Solar Hot Water Heater Design

Off Grid Solar Hot Water | Easy Low Cost Ways to Get a Hot Shower

June 29, 2020

Daniel Mark Schwartz Profile Picture

Today, while planning an upgrade to my off grid hot water system, I went looking for the best solar water heater options available at a reasonable cost. Solar electric based heaters are not cost efficient compared to direct solar heating, and so many off-gridders end up resorting to propane or natural gas based water heaters. But, there are many great lesser known solar water heating options out there.

How to heat water with solar. The most efficient way to heat water with solar is to directly heat heat the water using collectors and black pipe. Highly efficient solar water collectors tend to use vacuum insulation and multiples layers of insulation to reduced heat loss.

There are several styles of solar hot water heater out there, and knowing which type is right for you can take a little research. Below, I’ll cover the benefits of each type, including both ready made options and lower cost DIY options.

Commercial Vacuum Solar Water Heaters

The most efficient solar hot water heaters on the market are all based around evacuated solar heating tubes. These tubes work like a thermos combined with a green house, having two or three layers of glass with a vacuum between them. Inside, there is usually an integrated black layer, and then a copper tube inside which hold either the water (direct flow loops) or refrigerant (for indirect flow loops with heat exchangers).

While being the most expensive option on the list, vacuum tube based solar heaters can really put out the hot water, and can work well even on cold or cloud days. If you have a few hundred bucks to spend and want a solar hot water heater that you know will work, then I would recommend going with this option.

The most affordable option I have found to get a vacuum tube based solar hot water heater is to buy them online have them shipped to you directly from China. This is the model I chose to go with for an off grid solar hot water heater:

misol Solar Collector of Solar Hot Water Heater 10 Evacuated Tubes vacuum tube

DIY Vacuum Solar Hot Water Heater Panels

If you are interested in tinkering with the vacuum panels by themselves, without buying the whole kit, I found a few available on Amazon.

I decided not to go this route myself because these come without the internal copper pipe, which holds the water being heated. Since these solar water heaters can get quite hot internally, the safest option is to use copper pipe to guide the water. This requires a pretty good ability cutting, bending, and sweating copper pipe, which isn’t something I’m personally that good at.

But, if you are set on using vacuum solar tubes for the most efficient possible solar hot water system, and want to save as much money as possible, then this is probably the most cost efficient way to go.

DIY Solar Water Heater Panel

There are a number of low cost, low tech solar hot water panel designs floating around out there on the internet. I know of several people that use this hot water heater design, and so far it seems to be the simplest and lowest cost DIY solar hot water heater design out there.

The basis of the design is a coil of black PEX pipe inside an insulated box with a glass lid. The sun heats up the water just like a garden hose in the summer sun. In warm climates with lots of hot direct sun, this design is extremely effective, and can be built for about $50 of materials from the Home Depot or your local hardware store.

If price is your biggest concern, then these are absolutely the panels for you. Just be aware that in cold climates, are regions that don’t get much direct sun at times (like the PNW where I live) this type panels are more likely to leave you with luke warm shower than the vacuum tube based panels above. And, at some times you may have to resort to a wood or gassed based backup system.

Hot Water Storage Tanks for Panel Based Heaters

Panels designs do not store much hot water internally, and will usually not heat the water fast enough to allow for an “on demand” style water system. This means you will need some form of hot water storage tank. This can easily be made from a modified traditional hot water heater tank, preferably a used one sourced for free online.

If you tank is able to be placed physically above the panels, then you could use thermal siphoning to keep a flow of water naturally between the holding tank and the panels, keeping the water heating system optimally efficient. Since it is most efficient to heat colder water to warm rather than hot water to hot, constant circulation of water through the panels and holding tank is the best solution overall

For roof mounted panels, you will need to have some form of pump to circulate water through the panels. This usually takes the form of a small solar panel and DC pump, since the pump only really needs to run when the sun is out, although you could easily wire the pump in to your primary solar system directly.

DIY Batch Solar Water Heater from Used Water Heater

Another low cost DIY solar water heater option is the “tankless” solar hot water heater design. What this usually means is that the tank is the hot water heater itself.

One commonly recommended design along these lines is to use an old hot water heater tank like so:

  1. Remove any insulation, burners, electronics, etc from the water heater tank
  2. Seal off all but two water ports in the tank.
  3. Paint the outside matte black
  4. Build an insulated box to put the tank inside, with an insulated glass window exposing the tank to the Sun (preferably double or triple paned)
  5. Plumb the lowest tank port to your water source
  6. Plumb the highest tank port to your hot water lines

If you are good at finding scrap or used materials like old hot water heater and windows, then this could be the cheapest option on the list. This is also by far to simplest option, requiring no pump or separate holding tank.

However, it does not heat up water as quickly as the panel based or vacuum based options above, which makes it most applicable areas with lots of direct Sun, where heating up enough water won’t be a problem.

I showered with water heated up in this way at a Yoga school in Southern India, and found it extremely effective in that climate. I also know of people who use it with success during the warm summers of Southern Oregon, although during the cloudy winter months they usually resort to other methods of heating up their water.

Low Cost Camping Solar Water Heater Bags

One cheap and easy on the go option for hot water is simply a dark colored back or bucket. Installing cheap shower head on a black (or painted black) 5 gallon bucket in direct Sun works great to shower a few people at the end of a sunny day.

Online, they sell “camping showers” which work absolutely phenomenally. And this RISEPRO solar shower works absolutely phenomenally, holding up to 10 gallons and heating up to 113 degrees in less than 3 hours of direct sunlight. And, it includes a built in temperature gauge for safety and the convenience of knowing if the water is ready or not.

I have a friend in Colorado who uses this method to take showers all year long, by hanging the bag inside a sunny window in the winter.

Simple and cheap, camping showers should not be overlooked as a portable, all-in-one option for off grid showering.

Do solar water heater work when it’s cloudy?

The temperature of hot water from solar heaters are usually significantly reduced in cloudy weather. Highly insulated vacuum tube based solar water heaters may function just fine in cloudy conditions. While others may still produce a usable water temperature, their capacity will be much lower until the sun returns.

Do solar water heaters work in the winter?

The capability of most solar hot water heaters is significantly reduced in cold weather, although highly insulated vacuum tube heaters may still be effective. Most solar system in cold climates have a non-solar based backup, but in warmer tropical or subtropical climates solar water heaters may function well all year long.

Do solar water heaters actually get water hot?

Under good conditions, solar water heaters will get water extremely hot. Solar hot water heaters are highly effective and cost efficient when there access to direct sunlight. However, in cloudy conditions or winter time in some climates, they may not operate entirely as desired.


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