How Much Propane Does a Tankless Water Heater Use?

Propane tankless water heaters are a practical option for homes without natural gas service, providing hot water only when you need it. But when you're planning fuel costs or deciding on a propane tank size, one question naturally comes up: how much propane does a tankless water heater actually use?

There isn't one number that fits every home. Propane use depends mainly on how much hot water your household uses, the temperature of the incoming water, and the efficiency of the water heater.

For a practical example, assume a household takes four eight-minute showers per day—about the average shower duration reported by EPA WaterSense—using a 2 GPM showerhead. With incoming water around 50°F and mixed shower water around 105°F, those showers would use roughly 12 gallons of propane over 30 days, based on an 80% fuel-to-water efficiency assumption.

At an example propane price of $3 per gallon, that's about $36 for showering over 30 days. Other hot-water use, such as sinks, baths, and laundry, would add to the total.

What Does the BTU Rating Tell You?

A tankless water heater's maximum gas input rating, expressed in BTU/h, tells you how quickly it can consume fuel at full input. According to the U.S. Energy Information Administration, one US gallon of propane contains about 91,500 BTU.

Here are examples of propane consumption at different maximum gas input ratings:

The important part is at full input.

Unlike a storage water heater, a tankless unit doesn't keep a tank of water hot throughout the day. It heats water as it flows through the unit, so propane is used when there is a demand for hot water.

A 180,000 BTU/h heater may be capable of burning nearly two gallons of propane in an hour at maximum gas input, but that doesn't mean it uses two gallons every hour. Tankless water heaters fire when hot water is running, and many models adjust their firing rate to match demand.

That's why the BTU rating is useful for understanding maximum capacity, but it isn't a good way to predict your monthly propane use on its own.

Why Propane Use Varies From Home to Home

Your hot-water habits have the biggest effect on propane consumption. Longer showers, more people in the home, higher-flow fixtures, and frequent hot-water use will naturally require more fuel.

Incoming-water temperature also matters. A water heater receiving 40°F water in winter has to add more heat than one receiving 60°F water, even if both households take the same number of showers. This is why propane use may rise during colder months without any change in your daily routine.

Heater efficiency and your desired hot-water temperature make a difference as well. In general, the more water you need to heat—and the greater the temperature rise—the more propane the heater will use.

A higher BTU rating, however, doesn't automatically mean higher monthly propane consumption. A larger heater simply has more heating capacity available when demand is high. Homes with multiple bathrooms or several fixtures running at once may need that additional capacity.

What Affects Propane Tank Size and Refill Timing?

If your propane tank also supplies a furnace, range, fireplace, dryer, or generator, all of those appliances contribute to your total fuel use. This is especially noticeable in winter, when space heating may account for much more propane than the water heater.

Tank size should also account for the combined BTU demand of connected appliances, not just monthly fuel consumption. Your propane supplier can help determine an appropriate tank and regulator setup and recommend when to schedule refills.

Choosing a MIZUDO Propane Tankless Water Heater

Rather than choosing a tankless water heater based only on the lowest BTU rating, start with how much hot water your household needs at the same time.

A home where one shower is normally used at a time has different requirements from a household running two showers and a kitchen faucet simultaneously. Incoming-water temperature, the number of bathrooms, installation location, and available propane supply should all be considered.

Explore MIZUDO propane tankless water heaters to compare available capacities and installation options.

If you're still deciding which fuel type is right for your home, read our guide to Natural Gas or Propane Tankless Water Heaters.

Frequently Asked Questions

How much propane does a tankless water heater use per day?

Under the showering assumptions above, propane use would be approximately 0.4 US gallon per day for showers alone. Other hot-water uses would add to that amount.

Does a tankless water heater use less propane than a tank water heater?

It can. Tankless water heaters heat water on demand rather than continuously keeping stored water hot, which avoids the standby heat loss associated with traditional storage tanks. Actual fuel savings depend on the efficiency of the units being compared and your household's hot-water use.