare two panels by first deciding on the service category (A trough E) that represents your use. Then look up the output for the three sun conditions for each panel. Then divide the panel outputs by the area of the panel. This will allow you to compare the two panels output per square foot for sunny to cloudy conditions.
You should then figure out the price per square foot of the panels you are comparing by dividing the panel price by the panel are.
This tells you which of the collectors produces the most heat for the least cost. There may, of course, be other factors that enter in, such as quality of construction, warrantees, ... but, this gives you a good place to start.
You may find in comparing two collectors that one does better in sunny conditions while the other does better under cloudy conditions. All things being equal, it is more important for a collector to do better under sunny conditions than under cloudy conditions, just because there is more heat to capture on sunny and partly sunny days than on cloudy days. The exception to this might be if your full usage season is very cloudy.
Here is a full example done by Alan on comparing several candidate collectors for a solar water heating application he is designing.
Gary 1/15/07, updated 5/24/07, updated 5/25/07