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The Best Solar Water Heaters

An expert guide to the best solar water heaters on the US market in 2021-22.

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Carlos Huerta
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Access to hot water is surely one of the most necessary elements in any home. Traditional water heaters, or boilers, tend to be tanks of 30-50 gallons, powered by natural gas or non-renewable electricity. Tank water heaters heat water to somewhere in the region of 120 degrees Fahrenheit (50 degrees Celsius), and to do so requires huge amounts of energy. Water, once heated, is then sent from the water heater through copper piping to the rest of the home, in order to provide you with on-demand hot water.

Solar water heaters are the latest development in residential solar technology, drastically reducing our dependence on non-renewable grid electricity to heat our homes. Solar water heaters are specifically designed for use with solar panel setups, and are not only efficient, but cost effective, requiring nothing from you in energy bills once installation is complete.

In this article, we’ll take a look at a few of the very best solar water heaters on the market in 2021 and 2022. However, since buying these heaters tends to require an initial application for a quote, and since there are so many different combinations of solar water heater systems, each with their own unique applications, we’ll focus our attentions elsewhere. Namely, we’ll explain to you how solar water heaters work, what you should look for when you buy one, what pros and cons to expect, and finally, we’ll answer some of the most frequently asked questions regarding solar water heaters.

Solar water heaters installed on top of a public building. Source: Pixabay

How Do Solar Water Heaters Work?

Solar water heaters come in a wide variety of shapes and sizes, with an equally disparate range of operational mechanics. Whilst all solar water heaters (whether in domestic or commercial installations) comprise a collector and a storage tank, and all use the sun’s energy to heat water.

However, that’s where the similarities between models tends to stop. There are three different collector types, and four different types of circulation systems. Solar water heaters can be designed using any combination of these two elements. As such, it can be difficult to know what you’re looking at, or how it might work in your home.

Let’s take a look at collectors, circulation systems, and water tanks.

Solar Collectors

Solar collectors are, like solar panels, the units capable of converting sunlight into consumable energy. Unlike solar panels, they contain no photovoltaic cells, and do not convert sunlight into electricity. Instead, they use the natural thermal (heat) energy of sunlight to heat the building’s water. Sometimes they do so directly, other times they heat heat transfer fluid, which in turn transfers heat to the water.

There are three main types of solar collector: batch collectors, flat-plate collectors, and evacuated tube collectors.

Batch Collectors (AKA Integrated Collector-Storage, or ICS)

Batch collectors consist of dark tanks or tubes, contained within an insulated box. These types of collectors store water on the roof (or wherever they’re mounted) until the water is drawn. Made of especially conductive materials, the tanks or tubes transfer the sun’s warmth to the water, heating and storing it until it is needed.

These collectors can heat water to extremely high temperatures, and systems which use them tend to incorporate a tempering valve to avoid scalding temperatures at the tap. Batch collectors are incompatible with closed-loop circulation systems and, as such, are generally unfit for colder climates.

Diagram explaining the basic mechanics of a batch solar collector. Source: Wikimedia Commons

Flat-Plate Collectors

Flat-plate collectors look a lot like solar panels, and are mounted in the same way. They consist of copper tubing held between flat absorber plates (a little like a conventional household radiator). The flat-plate assembly is typically covered with tempered glass, and contained within an insulated box. Water or fluid passing through flat-plate collectors is heated by the sun, and then returned to a storage tank, or tankless water heater. Typically, flat-plate collectors can hold 40 gallons of water, enough to serve one person each.

Evacuated Tube Collectors

The most expensive, yet also the most efficient solar water heater collectors available today, evacuated tube collectors work, in principle, similarly to thermos mugs. A row of glass or metal tubes laid side by side is mounted at an angle similar to solar panels. These tubes contain water or heat transfer fluid, and are each encased within a larger glass tube. The space between the two tubes is a vacuum, ensuring that very little heat is lost from the liquid stored within.

Evacuated tube collectors work extremely well, even in overcast and cold conditions (operational at temperatures as low as -40 degrees Fahrenheit/Celsius). Maintenance is easy, since each individual tube can be replaced as necessary). They make the perfect solar water heaters for cold or overcast climates.

Circulation Systems

There are four main circulation system designs for solar water heaters: direct, closed-loop (AKA indirect), active (AKA forced-circulation), and passive. Each circulation system works in a unique way with each different solar water heater collector, and each is useful in different living situations and/or climates.

Direct System

In direct systems, water is heated directly by the sun as it passes through the solar collector. The hot water is then sent to a tank for storage, to a tankless water heater, or used immediately. Direct systems are best used in climates where the temperature rarely drops to freezing.

Closed-Loop or Indirect System

Closed-loop, or indirect, systems require freeze-resistant heat-transfer liquid to be stored in the solar collector. This heat-transfer liquid is heated in the collector by the sun’s energy, and is then passed through a heat exchanger located in the storage tank, thus transferring its collected heat to the water stored there. After the heat transfer is complete, the freeze-resistant fluid cycles back to the collector(s) for reheating. Closed-loop, or indirect, systems are preferable in colder climates.

Active or Forced-Circulation System

These types of systems utilize electric pumps, controllers and valves to actively move water from the collector to the storage tank. Active, or forced-circulation, systems are most common in the USA.

Passive System

A passive system does not, on the other hand, require any pumps or mechanics to move water from collector to storage tank. Natural convection moves water in these systems, created by the cyclical transfer of heat.

Water Tanks and Tankless Heaters

Last, but most certainly not least, it’s important to know how water storage tanks fit (or don’t fit) into this solar water heating equation.

Solar Water Tanks

Solar water tanks are tanks designed specifically to be compatible with solar collectors and arrays. They tend to be more insulated and modern than other, conventional water tanks, designed to minimise heat loss. Solar water tanks may also be built with any number of the following features, depending on the type of collector and/or system: heat exchanger coil, controller, thermometer.

Solar water tanks are often designed as hybrids, or used in conjunction with backup tanks ran on conventional energy (such as gas or grid-electricity). This is necessary as not all of the hot water required in domestic installations can always be heated by solar power alone. In hot climates, 80% of hot water required by domestic households can be heated by the sun (on average), whilst this percentage falls to 55-60% in colder climates.

Tankless Water Heaters

Today, there is not always even a need for a storage tank at all. Whilst systems utilizing tank storage heat water continuously throughout the day, storing it until it is drawn, tankless water heaters are designed to heat only the water needed, as and when it is called for.

An evacuated tube solar collector is the most efficient type of solar water heater. Source: Pixabay

How to Choose a Solar Water Heater

Having explained how each of the various components in a solar water heating system work, it’s time to look at the considerations you should make before you start shopping.

Climate

The climate in your area will have a massive impact on the type of solar water heater system you require (or can opt for). If you live in a hot climate which rarely (or never) freezes, then you can depend upon a batch collector, and perhaps even a passive system, to heat your home’s water. However, if you live in a colder climate where it does freeze regularly, or you experience overcast days throughout much of the year, you’ll need a different collector (such as flat-plate or evacuated tube) with freeze-resistant fluid. You’ll also need a different circulation system; most likely closed-loop (indirect) or active.

Tank or Tankless?

Tankless water heaters are technically more environmentally-friendly, since they only heat the water you need to use, rather than keeping a whole tank hot at all times. However, depending on your current setup (maybe you already have a new boiler which you wish to combine with a solar water heater, rather than replace), and your hot water demands, you may wish to stick with the more traditional tank setup.

Budget

And, of course, there’s always budget to consider. Evacuated tube collectors are, far and away, the most efficient and powerful solar water heater, but they are also twice as expensive per square foot when compared to flat-plate collectors. And both flat-plate and evacuated tube collectors tend to be more expensive than batch collectors.

Flat-plate solar water heater collectors are perhaps the most common and space efficient water collectors. Source: Wikimedia Commons

Best Solar Water Heaters Reviewed

Alright! You’ve read up on all the necessary components in the various solar water heater designs. You’ve considered the things you need to consider. It’s time to look at our two favorite solar water heaters on the market, and three other companies which we also recommend you check out (picking from their product list depending on your various needs, re: collector and circulation system type).

SolPak with Electric Backup: 80 Gallon Electric Solar Water Heater

Brand: Rheem

Features:

  • Rheem solar water heater tanks range from 65 to 120 gallons
  • This pack comes with the storage tank, collectors (flat-plate), a controller, adjustable speed pump & mixing valve
  • Collectors available in three sizes: 3ft x 8ft, 4ft x 8ft, and 4ft x 10ft
  • All collectors OG-100 certified by SRCC
  • Whole system OG-300 certified by SRCC
  • 6-year warranty
  • Back-up electric heating
  • Can be installed indoors or outdoors

Rheem is a well-known American brand based in Atlanta, GA, which has been developing water heaters for decades now. In fact, so dependable is Rheem, and so synonymous with American water heating are they, that you have most likely already lived in a building with a Rheem water heater. Rheem offer a huge range of solar water heaters, some hybrid (with gas or electric backup heating available, like this one), others in kits with solar collectors, and other standalone. A manufacturer you can trust, Rheem would be our first port-of-call when looking for a solar water heater.

Pros:

  • Wide range of capacities available
  • All heaters come with a 6-year limited warranty
  • This particular kit comes with almost everything you need to setup a fully independent solar water heater

Cons:

  • May require a lot of roof space for collector installation
  • Only flat-plate collectors available from Rheem
  • Rheem solar tanks only available up to 120 gallon capacity

Link: Rheem

Duda 300 Liter Solar Water Heater System

Brand: Duda

Features:

  • Duda Solar water tanks available in a capacity range of 100-1000 liters
  • Duda Solar 14mm heat pipe water collector (evacuated tube collector)
  • 45° mount
  • Food grade stainless steel solar water storage tank
  • SR961s Working Station (circulation pump, controller, pressure gauge, pressure relief valve, check valve)
  • Expansion tank
  • Automatic air vent (for purging air from top-most portion of the system)
  • Thermostatic mixing valve (for prevention of scalding)
  • Submersible water pump (for initial air purge/charging of fluid into system)
  • 1+-years warranty

Duda is well-known in the solar water heating industry, renowned for the quality of their products. They are based out of Alabama. Duda systems utilise evacuated tube collectors (the most efficient and dependable in the industry) to heat water on the rooftop, which they then store in large, specialized storage tanks. The solar collector comes with one rack of thirty 14mm solar tubes (less for smaller tanks, more for larger ones). These tubes are at a 45-degree angle to be mounted to a rooftop slant, but 37-degree angle mounts are also available. Due to the setup of the Duda system, it is perfect for a colder climate zone. It also comes with an 8-liter expansion tank to ensure that if there is an overflow due to too much water, there is backup to capture it.

Pros:

  • Capacity range is huge, with tanks from 100 liters to 1000 liters
  • Reputable US brand based in Alabama
  • Uses evacuated tube collectors, which are superior to all other collector designs
  • Comes with absolutely everything you need to establish a fully independent solar water heater system
  • Perfectly suited to colder climates, as evacuated tube collectors can work in overcast conditions and -40 degrees

Cons:

  • More expensive than most other brands
  • Warranty of 1-3 years not as impressive or assuring as those of other brands

Link: Duda

Further Recommended Solar Water Heater Manufacturers

As we’ve previously stated, there are so many different combinations of water heater collectors, storage tanks, heat transfer systems and circulation systems, that it would be impossible to list for you all of the best water heaters available. ‘Best’, in this case, is entirely dependent upon your own personal circumstances and requirements.

The two heaters we’ve detailed above are the finest examples of a hybrid flat-plate closed-loop solar water heater, and an active evacuated tube solar-only water heater. Below is a list of the three other companies we highly recommend you check out, if the kits offered by Duda and Rheem don’t meet your needs.

  • SunEarth offers a range of solar energy solutions for residential and commercial customers.
  • Sunbank Solar produces different parts for domestic solar water heating systems.
  • Apricus is also a very popular solar water heaters manufacturer.

Conclusion

Solar water heaters are complex things, delivering hot water to homes via a multitude of difference circulation systems, and via a range of different solar collectors. In hot climates, batch collectors work wonders, whilst in colder climates you’ll need either a flat-plate collector or an evacuated tube collector, and preferably with freeze-resistant fluid. Depending on your budget, your area’s weather, and whether you want a tank or tankless system, there are a range of considerations you need to make before buying. Hopefully, this buyer’s guide has helped you to make the most informed decision you possibly can. We highly recommend that you check out both Rheems’ and Duda’s product ranges before coming to a final purchase.

FAQs

Are solar water heaters cost effective?

There’s no doubting that solar water heaters are some of the more expensive solar products on the market. Nevertheless, they do, like all solar products, promise a healthy return-on-investment (ROI) over time. With most solar water heaters, ROI can be expected anywhere between 5 to 10 years post-purchase and installation. Ultimately, we think it’s worth it.

What happens when the sun is not shining?

Most solar water heaters have a backup system that is powered by electricity or natural gas, just in case you use more hot water than the solar collectors are able to generate on an overcast day. Still, some collectors (such as evacuated tube collectors) can operate just fine in overcast conditions. Alternatively, many solar water heater users have recorded changing their patterns of behavior to keep in-line with their water heater’s capabilities. For example, doing laundry during the day (when the sun is shining) rather than at night.

Is a solar water heater right for me?

Whether a solar heater is ideal for you or not depends on a few important factors: the amount of sunlight you receive, the amount of water you require daily, and your upfront budget for equipment and labor. Ultimately, we think a solar water heater is right for everyone, provided you choose a type of system and collector suited to the above factors.

Are there any federal tax benefits?

Yes! The US government’s Solar Investment Tax Credit program allows you to claim 26% of your entire solar water heater installation and materials cost in tax credits. The ITC will deliver this percentage of tax credits on any solar installation started before the end of 2022. Come 2023, the percentage is set to drop to 22%, and as of 2024 the initiative will have ended for residential projects (and be limited to 10% tax credits for commercial installations).

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Author Bio

Electrical Engineer with background in solar PV designs for residential and commercial projects as well as power systems development. Fan of renewable energy topics and projects. Technical writer for papers, articles and research in related topics to sustainability and especially solar power.

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