Tankless water heaters that run on propane are the real gas guzzlers, yet they are still around 41% more cost-effective than traditional water heaters that use tanks.

These on-demand heaters consume far more propane than any other equipment powered by propane that you are likely to come across.

What is the precise amount of propane that a tankless water heater requires?

The amount of propane that must be purchased (together with the associated cost in dollars) is determined by three primary considerations.

These include:

  1. The capacity of a tankless propane system. It goes without saying that a propane tankless heater with 11 GPM and 199,000 BTU will use more propane than, say, a unit with 7 GPM and 75,000 BTU.
  2. Hot water generation. A tankless propane water heater that operates for one hour every day will spend more money than the identical device that runs for twenty minutes every day.
  3. Efficiency in the use of energy. On-demand hot water heaters that run on propane are often quite energy efficient. The EF factor, which measures how efficiently propane is burned, begins at 80 percent. The most efficient tankless propane heaters have an energy efficiency of up to 99% (0.99 EF); these machines utilize the least quantity of propane for the largest amount of hot water production.

We can determine how much propane a tankless propane heater uses by calculating its consumption based on these three parameters.

The following is a summary of the output that highlights the usage of propane:

  • Smallest 53,000 BTU unit (lowest): 0.58 gallons of propane per hour of operation.
  • Maximum 199,000 BTU unit: 2.18 gallons of propane per hour of operation.

When you look at the labeling for Energy Star products, you’ll see that the annual use threshold for propane tankless heaters is set at 200 gallons.

The amount of money that one really spends over the course of one year to operate a tankless water heater that runs on propane might vary widely.

According to our research, the following are the minimum and maximum expenditures that might be incurred:

  • Low-end (53,000 BTU unit, 20 minutes per day, 0.99 EF): 71.22 gallons per year.
  • High-end (199,000 BTU unit, 60 min/day, 0.80 EF): 992.80 gallons/year.

That doesn’t tell us a whole lot, does it? About how much propane our very own tankless propane water heater consumes, right?

This is due to the fact that some homes have smaller units, while others have larger units.

Additionally, some homes use their propane on-demand water heaters for an hour each day, while others utilize them for just twenty minutes each day.

We have devised a calculator for tankless propane use so that everyone may get a more accurate estimate of the amount of propane that their particular device consumes.

You will need to enter the BTU capacity of your unit, its energy efficiency rating, and a reasonable estimate of the number of minutes your unit is on each day.

The calculator may be found a little bit farther on.

The following is how the calculation looks for a propane tankless heater with 95,000 BTU (9.8 GPM), 0.95 EF, which is operated for forty minutes every day:

In addition to that, we have computed the annual propane consumption of three different sized propane tankless heaters based on three distinct use scenarios: twenty minutes per day, forty minutes per day, and sixty minutes per day (results summarized in the table after the calculation).

This will provide you with a rough estimate of the number of gallons of propane that your tankless propane system consumes on an annual basis.

Let’s begin by determining whether or not everyone is capable of calculating the amount of propane that the instantaneous water heater uses:

How Do I Determine How Much Propane I Will Need For My Tankless Water Heater?

It is famously difficult to bring water to a boil.

This is because the specific heat is rather high at 4.19 kJ/kg°C.

Tankless heaters that run on propane can heat more than 10 or more GPM of water at a rate of less than 15 seconds.

Burning propane supplies the whole quantity of energy necessary to do this task, which is a significant amount.

To be able to accurately determine the amount of propane that these on-demand heaters use to heat water, we need to be aware of the amount of energy that propane has.

The Energy Information Administration of the United States is here to provide assistance with this energy content for us: Propane has a conversion factor of 91,452 BTU per gallon.

This indicates that the combustion of one gallon of propane yields 91,452 BTU (when operating at maximum efficiency).

These days, tankless water heaters that run on propane come in a variety of sizes.

If we look at Rinnai units, we can see that they offer units ranging from 5.3 GPM (these units produce 53,000 BTU heating output at 100% capacity) to 11 GPM (these units produce 199,000 BTU heating output at 100% capacity).

Rinnai is known for manufacturing the best propane tankless heaters, which you can view here.

As an example, let’s look at the largest home propane tankless heater available.

That is an 11 GPM Rinnai RU199iP; the ‘199’ in the model number indicates that the heating output of this propane tankless heater is 199,000 BTU/hr when it is functioning at its maximum capacity.

The Rinnai RU199iP is the largest domestic propane-powered heater available, producing 199,000 BTU (the limit for residential use is 200,000 BTU).

We are aware that the combustion of propane inside such a device will result in a heating output of 199,000 BTU.

Additionally, we are aware that the combustion of one gallon of propane will result in the production of 91,452 BTU of heat.

With this information, we are able to compute the amount of propane that a large tankless propane heater consumes in an hour.

The calculation is as follows:

Propane Used In 1 Hour = 199,000 BTU (Heater Capacity) / 91,452 BTU per gallon = 2.18 Gallons of Propane 

This indicates that the largest tankless water heater powered by propane will use 2.18 gallons of propane per hour when operating at full capacity.

This is how we go about doing that:

You should be able to adjust these estimates for almost any tankless propane water heater you come across.

Naturally, we also need to take into consideration the fact that propane does not burn with an efficiency of one hundred percent.

The EF (Efficiency Factor) of each individual heater determines how efficiently the fuel is used.

Now, all of this may seem to be really complicated to you.

Because of this, we have made the process much easier by providing a table that summarizes the information below the propane use calculator as well as an easy-to-use calculator.

You are allowed to use it in the following places:

Calculator for the Consumption of Propane in Tankless Water Heaters

Simply entering the size of the unit (in BTU), its energy efficiency (which is typically around 95%), and the amount of time per day that you are likely to use the unit at its maximum capacity into a calculator will allow you to determine how much propane your tankless propane heater will consume in a given year.

The last one, which is “running minutes per day,” is the one that the vast majority of homeowners have the greatest trouble with.

About forty minutes of the typical day will be spent using the tankless propane heater to its full capability in a typical home.

Here is the calculator for your use.

You also have the option of playing about with the numbers a little bit:

How Much Propane Do You Use in a Year If You Have a Gas Tankless Water Heater?

Tankless heater capacity measured in British thermal units (for instance, 120000 BTU): 120,000

Efficiency in the Use of Energy (between 80 and 99%): 90

How Many Hours Does The Unit Operate Per Day (for Example, sixty minutes each day): 60

As can be seen, the quantity of propane that is used by a tankless propane water heater is dependent not only on its size and efficiency but also on the amount of water that is heated each day.

To make matters even more straightforward, we have computed some numbers for the most popular sizes of tankless heaters that run on propane and compiled them in the following table:

How Many Gallons of Propane Do Different Types of Tankless Propane Water Heaters Consume Each Year? (Table)

In this context, we do all computations assuming an efficiency of 95% on average:

Propane Tankless Unit Size (in BTU): Yearly Propane Usage For 20 Min/Day: Yearly Propane Usage For 40 Min/Day: Yearly Propane Usage For 60 Min/Day:
53,000 BTU (5.3 GPM) 74.22 Gallons/Year 148.44 Gallons/Year 222.66 Gallons/Year
65,000 BTU (6.5 GPM) 91.03 Gallons/Year 182.05 Gallons/Year 273.08 Gallons/Year
75,000 BTU (7.5 GPM) 105.03 Gallons/Year 210.06 Gallons/Year 315.09 Gallons/Year
94,000 BTU (9.8 GPM) 131.64 Gallons/Year 263.28 Gallons/Year 394.92 Gallons/Year
100,000 BTU (10.2 GPM) 140.04 Gallons/Year 280.08 Gallons/Year 420.12 Gallons/Year
130,000 BTU (10.4 GPM) 182.05 Gallons/Year 364.11 Gallons/Year 546.16 Gallons/Year
160,000 BTU (10.7 GPM) 224.07 Gallons/Year 448.13 Gallons/Year 672.20 Gallons/Year
199,000 BTU (11 GPM) 278.68 Gallons/Year 557.36 Gallons/Year 836.04 Gallons/Year

It is my hope that you have a better knowledge of how much propane on-demand propane water heaters utilize now that you have read this.

Check out this link for an additional helpful article that discusses the issue of determining what size on-demand heater you need.

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Written by Bob Matsuoka
Bob Matsuoka is a blogger and founder of RVing Beginner blog. He has been blogging for over five years, writing about his own family’s RV adventures, tips for people who are interested in buying an RV or taking their family on an adventure by RV.