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How Much Propane Does a Tankless Water Heater Use?

Writen by Ronald Brown

Fact checked by Natalie Bridges

How Much Propane Does-a-Tankless Water Heater Use

Have you ever wondered if a tankless water heater is indeed more practical than a storage-tank water heater system? You will have a more definite answer if you know how much propane does a tankless water heater use in a month.

On-demand, tankless water heaters offer convenience and economy that other systems may not provide. Find out how much you need to allot for your propane budget if you want to make the most out of your tankless water heater.

Table of Contents

Why You Need to Know How Much Propane Your Tankless Water Heater Uses


The US Department of Energy says that tankless water heaters can save households with heavy hot water consumption about 8 to 14 percent of their energy expenditure.

Families with low to moderate hot water consumption can enjoy 24% to 34% more energy savings than those with traditional water heaters with storage tanks.

Tankless water heaters are also more durable than conventional storage tank heaters, lasting more than 20 years. Their parts are also easily replaceable, further extending their service life to several more years. These characteristics make on-demand water heaters more economical products in the long term.

Because tankless water heaters are energy-efficient and cost-efficient systems, you would also like to know how propane use can affect these parameters. In particular, you would want to know how much money you need to allocate for propane purchases every month.

Propane budget planning allows you to allocate your financial resources, using energy savings on matters that can improve your family’s quality of life.

How Much Propane Gas Does a Tankless, On-demand Water Heater Use?

Tankless water heaters come in different energy ratings, expressed in British Thermal Units or BTUs. BTUs are the amount of heat a system needs to increase a pound of water by one degree Fahrenheit per hour. The higher a water heater’s BTU rating, the greater and faster its heating power.

Since we buy our propane in gallons, it is essential to determine how many BTUs are present in each propane unit we purchase.

According to the US Energy Information Administration (EIA), a gallon of propane has 91,452 BTUs. With this knowledge, you can compute the number of propane gallons your tankless water heater will use. You only need to divide your heater’s BTU rating by 91,452 BTUs.

For example, suppose you have a 40,000 BTU-rated heater. In that case, you divide 40,000 BTUs by 91,452 BTUs to get 0.437 gallons.

Factors to Consider When Calculating How Much Propane Your Tankless Water Heater Uses


Computing a tankless water heater’s propane gas requirement is not always accurate. Several factors can also affect propane requirements.

1. Number of Hours of Heater Use

The 0.437 gallons of propane we computed above is only for an hour of hot water use. You need to determine how many hours your family will use hot water in a day.

Suppose your household uses hot water for about three hours per day. In that case, you will be consuming 1.311 gallons of propane per day or about 39 to 40 gallons per month.

2. Environmental Temperature

Ambient temperature can have a profound effect on your system’s water-heating capabilities. If you live in a cold climate region, it may take a while for your tankless water heater to heat a gallon of water. The longer you use the water heater, the higher the propane consumption is.

Colder regions have colder water temperatures. For instance, if you live in Minnesota, your average groundwater temperature is about 37 to 42 degrees Fahrenheit. It would take more time for your heater to bring your water temperature to a comfortable 105 degrees to 110 degrees Fahrenheit.

Compare this to a friend living in Southern Alabama, with an average groundwater temperature of 67 degrees Fahrenheit. It takes a shorter time for your water heater to reach the ideal temperature of 105 degrees Fahrenheit.

3. Household Size

The bigger the household is, the higher its propane consumption is. For example, a two-person household can use about 15 gallons of propane per month. On the other hand, a fiver-member family may need between 35 and 40 gallons of propane monthly.

4. Heater Capacity

Tankless water heaters come with different capacities, expressed in gallons of water per minute. In general, the higher the capacity, the lower is the propane consumption.

For example, the Rinnai V65eP Outdoor Tankless Water Heater has a rated capacity of 6.5 gallons per minute. Other systems can provide you with two or three gallons per minute.

The V65eP is more efficient than other systems because it can heat 6.5 gallons every minute using the same amount of propane energy that other systems use to heat two to three gallons per minute.

5. Water Hardness Level

Hard water can shorten your tankless water heater’s lifespan and negatively impact energy efficiency. It creates an insulating layer between the water and the unit’s heat exchanger, extending the time needed to raise the water temperature.

In effect, hard water requires more propane gas because of the prolonged time of water heating. Adding a water softener to your system can address limescale formation in your water heater at an additional cost.

How Much Are You Spending on Propane to Power Your Tankless Water Heater?

As of February 2021, American families pay an average of $2.304 for every gallon of propane gas. Suppose your household needs about 20 gallons of propane per month. In that case, you can expect your monthly propane budget to be about $45 to $47.

A high-hot water-consuming family with an average monthly propane usage of about 50 gallons can expect to pay $115 to $120 per month.


Learning how much propane does a tankless water heater use gives you a chance to make a more realistic budget for your family. You can use the savings you get for other activities that your family can enjoy.

Computing your monthly propane consumption requires considering the number of hours of heater use, environmental temperature, household size, water hardness level, and your heater’s rated capacity.

On that note, we recommend the Rinnai V65eP because of its exceptional energy efficiency. You should be able to get the most out of your propane use with this tankless water heater.

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