Solar Water Heaters: A Beginners’ Guide

solar water heater

If you’re looking for an eco-conscious way to cut down on your electricity bill or get onto solar power—but aren’t ready to put solar panels on your roof yet—then a solar water heater might be a good alternative.

Compared to installing a full solar panel system, a solar water heater or a solar hot water system is less expensive, but you can still reap pretty significant benefits because a hot shower requires quite a bit of hot water. (And if you have an off-grid home and have been making do with cold or lukewarm showers, then this will probably be a more attractive option.)

According to the US Department of Energy’s Energy Saver, having a solar water heater can cut down your water heating bill by a whopping 50 to 80%. Not surprising, given that water heating is the second largest expense in your home, accounting for 14 to 18% of your utility bill.

That being said, the exact amount depends on a few different factors, mainly how much hot water you use and how much sunlight you get.

In this post, we’ll look at how a solar hot water system works, different types of solar water heaters (some are better than others depending on the climate of where you live), and how the best solar water heaters on the market stack up against each other:

First, let’s look at how a solar water heater works. This will help you decide which type of heater might work best for you.

How does a solar water heater work?

A solar water heater typically works in conjunction with a few other major parts to make up your solar hot water system: the collector, storage tank, heat exchanger, controller system, and backup heater.

The solar hot water collector

The “collector” name is a bit misleading, because these aren’t really storage containers or tanks—they’re more like solar panels, and they’re usually placed on a home’s roof.

Whereas the typical PV (photovoltaic) solar panels that most people think of produce electricity, these solar water heating collectors produce—you guessed it—heat.

How it works: Sunlight, or solar radiation, gets absorbed through the collector. It then hits an absorber plate, which has a coating that captures that sunlight and turns it into heat. This heat is transferred to a “transfer fluid” (like antifreeze) through a heat exchanger—which we’ll talk about next.

☀️ Pro-tip: This antifreeze or transfer fluid will not physically come into contact with the water that’s being heated—there will always be a barrier between them (usually via the pipes), so it won’t affect the drinkability of your hot water. (Unless your water source is not potable to begin with!)

The heat exchanger

Once the antifreeze or transfer fluid in your collectors heats up, it goes into a “heat exchanger,” which is essentially a bunch of pipes that are inside your hot water storage tank. Once these pipes are filled with your (now warm) transfer fluid, that heat is then transferred from the pipes to your water.

This is essentially the process that heats up your water—and now you’re ready to take a hot shower.

The storage tank

To hold all that water, you’ll need a storage tank (ideally with good insulation). These tanks range from 20 to over 100 gallons, depending on how much hot water you think you’ll need.

The controller system

Most solar hot water systems have a controller system that makes sure the water in the storage tank doesn’t get too hot and also to protect it from freezing.

The backup heater

“But wait! What if it’s a cloudy day?” 

Even in places like Arizona, you’ll get a few cloudy or rainy days a year.  (And what if you need hot water at night?)

That’s why your solar water heater should have a backup. This backup system usually runs on electricity or gas, and it’ll fill in for your solar water heater when there isn’t enough sunlight to heat up all the water you need.

☀️ Pro-tip: A tankless water heater is a good backup for your solar water heater since it doesn’t require hot water storage—it heats cold water as you need it via electric elements (or gas).

On the other hand, you could also have a more conventional storage water heater, but those aren’t as efficient and you can waste a lot of energy keeping the water in the tank hot.

Types of solar hot water systems

Direct vs. indirect

Okay, now that we know the main components of a solar hot water system, let’s look at a few different types of systems.

There are direct and indirect solar hot water systems, which are different in how your water is heated.

Direct: Water gets heated directly by the sun—no transfer liquid needed

Indirect: Solar energy is collected and held in a special antifreeze fluid (sometimes called a heat transfer liquid), which gets circulated through storage tank, heating the water

As you can imagine, having the antifreeze makes your solar water heater system more resilient against cold or freezing temperatures. So, if you live somewhere with harsh winters, you’ll probably want to go for the indirect solar water system.

Active vs. passive

And then there are active and passive solar hot water systems, which are different in the way they move hot water through the system.

Active: Uses pumps to move antifreeze fluid or hot water; more expensive, but more efficient

Passive: Uses gravity to move antifreeze fluid or hot water; cheaper, but less efficient

Being dependent on gravity makes things a little tricky. For example, if you’re using a passive solar water heating system, you might need to make sure your storage tank is physically above your collectors, which could complicate your roof installation design.

How to choose the best solar water heater for you

Here are three questions to ask to narrow down your options:

1. Do you live in a warm or cold climate?

If you live somewhere that regularly gets to freezing temperatures in the winter, then an active + indirect solar water heating system would be better because it uses pumps to circulate your hot water and more importantly, antifreeze fluid, which is immune to cold temperatures.

☀️ Pro-tip: Note that a warm or cold climate influences the type of solar water heater you get, but doesn’t have as much bearing on the question of how much sunlight your home gets. Some areas are very cold but get many clear, sunny days in a year. This is fine! It’s that sunlight that the collectors in your solar hot water system need—not a really high temperature, to work.

2. How much sunlight does your home get?

As we just mentioned, if you live somewhere that gets lots of sunny days, then a solar water heater will be a good investment. (Even if that weren’t the case though, we’d personally still want to see if a solar hot water system with a backup would help cut down on utility bills.)

Again, there’s always the option of having a backup electric hot water system that kicks in when you don’t get enough sunlight to heat your water. Just turn off the electric water heater when you don’t need it and turn it on when it’s a cloudy day (or at night).

3. How much hot water do you use during the day?

If you’re the type of person who likes taking hot showers at night, then you might want to adjust your habits slightly so that you can take advantage of solar water heating while the sun is out.

Knowing how much hot water you use will also help you choose a system with a tank that can hold enough gallons of hot water for you and your family.

☀️ Pro-tip: Typically, a small household of one or two people would be fine with a 30-gallon storage tank, going up to 60 to 80 gallons for a household of three or four.

Disclaimer: This article contains affiliate links through which we may receive a small commission at no cost to you, if you choose to purchase anything from a link on this page. That being said, these tools were curated from extensive research and consideration of reviews of the best options we’ve found.

What are some of the best solar water heaters?

1. Sunbank 40 Gallon Solar Water Heater: Good for mild temperatures

sunbank solar water heater

  • Gallons: 40 (151L)
  • Type: Passive (thermosyphon)
  • SRCC-certified / Eligible for 30% federal tax credit? Yes, OG-300 and OG-100
  • Pros: A good starter option that’s easy to set up if you live somewhere warm-ish

If you live somewhere warmer and don’t see a lot of freezing temperatures, then this Sunbank solar water heater is a good option because it’s a passive solar water heater and doesn’t require pumps or antifreeze fluid. (Which also makes installation a little easier if you’re doing it yourself.)

At 40 gallons, this solar water heater’s stainless steel tank is a fine size if you’re living solo or with a partner (but you might be pushing it with three).

The hot water system itself is SRCC OG-300 certified, while the collector is OG-100 certified, which qualifies you for tax credits and lets you save a decent chunk of change.

☀️ Pro-tip: OG-300 is a certification that applies to the entire solar water heater system, whereas OG-100 only covers the collector. This matters because the other parts, like the heat exchangers, pumps, and storage tanks will affect the system’s efficiency. Having either is better than none, but between the two, OG-300 is the better one to have.

The collectors are 92% efficient, and the storage is insulated with high-density polyurethane to minimize heat loss when the temperature does get cool.

Because it’s a passive solar water heating system, it’s pretty affordable compared to active systems at about $2,500 (though it doesn’t come with the temperature controller), and there’s even a 10-year warranty in case anything happens.

2. GoSun Flow Ultimate: Best for solar water heating on the go

gosun flow ultimate solar water heater

  • Gallons:  (9L)
  • SRCC-certified / Eligible for 30% federal tax credit? No
  • Pros: You can bring it anywhere and not only have a hot shower, but also have clean water for drinking and washing hands

If your hot water needs aren’t that serious and you only really need to have a hot shower and wash up occasionally, then GoSun’s Flow is a good option to look into.

It comes with everything you need including a shower, sink, filter, and heater. Not only is the Flow a water purifier, it also, of course, heats up water:


It’s super portable since it’s small enough to fit into a backpack, and uses solar energy to filter a remarkable 99.99% of pathogens from water, making it safe to drink.

3. Duda Solar 200L Active Split System: A good winter-resistant all-inclusive solar water heater system

duda solar water heater

  • Gallons: 53 (200L)
  • Type: Active (indirect)
  • SRCC-certified / Eligible for 30% federal tax credit? Yes, OG-100
  • Pros: Comes with everything you need; resistant to freezing temperatures since it’s an active indirect system

Duda is known in the solar water heating industry as being a reliable and high-quality brand.

What’s good about Duda’s solar water heater is that because it’s an active heating system, the storage tank can be placed indoors—great if you live somewhere with harsh winters. It’s convenient too because you can place the tanks near where your hot water will actually be used so it takes less time for the water to come through your taps.

At 53 gallons, it’s good for a small family of two to four. It comes with everything you need to set up a solar water heating system: the tank, propylene glycol (the antifreeze fluid), and even an eight liter expansion tank.

Duda Diesel’s popular solar water heater comes in a variety of sizes, but the 200-liter (53 gallon) system is the best for residential use.

Its water tank rivals standard electric or natural gas systems but eliminates the need for costly energy consumption as it’s equipped with a solar water heater collector. The storage tank itself consists of food grade stainless steel.

The solar collector transfers the heat of the sun to your water pump through an indirect circulation system. The Duda kit includes the necessary two gallons of food-grade inhibited propylene glycol used in the process.

Other safety systems include an automatic air vent for purging air from the top portion of the system, and a submersible water pump for the initial air purge and charging of fluid into the system.

4. Heliatos Standard Solar Water Heater Kit: Best for hooking up your existing water heater to solar

heliatos solar hot water kit

  • Gallons: N/A; it’s meant to attach to your existing solar hot water system
  • SRCC-certified / Eligible for 30% federal tax credit? No
  • Pros: It supports your existing hot water system

This is a bit of a different type of solar hot water system, but we thought it was an interesting alternative to your typical solar water heater that works for both houses and cabins.

As opposed to the other systems on this list, this Heliatos kit is meant to hook up to your existing (gas or electric) water heater and power it with solar energy.

If you think you’ll need an alternate water heating source at night or during cloudy periods, then you’ll need some kind of two-way setup like this anyway.

This means that unlike the other systems, it uses small solar panels to heat your water. Essentially, the Heliatos PV (photovoltaic) solar panels pump the cold water from your water heater through your solar water heating panels, which will heat the water.

To determine many solar panels you need, you’ll need to figure out roughly how much sunshine you get—and how many gallons of hot water you need a day. Heliatos has come up with a handy table to give you an idea of how many solar panels you might need:

heliatos solar water panels needed

The Heliatos kit comes with:

  • solar panels (made of ⅛ inch thick aluminum and polycarbonate glazing, which is also used for bulletproof glass and airplane windows),
  • adaptors,
  • a stainless steel pump,
  • and pretty much everything you’d need to hook it up to your system

heliatos solar water heater kit

FAQs about solar water heaters

1. What would you typically need hot water for?

Other than a nice hot shower, other appliances around the home that usually require hot water include your laundry washing machine (although washing clothes in cold water is better for your clothes anyway) and dishwasher.

There’s also the occasional hot water you may need from your faucets, but that amount is usually much less compared to these appliances.

2. Does a solar water heater still work when it’s cloudy?

Just like with photovoltaic or PV solar panels (the kind you see a lot on the roof of a home), solar water heaters work best on sunny days with unobstructed sunshine hitting it.

If you live somewhere that regularly gets cloudy days, then definitely get a backup electric water heater (which can use electricity from the grid or from any power stored in your solar generator or battery). Another option is to get an additional storage tank that can hold hot water for a certain period of time as a backup.

3. Should I get a solar water heater or solar panels (or both)?

It really depends. A home solar panel system can power multiple appliances and devices in addition to an electric water heater. It does cost more to install solar panels than a solar water heater, and you’d likely need a bigger roof as well. If you want to invest more seriously into solar, then you could look into installing solar panels.

These would typically just be regular electric water heaters though, not solar water heaters—they’re just powered with solar energy from your solar panels instead.

4. What’s the maintenance for a solar water heater like?

Because passive hot water systems are more basic, they don’t take a lot of upkeep or work.

Active systems might need a bit more work because they have more components and pumps, but your owner’s manual should tell you the fundamental things to look out for.

Other than that, just keep an eye on the usual things you would monitor (like making sure your pipes don’t freeze in the winter).

5. How much does a solar water heater cost?

Smaller, more basic solar water heater systems are pretty affordable and cost around $2,500 (like the Sunbank option above), while larger systems could go up to $10,000 (or more). Passive systems tend to be less expensive than active systems.

Solar Water Heaters: A Beginners’ Guide
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