All About Solar > Evacuated Tubes or Flat Panels?

The solar heating collector's job is simple – it sits in the sun, absorbs the heat, and transfers it to where you need it. To do this efficiently, solar heating collectors need to absorb – and retain – a large volume of sunlight every day. There are two main technologies that have both been time tested and accomplish this, and as a result there has been a practically endless debate over which is considered “best”. These two main technologies are the Solar Flat Panels, and Solar Evacuated Tubes.

As an American manufacture of solar heating products, Solar Panels Plus has tested, designed, and supplies both flat panel and evacuated tube systems.

Flat Panel Collectors

Flat panel collectors have been on the market and in use since the early 1900's, and are one of the most time tested and well known technologies. They consist of an absorber panel – generally a painted metal, such as copper – attached to copper pipes where water or a heat transfer liquid passes through. This is encased in a metal frame, surrounded by thick insulation to help retain the collected heat, and protected by a sheet of glass or glazing, which also provides an insulating air space.

Evacuated Tube Collectors

Evacuated tube collectors are a slightly more recently developed technology, introduced to the market in the 1970's. There are several varieties of evacuated tubes, however the most commonly used type employs the use of a heat pipe surrounded by a glass tube that is under a vacuum. The glass tube actually consists of two walls of glass. In between the two walls, all the air is removed, resulting in a  vacuum in the same manner as a clear Thermos bottle would function. This vacuum is the best insulation one could ask for, and gives the evacuated tubes a much better heat retention than air space.

The heat pipe is also pressurized, allowing the liquid (usually water) to boil very rapidly, at a very low temperature (usually between 75F and 80F). As the water boils, it carries the collected heat to the top of the collector, where the heat is then collected by water or heat transfer liquid that flows around the top of the heat pipe, and then transferred to a storage tank or elsewhere in the system.

Evacuated Tubes vs. Flat Panels: Which is Best?


Cost is typically the primary consideration. Collector for collector, evacuated tubes can cost around 20% to 40% more to buy than flat panel collectors. However when comparing price one should consider cost per BTU capacity, and consider year round performance. In cool climates, evacuated tube collectors will have a lower cost per BTU.

Shipping costs can be more with flat panels than with evacuated tubes as well, especially when ordering a package system. Evacuated tubes are modular, and can be shipped vertically, maximizing the usable space on a pallet. It always takes 2-3 people to install a flat panel collector whereas a evacuated tube collector can be installed by one person.

Location is also an important consideration to cost. In some regions, it make take more or less of one type of collector vs the other to heat the same amount of water. For example in a cool climate, you could need 2 or 3 flat panel collectors to produce the same heat as 1 evacuated tube collector. In really cold (under 50F) weather, flat panel collectors collect little or no heat.


Generally, evacuated tubes perform better in colder and/or cloudier conditions than their flat panel counterparts. This is because of the vacuum in the glass tube, which allows tube collectors to retain a high percentage of collected heat. They work well in freezing conditions where flat panels will not work.

However, in areas where heavy snowfall can be an issue, evacuated tube collectors will not leak much heat from the collector, and therefore will not melt snow and heavy frost as quickly as a flat panel collectors. Evacuated tube collectors in cold climates can be installed at a higher angle to better face the sun, and this, along with a separation between the tubes, allows snow to slide off more easily. A flat panel collector, on the other hand, will collect some heat through the reflected sunlight off snow & ice, rising above freezing and therefore melt the snow or heavy frost much quicker, even though it may not be able to produce any hot water in cold conditions.

For customers needing really hot water, for example, laundromats, car wash, manufacturing process etc. note that flat panel collectors will not reliably perform above 130-140F. Evacuated tube collectors can produce hot water up to 200F.

Due to the self-tracking design of evacuated tube collectors, they collect heat fairly evenly throughout the day starting within minutes of sunrise. Flat panel collectors must collect nearly all of their heat in the middle part of the day.


Flat panels are typically designed with an unsealed enclosure. This can make them prone to condensation over time, which can result in corrosion. However, this largely does not impact the actual performance of a flat panel unless corrosion results, and is mainly a cosmetic downfall.

Flat panel collectors – if damaged, will continue to function, and can at times be repaired. Other times, the entire flat panel must be replaced.

Evacuated tubes, on the other hand, are sealed with a vacuum. This gives them their high heat retention properties, however, without this vacuum an evacuated tube collector performs very poorly. If a tube were to lose it's vacuum, it is generally very easy to correct, and can be done easily by simply replacing the tube.


Evacuated tubes are typically less sensitive to sun angle and orientation than their flat panel counterparts. Their circular design allows sunlight to pass at an optimal angle throughout the day – from morning to night.

Flat panel collectors are more sensitive to sun angle, and may require the use of racking systems, or other elevations to maximize their production.

When considering which technology to use, consult your local dealer, or contact us directly. We will be glad to look at both technologies and see which is the best fit for your specific application.


Flat panel collectors are best for users in southern climates or for northern seasonal homes only used during the summer. Evacuated tube collectors are best for areas where winter temperatures frequently drop in the 40F range or below. Customers  needing hot water at higher temperatures in all climates should consider evacuated tube collectors.