How Much Propane Does a Tankless Water Heater Use?

Last Updated On October 19, 2021
How Much Propane Does a Tankless Water Heater Use

Do you want to monitor the propane usage of your tankless water heater to manage your gas bill and ensure you have enough fuel on hand? 

Propane gas tankless water heaters provide an excellent water heating option. However, it is best to plan appropriately by learning how much propane you should expect to use and how much it will cost. 

In this PlumbingNav guide, you will learn:

  • What are the differences between propane gas vs. natural gas water heaters?
  • How many showers can you get from one full propane tank?
  • How do I calculate how much propane a tankless water heater will use?

What's In This Guide?

      What You Need To Know About Using Propane vs. Natural Gas Tankless Water Heating 

      When you purchase a gas tankless water heater, you have the choice between natural gas and propane gas. Many people mistakenly assume natural gas and propane refer to the same thing. However, natural gas and propane have different qualities that make them unique. We cover the cost of those and other tankless units here.

      Natural gas tends to overshadow propane since it costs significantly less to burn due to its abundance and higher BTU levels when used to heat a water heater. The lower cost of natural gas makes it the most popular option, but propane provides benefits that make it the more desirable choice in many situations. 

      Propane has a higher octane level, meaning less maintenance since the tankless water heater stays clean longer. It also falls under the category of an “alternative fuel” under the Energy Policy Act of 1992, meaning it emits fewer toxins into the universe.

      It also burns more efficiently, making up for the higher cost. Despite the benefits, propane only makes up about 2% of the fuel used in the United States. 

      Looking for Actual Units? We have written extensively and offer detailed posts covering major tankless brands and those best “by use” for your convenience. See below:

      How To Calculate How Much Propane a Tankless Water Heater Will Use (5 Steps)

      You need to know how much propane your tankless water heater will use to ensure you have enough propane on hand and budget accordingly. Naturally, this varies from household to household based on your water heater and water usage habits. 

      We will help you accurately calculate how much propane you need for your water heater in five easy steps. 

      WARNING: MATH AHEAD!

      (Read the detailed explanation or skip to the bottom for the simplified answer)

      Step 1: Understand Propane Measurements

      People measure propane in gallons. You can have people deliver propane tanks to your home, or you can fill up your propane tank yourself at hardware stores and gas stations. Learn the size of your propane tank and keep track of how much propane you have at any given time.

      Propane does not come with a short-term expiration date, so feel free to stock up for the year (once you finish your calculations). 

      Step 2: Learn Your Water Heater 

      Your tankless propane water heater runs on British Thermal Units (BTUs). BTUs measure heat, and the heat from the water heater burns the propane to heat your water. 90,500 BTUs use up one gallon of propane

      Now, you need to read up on how many BTUs your specific propane tankless water heater uses. The higher the BTUs, the more hot water. However, you will use up the propane more quickly. 

      For example, the Rinnai V65iP tankless propane water heater has a maximum capacity of 150,000 BTUs/hr. If you run the water for an hour straight, you will use about 1.65 gallons of propane in an hour. Of course, you likely won’t have the hot water running for an hour straight.

      did you know how much propane a tankless water heater uses

      Step 3: Analyze Hot Water Usage

      Your hot water usage varies depending on the number of people in your home and everyone’s habits. 

      Ask everyone in the household to come together to analyze your combined hot water usage by asking everyone to write down how many showers they took during a week and how long each shower took. Also ask them to record how many times they used the washing machine or dishwasher. 

      Don’t forget to add to your final calculations for hot water from the sink. If you have the exact information about your appliances, great! 

      Otherwise, for standard appliances, use the following to estimate water usage:

      • 8 minute shower – 16 gallons
      • Full tub – 36 gallons
      • Dishwasher – 6 – 16 gallons
      • Laundry machine – 25 gallons
      • Sink usage – 1 gallon

      For a family of four, we will assume each person will take a shower every day. We will also assume that each person will use the hot water in the sink once a day. Finally, we will assume that they run the efficient dishwasher and laundry machine every day. Using the estimates above, this family uses 99 gallons of water in a day (but let’s round up to 100 gallons per day). 

      Calculating water usage can get tedious, and people don’t always stick to a schedule. Luckily, many water heaters offer a solution. 

      If your water heater has the technology, it can monitor water usage for you. Most new water heaters allow you to connect it to WiFi and view your numbers through an app. You can check the app whenever you want, and you can have regular reports sent to you. However, this only applies to newer models equipped with advanced features. 

      Step 4: Determine GPM of Water Heater 

      The gallons per minute (GPM) of your water heater varies based on the temperature rise. The rise refers to the water temperature as it enters the water heater and how much you need to heat it. 

      According to the Department of Energy, water usually comes into the water heater at 50° F. Most homeowners and professional plumbers set the water heater at 120° F. This means, on average, the water heater needs to heat the water to 70° F. 

      The Rinnai unit in our example has a maximum water flow rating of 6.5 GPM. However, it won’t always provide those water flow levels. A quick examination of the product’s specs informs us that ground temperature water of around 50° F produces a lower water flow rate of 4.3. Warmer climates will have a higher GPM than colder climates.

      Step 5: Do The Math

      Sorry, but we do have to do some math to get to the bottom of how much propane your tankless propane water heater will use. 

      To make it as easy as possible, let’s recap the numbers from the water usage from our example:

      • 100 gallons of hot water per day
      • 1 BTU heats a gallon of water 1° F
      • Water temperature rise of 70° F with 4.3 gpm flow rate
      • Water heater runs at 150,000 BTUs/hr 
      • 90,500 BTUs = 1 gallon of propane

      4.3 GPM generates 258 gallons of hot water in one hour, but for our example we only need 100 gallons. So, it will generate the required amount of water in 24 minutes. (24 x 4.3 = 104 gallons). 

      BTUs generated by the water heater (150,000) divided by BTUs in one gallon of propane (90,500), then multiplied by hours the water heater will be in use (24 minutes or .4 hours), equals propane used in a day (.66 gallons).

      Gallons of propane per day (.66) x 30 = propane usage per month (19.8 gallons)

      Gallons of propane per day (.66) x 365 = propane usage per year (240.9 gallons)

      These calculations assume that everything is running at top efficiency. To ensure that you have enough propane for all of your needs, round up. Remember, propane doesn’t expire.

      How Many Hot Showers Can You Get From One Full Propane Tank?

      Tankless units provide endless hot water, but only as long as you have enough propane to fuel the water heater. 

      You’ve bought your propane tank. Now, how long will it last? Many people like to get the answer in the amount of showers. 

      The answer depends on two main variables:

      • Shower Habits
      • Tank size

      Shower Habits

      First, we need to establish the length of each shower and how much water you use during that time. The average showerhead provides 2.1 gallons of water per minute, and the average shower lasts 8 minutes. This means you can expect to use just above 16 gallons of water during one shower. 

      Note: You may need to double the number of gallons per shower for people who like to stand under the hot water and sing (poorly) for 15 – 20 minutes every morning. 

      Tank Size

      Next, we need to clarify the size of the propane tank. Most homes use either a 40 gallon or 100 gallon tank. The burn rate refers to how much time (in hours) it takes to use up the propane tank. To calculate the burn rate, you will use the information from the metrics we already established.

      Take the amount of BTUs required to use up a pound of propane (90,500) and divide it by the BTUs of the water heater (150,000), then multiply it by the size of the propane tank (40 gallons/100 gallons). 

      Burn rate for 40 gallon tank – 24 hours

      Burn rate for 100 gallon tank  – 60 hours

      As a general rule, you want to order more propane when your tank gets down to about 20%. 

      Still confused? Don’t worry! Read the following questions for simplified answers!

      Frequently Asked Questions (FAQs)

      See these simplified answers to your questions about how much fuel your water heater will use and how much that will cost. 

      How much propane does a propane tankless water heater use?

      Most families use an estimate of one gallon of propane per day. One gallon of propane should supply more than enough for a four person household. Adjust this number based on the number of people in your home. 

      How much gas does a tankless gas water heater use?

      We measure natural gas in therms, which equates to 100,000 BTUs. A family using 100 gallons of water a day will utilize about 395 therms per year to heat a gas water heater or about 1.1 therms per day. With the price of natural gas at $2.62 in March of 2021, you can expect to spend $2.88 a day or $1,034.90 a year to heat a gas water tank. 

      How much propane does a tankless water heater use in a year?

      Most families need about 350 gallons of propane every year.

      Want to use less propane when running your water heater? To help reduce your propane expenses, start by purchasing a properly sized and energy-efficient water heater and energy-efficient fixtures on your sinks and showers. 

      Next, you should ask about buying propane in bulk, especially when propane costs are low. 

      Finally, reduce how much you use your water heater by taking shorter showers and drying clothes outside on warm days.

      How much does it cost to run a propane tankless water heater?

      Propane costs vary greatly based on several variables, including time of year and location. Propane tends to cost less in the summer months. Also, the Midwest has the lowest regional propane rates, while New England has the highest regional rates. 

      To determine how much you will spend, multiply the amount of propane you will use by the cost. 

      For example, Americans paid an average of $2.378 per gallon in March of 2021. Someone who uses 350 gallons of propane per year can expect to pay roughly $832.30.

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      I'm passionate about all things plumbing, and love sharing tips, "how-to", and reviewing the latest products to help make your project a success!

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      I'm passionate about all things plumbing, and love sharing tips, "how-to", and reviewing the latest products to help make your project a success!

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