If you own your own home, you know from experience your power bill always spikes during the summer and winter months. In large part, that’s because your air conditioning and heating units have to work harder to maintain a comfortable temperature for your home. And whether it’s a reaction to higher bills, concern over the environment or a combination of both, you’ve probably started looking into ways to reduce the energy dependence that comes from living in a modern household.


Fortunately, there are a number of effective solutions for reducing power consumption you may have tried. Maybe you switched to fluorescent or LED light bulbs or put down an extra layer of insulation in your attic and have gotten positive results.


However, a big culprit of energy consumption is your water heater, and unless your water heater is ready to be replaced, you probably haven’t given serious consideration to switching to a solar hot water system. Either way, if you’re currently using a low-efficiency electric tank-type water heater, you may want to consider changing.


How Much Electricity Does Your Water Heater Use?

Most U.S. homes have a tank water heater that runs warm water using either electricity or some sort of fossil fuel, like oil or gas. These types of water heaters are inexpensive to purchase, so they’re popular. Unfortunately, tank-type systems require frequent heating for hot water on demand, which ends up costing more over their lifetime. According to the U.S. Energy Information Administration (EIA), the five biggest electricity uses in the average American home are:

Space cooling – 13%

Lighting – 11%

Water heating – 9%

Space heating – 9%

Refrigeration – 8%


Assuming you’re using an electric tank-type water heater in your home, it’s responsible for nearly a tenth of your power bill.


Electric Tank vs. Solar

Part of the reason electric tank-type water heaters draw so much power is because their design is inefficient. As you use the hot water that’s sitting in your tank, relatively cool water has to replace it. This is usually sourced from your local utility and pours into the tank at ground temperature. The tank switches on to heat the cool water inside until it reaches the desired temperature — usually between 120 and 140 degrees Fahrenheit to prevent scalding and to reduce the chances of microbes forming inside. But what happens when you aren’t using the water?


Electric tank-type systems are designed to maintain the water at the desired temperature, but due to heat seepage, they have to continually warm the water. That means your tank system will periodically switch on, even when no water is being used. Because they’re relatively inexpensive to purchase and install, tank-type water heaters have maintained their popularity, but when you consider that nearly a tenth of your power bill comes from providing electricity to your water heater, an electric or gas tank system may not be quite so cheap in the long run.



Solar water heating systems use the thermal energy from the sun to heat the water. With the right system, you can enjoy monthly savings and consistent year-round hot water. And as an added benefit, solar energy is renewable and has no negative environmental impact.


Solar Hot Water Systems

Because solar technology relies on sunlight, not heat, you don’t have to live in a warm-weather state like California or Texas to enjoy the benefits of a solar hot water system. However, not every type of solar water heater is right for every situation. Before you commit to purchasing and installing a solar hot water system, you should familiarize yourself with the various types that are available in your area and contact a licensed professional with any questions you have. In the meantime, you can familiarize yourself with different types of systems and the various options available so you can make an informed choice.


Solar Hot Water System Concepts

Despite the fact there are dozens of types of hot water systems, they all rely on a few similar principles. To understand your potential investment in a solar water heater, it’s important to know a little bit about how each one works. For instance, you don’t want to purchase a solar water heater that requires a tank to be mounted on your roof if you live in a city or community that doesn’t allow it. If you live in a winter climate, you probably won’t be able to use an open-loop water heater, as they’re prone to freezing.


In the following sections, we’ll discuss some of the specifications and operating principles of solar water heating



Collectors – A collector or solar collector is a large flat panel that sits on your roof for maximum exposure to the sun. To the uninitiated, and maybe at a distance, the collector may look like a single photovoltaic panel you’d use to power a residential electrical system. Because most solar technology relies on maximizing the surface area of the collectors to increase solar exposure, the panel shape is common.


But unlike a photovoltaic cell, the collector usually has water or a liquid chemical moving through it. One of the benefits of solar hot water systems is that they don’t take nearly as much space as an array of photovoltaic solar panels, which can cover an entire rooftop. If you don’t have the surface area for a solar power system, you still may have room for a solar collector. There are two main types of collectors: flat-plate and evacuated-tube. We will cover these individually in other sections.



Passive systems – The design for passive water heaters is remarkably simple — which may be one of the reasons passive systems have been around longer than other types of solar water heaters. A flat panel containing tubing sits on your roof. Water enters through the bottom of the panel. The sun shining on the panel heats the water inside the tube, and the insulated housing prevents it from leaving the panel.


As water heats, it expands, which causes it to circulate through the pipes. Hot water is lighter than cold water, so it rises and exits through the top of the panel and connects to a hot-water tank where it’s available for home use. If you’ve ever picked up a garden hose that’s been lying on the ground in the sun, you may have noticed the first few gallons of water that comes out of it are hot. That’s because the sun has warmed the water inside of the hose. Passive hot water systems work on the same principle. Instead of a using a pump, the systems depend on the laws of thermodynamics to move water, which is why the system is referred to as “passive.”


These types of systems work well in warm, sunny climates, but the pipes are prone to freezing when the temperatures drop. For this reason, passive solar water heaters are nearly non-existent in northern climates — at least, not as primary residential hot water systems. Thermosiphon water heating models require the tank to be higher than the panel, with both a tank and the collector on the roof. We’ll discuss these in another section.


Active systems – Active solar hot water systems use one or more pumps to move water through the heater. The pumps utilize electricity, but this only occurs when you turn the hot water on. Some active systems use photovoltaic cells to provide the electricity, so there isn’t any utility power consumption. Active systems have a number of features that make them more attractive than passive systems. Some of these include:


  • The storage tank doesn’t have to be placed above the collectors
  • The storage tank can be located in a more conventional, hidden location, like a storage room or a garage.
  • Active systems tend to be more efficient
  • The storage tank can be placed in a space where it’s less likely to experience heat loss.


Open-loop vs. closed-loop systems – In an open-loop water heater, the water that’s being heated moves directly through the hot water solar collector. Contrast this to a closed-loop system, which uses another type of fluid chemical — known as the heat transfer fluid (HTF) — to absorb and give off heat.


In a closed-loop system, the HTF is contained in tubes or piping, which warm the water. This is called a heat exchanger. The walls of the tubing serve as a barrier between the chemical and the water itself, which is why the loop is considered closed. Because closed-loop solar hot water systems utilize chemicals with much lower freezing points than water, they can be used in climates where open-loop systems are prone to freezing.


Flat-plate collectors These panels contain an array of tubes that water passes through. The tubes and the water inside are heated by sunlight. The surface materials of the panel are non-reflective and are designed to trap in the heat. In the Northern Hemisphere, these collectors are usually canted to face the south, so they can capture the greatest amount of sunlight on any given day.


Evacuated-tube collectors – This type of collector panel contains a series of completely empty translucent tubes that are designed to trap heat. They are almost always closed-loop systems because the heat rises to the top of the tubes where it meets a channel containing the chemical that will heat the water. Because they aren’t as durable as some of the other systems, evacuated tube collectors are not recommended in areas with high winds.


Batch systems – This may be among the simplest solar hot water heating systems as well as the easiest to construct. In a batch system, a tank of water is exposed directly to the sunlight. As in all passive systems, the warmer water moves to the top of the tank where it connects to the house. Some do-it-yourselfers have even been successful constructing these from old electric water heater tanks.


To summarize solar water heating system features:


  • Solar water heaters are either active (with a pump) or passive (without a pump)
  • Collectors are the panels where water is heated and are either flat-plate or evacuated-tube
  • Solar hot water systems can be open loop where the water is heated by the sun, or closed loop where a chemical passing through pipes is heated by the sun, and in turn, heats the water.


What Are the Types of Solar Hot Water Systems?

Now that you have a basic understanding of how an appliance can use solar energy to heat your water, it’s time to put the various features together so you can see a few types of hot water systems and how they work:


Drain-Back Solar Hot Water System



A drain-back system utilizes both a pump and a heat-transfer fluid, so it is an active, closed-loop water heater. In a drain-back water heater, distilled water is used as the HTF in the coils that heat the water. A pump pushes the distilled water from a tank into the collector where it’s heated by the sun. The hot distilled water in the pipes then heats the water for the house.


When the optimum water temperature is reached, the pumps switch off, and the distilled water drains back down into the tanks, which is where the term “drain-back” comes from. It may seem that using distilled water to heat tap water is counter-intuitive. After all, distilled water freezes at the same temperature as regular H2O. However, because the water is only exposed to the elements in the collector when the pumps are on, drain-back systems avoid freezing.


Among the advantages of a drain-back system are:

  • They work in freezing temperatures.
  • They can’t overheat.
  • Distilled water is a safe, non-toxic heating thermal fluid.


Because drain-back systems rely on gravity, the collector must be kept above the tank. This is not usually a problem since collectors are frequently placed on roofs.


Active Open-Loop Solar Hot Water System



Because this is an open-loop system, there is no heating exchange, and no HTF is used. It does, however, use a pump, so as the name indicates, it’s an active system.


This system employs the use of two sensors: one on the solar collector and one at the bottom of the water storage tank. When the sensor on the collector is four to five degrees Fahrenheit greater than the one on the bottom of the tank where the cool water sits, a pump will draw water from the bottom of the tank into the collector. The warm water in the collector is pushed to the top of the tank. The sensor will also pump warm water through the system when temperatures drop to avoid freezing. By repeating this process, this open-loop system is able to heat water to the desired level of 120 – 140 degrees.


The advantages of this type of system are:

  • The simple design reduces the chance of a malfunction and lowers maintenance costs.
  • They’re highly efficient.
  • The tank can be lower than the collector panel so a rooftop tank isn’t necessary.


Active Closed-Loop Solar Hot Water System


Unlike the drain-back solar water heaters, this system uses a chemical HTF. Because there is always a chance of ruptures in the heat exchange, the fluid used must be non-toxic. The U.S. Food and Drug Administration considers glycol fluids safe if ingested by humans. Because of their non-toxicity and because they have a freezing point well below that of water, glycols are often used as the HTF in closed loop systems.


Because the glycol fluid and similar heat transfer fluids don’t freeze during typical winters, this type of solar water heating system can be used in colder climates year round. One of the drawbacks to closed-loop systems is that the HTF sometimes needs to be replaced. In the case of glycol, it needs to be changed every several years depending on the unit manufacturer’s specifications.


Thermosiphon Open-Loop Solar Water Heater



Thermosiphon water heaters aren’t as popular as some of the active closed-loop models for two reasons:

  • They’re prone to freezing.
  • The solar collector has to be higher than the tank, which usually means a roof-mounted tank.


Colder water settles to the bottom of a tank, and hot water rises. This creates a flow as the hot water transfers upward. Cold water exits the tank via the bottom and enters the collector, where it starts to heat and rise through the tubing. At the top of the collector, it cycles out and back into the top of the tank where the warmer water is. The advantage to this system is that it’s simple and requires no electricity.


Frequently Asked Questions About Solar Hot Water Systems

Understanding the various approaches to home solar water heating systems should help you focus your approach to identifying the proper type of system for your home. If you have concerns about whether or not your home or rooftop is suited for a solar water heater, you should contact a professional installer. Here are some of the questions that we are frequently asked by our clients during our consultations:


What’s the difference between solar panels used to generate electricity and the ones used to heat water?


Although the shapes are similar, the construction of the two types of panels is quite different. Photovoltaic cells —  the ones used to generate electricity — use a photoelectric material, like monocrystalline silicon, that releases electrons when light shines on them. This generates an electric current. The collectors used in solar water heaters focus the sun on the elements that contain water or a HTF to create thermal energy instead of electricity.


Are solar hot water systems more efficient than tankless heaters?


Yes. Tankless systems only warm water when there’s an immediate need in the household. Because they aren’t continually drawing electricity or gas, there is an energy savings over traditional electric tank-type heaters. Solar heaters, however, don’t require an energy source to generate heat. They’re by far the most energy efficient hot water systems on the market.


What types of homes aren’t right for solar hot water systems?


Most single-family residences can accommodate some type of solar water heater. There are some factors that may make a home less suitable for this type of system:


  • There’s an old and decaying roof that can’t support a collector or tank.
  • The roof is blocked by trees or buildings.
  • The residence is in a condo or apartment building and there’s no access to the roof.
  • The Condo or homeowners’ association doesn’t allow rooftop structures.

If you have concerns that your residence isn’t right for a solar hot water system, contact an installer in your area for a professional opinion.


What are the monthly savings for a solar water heating system?


Because there are so many different models of solar hot water systems, coming up with an approximate savings is difficult. If you choose a passive water heating system, there is no electricity involved. Consequently, the portion of your power bill that currently accounts for water heating should disappear. Active systems are more reliable, but do, in fact, use some electricity to power the pumps, so there’s a small energy cost. Again, with an active system, the savings will be consistent with your hot water usage.


Are there any tax incentives for switching to a solar water heating system?



Currently there’s a 30% federal tax deduction for systems installed before the end of 2019. The credit only applies if the improvement is on your personal home.


What is the lifespan of a solar water heating system?


There are both internal and external factors that can lengthen and shorten the lifespan of any type of water heating system — solar isn’t an exception. But with the proper care and treatment, some solar water heating systems are expected to last as long as thirty years.


Contact a Solar Hot Water Professional

No one should purchase any major appliance without first discussing its features with someone who is familiar with the appliance’s operation. If you’re considering moving to solar water heating, contact a licensed, professional installer with a known track record. Hydroflex Systems provides a free home assessments and cost estimate. Contact us today.