How Much Propane Does a Tankless Water Heater Use? Simple Calculations

From explaining the info about important heating parameters to providing a simple-to-use formula – in this article we’ll show how much propane does a tankless water heater use
Rodney Nestor
Rodney Nestor
Research Writer
Rodney holds a Master’s degree in Arts and apart from working as a freelance writer, is a rather successful author of science fiction stories published in several literary read more
Last updated: August 16, 2023
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A tankless water heater, also known as instantaneous or demand-type water heater, heats water instantly without a storage tank. When you turn on the faucet, cold water hits the heat exchange unit where a natural gas like propane heats it.  The best thing about heating water this way is that it saves time; you don’t have to wait for long to get enough hot water.

How much propane does a tankless water heater use? First off, understand that heating water instantaneously requires a lot of energy; all of which comes from combusting propane. How much propane these heaters use isn’t a concern for some people, especially those who live in extremely cold regions. All in all, they are a must-have in every home because they supply hot water on demand. Let’s discuss basic information about how the tankless water heater works, to help you decide whether it’s right for your home.

Tankless Water Heater Propane Use Considerations

How Much Propane Does a Tankless Water Heater Use? Simple Calculations

According to the department of energy, propane has a high fuel stability, or  simply higher octane rating. This means that it can combust easily and quickly even under low pressure. Since it doesn’t harm the environment when it spills, propane makes the best green fuel for both domestic and commercial use.

To figure out how much propane you need for your home, you need to first think about your gadget needs. How many tankless theaters do you need? How much gallons of propane does each heater consume? The answers to these questions vary to some extent, depending on how often you use the heater, and the size of your household. For a small household, buying a small but super-efficient heater such as the Hotaitai Portable Propane Tankless Water Heater will help cut your propane bills.

If you can’t estimate how much propane you need, seek the help of a propane supplier. Now, here’s what you need to know about tankless water heaters as far as propane use is concerned:

BTU output

BTU output is one of the most important considerations when buying a tankless water heater. BTU is simply the heating power rating. It denotes the amount of energy required to raise one pound of cold water by one degree Fahrenheit. This rating is often given based on how much power a heater can provide in an hour.

If a heater cannot generate sufficient heat, the water entering the heating exchange unit may not be heated enough. A typical propane tankless water heater (9 or 10 gallons of propane per minute) may generate anything between 25,000 and 200,000 BTUs per hour. This energy is sufficient for three bathrooms.

The more the rooms, the more the gallons you’ll need every day.

EF factor

Energy factor (EF) is a heater’s overall efficiency. If a heater produces a lot of hot water per unit of fuel per day, it’s safe to conclude that the heater is efficient.

Propane tankless water heaters offer efficiency by helping you save hundreds or thousands of dollars every year compared with other forms of energy. It transforms the thermal energy in propane into heated water by at least 0.99 EF. Most propane-based heaters use a condensing design to extract additional thermal energy from the combustion chamber, which is used to pre-heat water. This in turn boosts the efficiency of the system.

Additionally, these systems reduce energy losses because they only heat water on demand. They therefore make a great alternative to storage tank heating systems.

Hot water production

A propane heater can deliver 200+ gallons of hot water per hour, which is way far from the 50-60 gallons a standard electric water tank delivers. The superior performance and increased efficiency makes these systems a smarter choice for everyone’s economic needs.

How to Calculate Propane Usage of Tankless Water Heater

How Much Propane Does a Tankless Water Heater Use? Simple Calculations

That different households naturally have varying energy needs is a no-brainer. That’s why you should know how much propane you need, to ensure you don’t run out of fuel just a few minutes into taking a shower. Knowing your fuel needs will also help you to budget accordingly. More importantly, you’ll get an opportunity to compare the efficiency of propane tankless water heaters with other types of heaters.

On that note, understand that although propane-fueled heaters consume more energy than other appliances that use the same fuel, they’re still more economical than the traditional water heaters.

To calculate tankless water heater propane usage, take these approaches:

Water heater GPM rate

Determining the gallons per minute (GPM) of your tankless water heater can help you to accurately figure out your propane needs. Also known as flow rate, GPM is the number of gallons of hot water that flow out of a heating system per minute. The figure varies depending on water temperature before it enters the heat exchange unit.

Water usually enters the heater at around 50 degrees Fahrenheit. Most people heat water until it’s 120 degrees Fahrenheit on average. As you can see, heating water isn’t an easy feat, but subjecting it to a considerable amount of heat can do the magic.

According to the Energy Information Administration, a gallon of propane can produce 91,452 BTU at 100% efficiency.

Now, let’s calculate propane usage based on the GPM rate of your heater. Assuming you’re using a high-efficiency tankless heater like the Rinnai V65EP, you’ll have a GPM of 6.5. Next, calculate the water temperature rise. This is the difference between the current temperature of the water and how hot you want it to be. Let’s use 50 degrees Fahrenheit as the current cold temperature, and 100 degrees Fahrenheit as your desired temperature.

Subtracting the two, you get a 50 degrees Fahrenheit temperature rise. So you’ll need a water heater suited for 6.5 GPM at 50 degrees Fahrenheit. If you run such a heater for 20 minutes, you will generate 130 gallons of hot water. Divide 150, 000 BTU generated by the heater by the BTU in a single gallon of propane, something like 90,000, and multiply by the number of hours (20 minutes = 0.33 hours). From this assumption, you need 0.54 gallons of propane to produce 130 gallons of water every day.

Use the formula

How much gas does a tankless water heater use? If one fuel unit produces 150,000 BTU (heater capacity) and a gallon of propane produces 90,000 BTU, it’s safe to say that the amount of propane used in an hour is:

= 150,000 BTU ÷ 90,000 BTU per gallon;

So we need 1.6 gallons of propane per hour.


Now, let’s answer a few common questions regarding propane usage in tankless water heaters.

How much propane do I use for a single shower?

A shower lasts 8-20 minutes. You’ll therefore need 0.21-2.1 gallons of propane for a single shower.

How to reduce propane use?

There are numerous ways through which you can save on propane usage:

  • Set your heater at a lower temperature: According to gov, most households set water heater temperature at 140 degrees Fahrenheit, but you may set yours at 120 degrees Fahrenheit or thereabouts.
  • Use alternatives: You might want to trap solar energy and use it to run water heaters. This can greatly save you a lot of money every year.
  • Minimize the time you spend in the shower: If you can clean yourself in five minutes, there’s absolutely no need to do 20 minutes.
  • Use cold water: Although this may sound weird considering the work of water heaters, you just don’t know how much propane you could save if you have a cold shower on a hot summer day.
  • Regularly maintain the heater: Inspecting your water heater one a week will help you to detect system flaws and have them fixed before it’s too late. Remember, a properly working heater is not only efficient but is also a money-saver.

Final Thoughts

To this end, there’s no doubt we have answered the question: How much propane does a tankless water heater use. Although the process may seem easy, it’s a bit difficult for most people because different information is scattered all over the interwebs. You’re lucky because we made a lot of calculations and considerations to simplify things for you.

Keep in mind that knowing how much propane you need is a great first step, but knowing how to reduce usage and save on costs is more important. You could simply lower the thermostat, reduce the time you take below a shower, or embrace other habits that can cut your propane bill.

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