DIY Solar Hot Water – Sizing your system


This is the second article in our series on the DIY Solar Hot Water heater.  Solar hot water  heating is one of the most cost effective forms of utilizing the suns energy.  This is because heat and light energy from the sun is directly captured and stored at the point of use.  Unlike solar electric power generation, there is no lossy conversion from light into electricity.  When water is heated via a solar panel, the heat energy is absorbed by the transfer fluid and stored or used directly.  This article will describe a cost effective and completely DIY solar panel for a hot water heating system.

Understanding your hot water usage:

How much hot water does your household actually use in a day?  That’s a great question and one you should find out.  In general, the following average usages can help give you a starting point:

Assuming a 4 person household:

Usage Average consumption Uses per day Total gallons used
Shower 10 gallons 4 40
Dishwasher 6 gallons 2 12
Clothes washer 10 gallons 2 20
Hygiene (handwashing, shaving, etc) 2 gallons 4 8
Food prep 2 gallons 2 4
Total 84

These are just rough estimates, add or remove hot water usages as you require for your household.  If you want to accurately measure the hot water usage for your shower, simply hold a 5 gallon bucket under your shower, turn on the water all the way to hot and time how long it takes to fill up the bucket.  Once full, stop the timer and divide that time by 5.  This will give you the time it takes to deliver 1 gallon of hot water to your shower.  While most of us don’t take a shower that is pure hot water, this does provide a worst case shower usage scenario.  You can also use this technique at the bathtub and at the sinks.  For example:

Location Minutes to Fill 5 gallon bucket Flow rate (gallons / min)
Master shower 3  1.6
Guest shower 2.5 2
Kitchen sink 4 1.25
Bathrooom sink 1.25 (1 gallon milk jug) 0.8

If you can’t fit a 5 gallon bucket in your sink, you can use a 1 gallon milk jug.

Easy System Sizing

In order to reach peak efficiency, the entire system, including panel(s), storage tank(s) and heat exchangers should be matched to the expected hot water usage of the home.  We will get into a little more component sizing details in a bit, but for starters, here are some good rules of thumb that can be considered when you design your system:

  • Size the system in order to provide for ~90% of your houses hot water usage during the summer
  • Provide 20 square feet of collector area for the first 2 people in the home and 15 square feet per additional person
  • For two people, a 50-60 gallon storage tank is usually sufficient while an 80 gallon tank will work well for up to 4 people.  For 5 or more people, a 120 gallon storage tank may be what you decide on.
    • If you are building your own solar hot water system from scratch, you are probably cost conscious.  This may rule out the larger tanks, which often times can be extremely expensive.
    • For actively circulated systems, it is often recommended to provide 1.5 gallons of storage per square foot of collector area.

When you design your system, you are not required to follow the recommendations, rather do what your budget and available time allow for.

A Modular Approach to System Sizing

While some folks may wish to build their entire system based on calculated usage, others may wish to start out small and grow their system as time and finances allow.  This was the approach taken for my home.

I started out with a single 4×8 panel which provides 32 square feet of collector area and a 40 gallon storage tank.  For 3 people, this is a bit undersized, but with a limited hobby budget and not wanting to spend too much time, this seemed to be a good place to start.  Even with this small system, our household has seen our propane water heating usage cut almost in half.

Since all of the plumbing, pumps, sensors and infrastructure had to be put in place for the small starter system, I can easily add more collector panels or storage without changing the basic system design or re-plumbing everything.  This system has been in place for 2 summer seasons now and has worked without a problem.

Due to the limited storage capacity, the heat transfer capability of the system collapses after about 20 minutes of high hot water usage, such as a bath or long shower.  I consider the system to be fully charged when the storage tank is at ~145 deg. Fahrenheit.  This is a little hot for a storage tank, but since we have a small tank compared to the collector area, I want to get as much heat energy into storage as possible.

With my DIY coaxial heat exchanger, and the system fully charged, I see approximately 15-20 deg. F temperature rise of the domestic hot water.  Recall that my system pre-heats the well water prior to it going into the domestic propane heated hot water tank.  This results in 80-90 deg. F water entering the domestic hot water tank.  When lots of hot water is used for an extended period, the ability of the solar hot water system to provide the 15-20 deg. F temperature rise collapses and only provides a 5 – 10 deg F temperature rise.

Planned improvements for my system

Now that I have 2 seasons worth of real world, experiential data, I plan to add a second 4×8 panel and a second storage tank.


  • Prior to building your system, consider your real world hot water usage
    • Gather data
    • Don’t be overwhelmed by large hot water usage…go with a modular approach if needed
  • It is ok to experiment with system size
  • You can start out small and grow your system as your budget and available time allow
  • Getting started sooner, with a smaller system can start saving money sooner, which can help pay for system expansion