How the Solar Hot Tub Kit Works

The Sunbank Solar Hot Tub Kit is the least expensive way to heat a hot tub, but how does it work?

It’s pretty simple, really. The Sunbank flat plate collector traps the heat from the sun, and copper pipes running through the collector allow the hot tub water to circulate through the collector, via a solar powered pump, transferring that heat into the tub. And just like that, you’re heating your hot tub with the sun instead of fossil fuels.

How It Works

The solar kit can be added to any hot tub, but it’s good to know how it works and what to expect.

The solar hot tub kit will start heating the tub in the morning, raising the temperature above your electric or gas thermostat setpoint during the day. Without the solar hot tub kit, the tub’s thermostat would kick on multiple times an hour to keep the tub at temp, but with the kit, on a sunny day, the backup heater will stay off. This is how you save energy. Depending on the weather, the volume of the tub, how well insulated it is, how many collectors you have installed, etc., the tub may heat from a 102F setpoint (for example) up to 104F, 108F or higher during the day. This is all free energy. If you don’t use the tub that day, the tub will slowly cool down after sunset back to 102F and will be back on electricity for the evening. If you decide to have a sunset soak, you can easily dissipate any excess heat by turning on the jets. With the insulated cover removed, spa jets can dissipate heat at up to one degree per minute with no heating element turned on.

Some people who are either off-grid or want to run their spa completely from solar, will need to install two or more collectors depending on the size of their hot tub, the local climate, etc. Using the system like this works well for sunset soaks, as the system is typically at it’s hottest at the end of the day, cools overnight, and heats again the next day. Using the solar hot tub kit like this means, of course, that you will only have a hot tub on sunny days.

Mounting Options

The preferred mounting option is to mount the collector(s) at a 30-45 degree tilt adjacent to, but just above the hot tub. We’ll refer to this as the “drain back option,” as this allows the hot tub water to drain back into the tub when the pump turns off. The collector can be mounted on a platform specifically built for this purpose, a pergola type structure, or even a nearby roof. If the top of the collector is more than 9 feet above the water line, you will need a second pump mounted in series to increase the head height. Mounting the collector above the hot tub and allowing it to drain back provides freeze protection at night. This drain back feature can also provide overheat protection. For example, if you set your electric thermostat to 102F and only want the solar to heat the tub to 106F, you could install our solar controller to turn the pump on and off based on the temperature of the tub. This option requires the installation of a temperature sensor, requires AC power to run the controller, and adds unnecessary complexity for most installations. But it may be right for some applications.

You can also mount the collector using our Vertical or Horizontal Mounting Bracket at the same level as the hot tub, with the collector below the water line. This is only recommended in climates that don’t have prolonged hard freezes that are capable of freezing an outdoor copper pipe, or for customers who use their system seasonally. In this configuration, the collector is below the water line and therefore stays full of water. With this option, the pump should be powered by a solar panel (provided in the kit) so that the water in the collector does not overheat. It is possible to achieve thermosiphon circulation with this mounting option, but using the pump is recommended.

A flat plate solar collector mounted above a hot tub in order to drain back into the tub when not pumping
Diagram showing a solar powered hot tub

Plumbing Integration

Our redesigned kit runs the hot water directly through the collector. There is no problem running chlorinated water through a copper pipe. In fact, copper has a disinfecting property and could allow you to use slightly less chlorine while keeping your water clear. There are two options for how you will get your hot tub water into the collector(s).

The first option is to integrate with the hot tub’s plumbing. Hot tubs typically have PVC plumbing, and you will cut a section of this piping out and add in two tees, a supply and return to the collector. The pump will be mounted here, inside of the hot tub housing, where it is dry and safe from freezing. If mounted outdoors, the pump will need to be housed in a weatherproof box. The pump will turn on either when sun hits the solar panel mounted next to the collector, or when the controller turns it on, according to your settings. Hot tub piping varies, and so these pipe and fittings are not provided in the kit. Check with your spa manufacturer if you are concerned about voiding your warranty. 

The second option, is to run the pipe or hoses from the collector directly into the hot tub water. In this scenario, the pump will be mounted in a weatherproof box. Make sure that the supply pipe/hose always stays below the water line in the hot tub, so that the pump can always draw water. And for those with freezing concerns, you will want to follow solar thermal installation best practices, one of which is to be sure that the lines are sloped to drain and have no places where water can accumulate and freeze.


How many collectors you can use depends on a number of factors, including: how much space you have for collectors, your local climate, collector shading, how well insulated your tub is, the size of your tub, how often you use your tub, whether or not you use jets (they quickly dissipate heat), etc. There are two methods to see how big of a system you need.

  1. If you can isolate the energy use from your spa or hot tub heater, either by looking at your utility bill (which is difficult unless it is the only appliance that uses gas, for example) or by metering the electricity consumption.
  2. Heat your hot tub up to temp and then turn off the heater. Open the cover and turn on the jets for the amount of time that you would normally use the tub. After 24 hours, measure the temperature of the tub. If you know the temperature difference and the volume of the tub then you know how much heat input it requires in a day. This is not as precise as metering, but is a close approximation.

If you have a functioning tub, it’s probably best to start with one collector. You can always add more later. If you are using solar as your only heat source, you should start with at least two collectors.


If your hot tub is located in a place that gets a lot of sunlight then you can mount your solar hot tub kit directly adjacent to the hot tub. Not everyone will be able to mount the solar collector right next to the hot tub. You might want to mount the collector on your roof, a nearby fence, or the pergola over the hot tub. Let us know your mounting location, and we may be able to help you get the correct mounting hardware.

In Summary

The hot tub kit may be a good fit for you if:

  1. You want to use less electricity or gas to heat your hot tub.
  2. You feel like you have a good understanding of how the system works and are confident doing basic plumbing work.
  3. You have a good location to mount the collector(s).
  4. You are fine with slightly overheating your tub in order to save energy.
  5. You understand how to properly freeze protect your system, if necessary in your climate.
16 reviews on
nic maier
nic maier
I don't write reviews but I have to tell others about James from Sunbank! We got an 80 gallon solar water heater and installed it ourselves. It works very well in Southern California. The installation was a little challenging for us, but James helped every step of the way and was even available to give us advice a full year after we had purchased the unit. He was always quick to respond with a thorough and helpful answer! I wish all businesses were run like Sunbank with people like James!
Bob Billings
Bob Billings
James, thanks for your continued support. Our 40 gallon Sunbank water heater is the best. Installed in October of 2021 and we would not trade it. Our life off-grid has worked out well. Bob Billings
Jonathan Bishop
Jonathan Bishop
My only regret is not getting the Sunbank sooner! I installed the 40 gallon system which has been perfect for 3 people. It has a 1500 watt element to heat the tank when the sun isn't shining and because of the low power demand, we are able to run it completely off-grid. James also took time to answer any questions I had and has been excellent to work with!
Robert Thorlin
Robert Thorlin
James and the team at Sunbank are exceptionally knowledgeable and willing to help. Pre-sale and post-sale support was wonderful, even to the point of shipping a bunch of extra T-bolts I needed for my unique installation, bolts and shipping all at their cost. I would bot hesitate to work with them again on anything... 10 stars if I could... Mike
Elsa Hennings
Elsa Hennings
Living in the Mojave desert, I've wanted to "go solar" for our home's energy needs for a long time (I like free stuff). We put in a solar electric system in 2004, which has long since paid for itself, and bought a plug-in hybrid car to use our "free gas" several years ago. I finally decided to heat our water with the sun, and after looking at a number of systems, I chose SunBank Solar based on it having no moving parts (I don't like to fix stuff). I mounted it on the ground since we have no shading on the other side of the wall from our old electric water heater, which made the plumbing pretty simple. For overheating protection in the desert, I painted half of each heating tube with reflective silver paint as James recommended (see photo on right), so when we leave on vacation I just rotate each tube to prevent heat absorption, turning the system off. The instructions provided, along with some how-to videos on youtube for using pex tubing, allowed me to get it all put together with no prior plumbing experience. James was an invaluable resource when I had questions, which was often! The system has now been running for over a month, quietly generating all the hot water we need! I highly recommend not only the system but the company for the high quality materials and excellent product support!
debra covey
debra covey
Installed the Sunbank 40 water heater 12/2019. We didn’t install the controller until 8/2020 in case we needed it in the coming winter, but still haven’t needed supplemental heat in south Florida as of 4/2021. Love this thing! Works great for the 2 of us in our small (1000 sq ft) home. So glad we installed it. Company was very responsive to all questions prior to ordering through install. Highly recommend!
Jim Feldhan
Jim Feldhan
I rarely write reviews; however, I am very happy and impressed with the Sunbank solar hot water heater. It appears to be made from top quality material. I was replacing a Solarhart which was not a stainless steel tank and the heat exchanger developed a leak. I started with a 40-gallon system and installed it myself. I’m very handy with DIY projects. The Instructions online were clear, and James was very helpful with any of the questions I had. Being in Phoenix I thought a 40-gallon system would be enough. Once Witter arrived, I realized that I use more hot water than I thought and wanted to have 100% solar hot water. Thus, I installed a second 40-gallon system to boost my capacity to 80. I also install a couple of radiators to heat the master bath and home office allowing me to heater those room in the fall, winter, and spring with excess hot water. In summary, I am very happy and give Sunbank 5 stars for quality and service.
Timothy Clayton
Timothy Clayton
I installed the Sunbank and the water temp went from 50F to 101F in 4 hours the 1st day. By the 3rd day I was up to 160F. It only loses approx 5 degrees overnight. It is awesome! James at Sunbank is one of the best support guys I've dealt with. Definitely recommend buying this for solar hot water.
Cuyler Smith
Cuyler Smith
Our family purchased a 40 Gallon Sunbank Solar Water Heater just over a year ago and we are VERY pleased with its performance, ease of installation, and energy savings! We have it installed on top of our spring house roof, located about 125 ft. away from our house. When combined with the Federal Tax Credit and the savings associated with the solar offset of electrical usage for heating water, this Sunbank system quickly pays for itself and is a great investment. I was able to install the system on my own and it has performed flawlessly during its first year of use. James at Sunbank Solar has been more than helpful in responding to any questions I have, even after the initial purchase, which says a lot! I would recommend the Sunbank Solar Water Heater to anyone wanting to save money and reduce our environmental impact!
tutti hacking
tutti hacking
I recently installed two 40 gallon Sunbank solar water heaters - one on my garage and one on my ADU. They have been awesome! Water is super hot and now I don't have any qualms washing my laundry with warm, solar heated water! I have wanted a Sunbank system for almost four years now, and James has patiently helped me through the process. I used my local solar panel installer to get the job done. Took 3 days to fully install (I did need some reinforcement for 2x4 rafters) and inspection was quick and easy. James steered me to a great structural engineer to get what I needed for City Planning. It feels great to have gotten rid of my natural (fracked) gas water heaters! And the 30% tax credit, along with the California Solar Initiative rebate, really makes it affordable.

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