Top 50 Most Efficient Solar Thermal Collectors on the Market

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  • Travis Thompson

    This is very misleading and there is no way to identify a single most effficient collector. One has to take into account the operating and ambient temperatures (among other factors) to determine efficiency. There are applications when an unglazed plastic collector would be more efficient than any of the collectors listed on this chart.

    • Brian Hayden


      That’s the same feedback we got from a lot of people when we first started talking about this sort of ranking. Any sort of ‘ranking’ will always come with some debate. The reason we felt good about putting this out is because SRCC data is so widely accepted as the standard within the solar thermal industry. Here’s a quote from the SRCC website about how to interpret these ratings:

      The information in the directory will provide you with reliable and comparable data for solar water heating collectors you may be considering buying. The rating information is a helpful tool for comparing the efficiency of the various solar collectors on the market. While you can, and should, compare collector ratings, you cannot compare collector ratings with system ratings. All collectors which have been certified by SRCC will bear the SRCC label, which is your assurance that an independent party has verified the performance and basic durability of the solar product you are considering. Copies of SRCC labels are shown in the directory.

      The directory contains descriptive information about the solar collectors and also “performance” information about them. “Performance” data relates to the energy output of the collector. The SRCC performance information contained in this directory provides a way to compare the relative performance of different solar water heating collectors, not the actual performance you can expect from a given collector. This is because the collectors and systems are tested under standard laboratory conditions which are certain to be different from those in your home. Think of the SRCC ratings as you do the MPG ratings for cars — a benchmark, but not necessarily the same performance you will experience. Remember, too, that performance (or energy output) is only one cri­teria in choosing a solar energy collector. Quality of installation, cost, availability of service and parts, and the expected life of the equipment are also important points to consider. Equipment which is well-designed and well-built, but poorly installed, cannot perform according to the manufacturer’s specifications.

      • Steve Pitney

        Good Answer, Brian. It takes knowing the client’s needs and locale and matching that with the equipment.

  • Tim Gulden

    Is there a way to add another column to distinguish the vacuum tube from the flat plate collectors? Maybe another column showing collector retail cost per square foot? Also, what Category (Ti-Ta) did you use? Thanks Tim

    • Brian Hayden


      Great questions! #21 and #24 are vacuum tubes and the rest are flat plate. I’d love to be able to overlay the cost data, but we’ve always had trouble getting an apples-to-apples comparison of retail costs. It’s hard to come by that information and it’s almost never comprehensive. Not sure about the Category question – can you elaborate?

      • Jim

        Category question…he did answer (t1-ta)

        “not sure how valid information is when the first sentence has a grammatical error. “Who sell the best equipment?”

        • Brian Hayden

          Jim – thanks for pointing out that spelling error. It’s always a little embarrassing when things like that slip through the cracks.

  • Steve Pitney

    I’m with Travis on this one. What temperature ranges are we looking at? The SRCC has 5 operating conditions denoted A – E. A is for applications like pool heating, where an unglazed collector can operate above 90% if the ambient temp is near or above the fluid temp in the collector. B and C cover most solar hot water applications, summer and winter respectively, simplistically speaking. D & E are generally for space heating and air conditioning, for which you might use a vacuum tube collector. What might be more useful is to rank registered HW systems, since that is the market. This can still vary. I sell a different system on Cape than farther north, because of the climate difference.

    • Brian Hayden


      We agree – the SRCC data is taken in a lab, and those conditions almost never reflect what’s happening in the field. Installers need to understand how these collectors will operate in the field and select equipment based on the design for that specific site. Using these rankings to select equipment would be naive and we don’t recommend it. Thanks for pointing this point out – it’s really important.

  • Glenn Koenig

    My evacuated tube collectors are completely covered with snow at the moment, even though the entire day is listed on the weather report as bright sun (just confirmed by looking out my window). So my efficiency is zero and will very likely be zero tomorrow and zero the next day (also supposed to be full or mostly sunny days). This is the third winter, by now. Where are the helpful articles about what to do to get the snow off there without damaging the collector array? My roof is easily 30 feet above ground (a classic 2-family ‘box’ here in Eastern Massachusetts), so extension ladders are just too dangerous to put up (there is a lot of ice on the ground, of course, so not enough footing). We spent over $30,000 for the installation of the entire system. Yikes! That’s what I get for being an early adopter, I guess. I’m just paying the gas bill now.

    • Brian Hayden

      Hi Glenn,

      This reminds me of a great story that Bob Ramlow tells in our Solar Thermal Boot Camp. Bob worked in technical support for ‘Real Goods’ in the 80s. When he started in the role, they sent him a big file cabinet with more than 700 problem systems to fix all around the country. He tackled them one by one and kept some good data on what the problems were. The #1 most common problem was exactly what you’re describing – poor snow melt on evacuated tube systems in northern climates. I know what you’re looking for is a fix, so this story doesn’t really help. It really just underscores the problem and how common it is.

      Troubleshooting this stuff is outside my area of expertise (I don’t teach the courses), but lets see if anybody who reads this has a solution. I’ll reach out to some people and see what suggestions we can pull together. Thanks for sharing your story! Sorry to hear it’s not a happy one.

    • Eric Skiba


      Any collector that gets completely covered in snow is not going to produce any energy that day. You do have an advantage however that the gaps between the tubes will allow the wind to blow some of the snow off.

  • Eric Skiba


    I would have to agree with the above that publishing a list like this is very misleading. Which category did you use for the ratings? The efficiencies of these collectors under optimal conditions are one thing but a majority of the country spends most of their time outside of the SRCC Clear C rating. The best way to look at a collectors efficiency is to compare it’s operation over the entire range of conditions it will see throughout the year. For example one could argue that the best rating system for efficiency would be to take the SRCC data for the best day the collector will see and compare that to the worst day it will see.

    The second issue is that using gross area does not give a true representation of efficiency since aperature area is the portion of the collector that is doing the conversion.

    • Brian Hayden

      Eric – these are all great points, thanks for contributing to the conversation.

      Yes, these are based on Clear C ratings, which aren’t the conditions people will experience all that frequently. And your comment about net aperature area being more relevant than gross area was brought up by another practitioner that I really respect. So these are definite limitations to the usefulness of these rankings.

      Based on this comment we went back and ran the numbers based on net aperature area and it’s striking how much the rankings change. Almost all of the Top 50 on this list are glazed, flat-plate collectors and when you use aperature area it flips that around – the revised Top 50 would be almost all tubular collectors. Given the debate within the industry about flat-plate versus evacuated tubes, this is clearly an important distinction that manufacturers, suppliers, installers, and consumers should be aware of.

      Eric’s company, Apricus, has a nice little section on Collector Efficiency at their website that is worth checking out for anyone looking to go deeper on this topic.

      Eric (or any other readers) – if you’re open to expanding on this topic in a follow-up article in HeatSpring Magazine, please email me at Our mission is to promote access to good information, so we’d appreciate your further contributions to the conversation.

  • Richard

    The only meaninful solution to comparing different untis is to utilize a standardized testing facility so that each unit is examined and tested under the exact same conditions. This is not that hard to do. I have heard that St. Cloud, MN is amoung the top 10-15 cities for sunny days. Does that mean that a thermal unit is as efficient there on a sunny day in January as the same unit in Miami. A heat pump provides economical heat when in used in the south but not when in St. Cloud. Again if you want to compare solar thermal equipment then they must all be tested under the same conditions and this would then provide relative and meaningful numbers for comparison.