When people start looking into gas tankless water heaters, one of the first things they wonder about is PROPANE CONSUMPTION. It’s a valid concern, as propane isn’t exactly cheap. The appeal of endless on-demand hot water and better energy efficiency is strong, but nobody wants to trade lower electricity costs for surprise bills.
The truth is, this isn’t the same for every household. In this article, we’ll break down what you can realistically expect when it comes to gas usage, so you’ll know whether a gas tankless system makes sense for your home and budget.
How Much Propane Is Needed For A Tankless Water Heater?
Propane usage depends on several factors, including the efficiency of the unit, how frequently hot water is used, and the temperature of the incoming water. Homes with higher daily demand or colder groundwater will naturally see greater gas consumption.
Even so, propane-powered tankless systems typically deliver better long-term savings and energy efficiency compared to electric models. I have made this calculator to easily estimate hourly, daily, monthly, and yearly propane usage and involved costs. Check it out here:
To understand the calculation, we first need to figure out how much propane a gas tankless water heater actually burns. This is straightforward, but before we get there, two key details are needed:
- BTU rating of the water heater: This shows how much heat energy the unit can produce per hour. BTU, or British Thermal Unit, measures how powerful your geyser is and how much energy it uses to heat water. More about BTU is here.
- Average runtime per day: how many hours the unit typically operates.
You’ll usually find the BTU rating listed on the product label or in the user manual. For example, the Rinnai RX130iN model has a BTU rating of 130,000.
For our calculation, we’ll consider this Rinnai water heater and use the standard energy content of propane: 91,452 BTUs per gallon (rounded here to 91,500 BTUs for easier math).
A. Propane Usage Per Hour
To calculate hourly consumption, divide the BTU rating of the heater by 91,500.
130,000 ÷ 91,500 = 1.42 gallons
So, this particular tankless heater uses about 1.42 gallons of propane every hour it’s running.
B. Propane Usage Per Day
Next, let’s factor in runtime. If the heater operates an average of 4 hours a day, the daily usage would be:
1.42 × 4 = ~6 gallons
If it only runs for 2 hours a day, then:
1.42 × 2 = ~2.84 gallons
C. Propane Usage Per Month
Extend that daily number over 30 days:
- 4 hours daily → 6 × 30 = 180 gallons per month
- 2 hours daily → 3 × 30 = 90 gallons per month
D. Propane Usage Per Year
Now for the yearly numbers:
- 4 hours daily → 6 × 365 = 2,190 gallons per year
- 2 hours daily → 3 × 365 = 1,095 gallons per year
Cost Estimate
With propane averaging about $2.50 per gallon in the U.S., that works out to:
- $2,730 per year (running 4 hours daily)
- $1,370 per year (running 2 hours daily)
These numbers aren’t exact, of course. They’re estimates to give you a clear picture of how propane use is calculated. Your household’s actual usage will vary based on demand, water temperature, climate, and the efficiency of your particular model.
How Long Will 100 Gallons Of Propane Last For A Water Heater?
This is a bit of a tricky question, because there’s no fixed timeline for how long 100 gallons of propane will last in a tankless water heater. It really depends on a few CORE FACTORS—most importantly, the efficiency of your unit. Some geysers are simply better at squeezing more heat out of every gallon of gas. Beyond that, your household’s daily routine and local climate conditions also play a big role. We’ll look at these factors more closely in the next section.
That said, we can still make a rough estimate based on the example we used earlier. If your heater runs about 2 hours a day, 100 gallons of propane should last around 33 to 35 days. If it’s used more heavily, that number shrinks. For instance, with 4 hours of daily use, you’d be looking at closer to 15 to 17 days before the tank runs dry.
Factors Influencing Propane Usage
We’ve finally reached the section I mentioned earlier.
I often hear from people who are surprised that their actual gas usage doesn’t quite match the estimates they calculated. And that’s perfectly normal. Those numbers are just estimates, not guarantees. Real-world propane consumption is shaped by a range of everyday variables that can shift the final outcome quite a bit.
Here are the main factors that influence usage:
Duration Of Use: This one’s obvious, but it’s often overlooked. The more often you and your family use the tankless heater, the MORE propane it burns. For example, daily usage tends to spike in the winter when hot water demand naturally goes up, which can drive propane costs higher than in summer. In other words, your estimates should always factor in seasonal and lifestyle variations if you want them to be realistic.
BTU Rating: Gas tankless water geysers are rated by BTUs, or how much heat they can produce in an hour. Typical units range from 120,000 to 199,000 BTUs. A higher BTU model can deliver more hot water at once—great for bigger households with higher demand—but it also uses more gas when running at full capacity, sometimes as much as 2 to 3 gallons per hour.
Water Flow Rate: Tankless heaters aren’t measured by tank size (since they don’t have one!) but by water flow rate, expressed in gallons per minute (GPM). The higher the GPM, the harder the heater works to maintain water temperature, which means higher propane consumption. For example, a unit rated at 11 GPM will burn through more fuel to hold the same temperature compared to one rated at 2.7 GPM.
Household Size: The number of people under your roof plays a huge role in propane usage. When it was just me and my partner, our consumption was manageable. But once more, people moved in, the numbers shot up quickly. Daily routines are key here—small adjustments like shorter showers or spacing out laundry cycles can noticeably reduce propane costs.
Climate Conditions: Your location also matters. In colder regions, tankless heaters need to work harder and longer to heat up colder incoming water, which increases gas use. Plus, since on-demand geysers rely on outside air for combustion, colder air can make the system less efficient. So, if you live in a chilly area, expect higher bills. The good news? Even in colder climates, propane tankless heaters usually remain more cost-efficient than electric models.
Hard Water: If you live in an area with hard water, mineral buildup can reduce your heater’s efficiency. That forces the unit to work harder and burn more gas to produce the same amount of hot water. It can also increase heat-up time, further raising fuel consumption.
Does A Tankless Water Heater Use More Gas Than A Tank?
In my experience, a tankless water heater tends to use less gas overall than a traditional tank model, even though it burns hotter and faster when it’s running.
The difference comes down to HOW THEY OPERATE. A storage tank heater keeps water hot around the clock, which means it’s constantly burning energy—even when no one is using hot water. A tankless system, on the other hand, only fires up when you turn on the tap. That on-demand setup is what makes the real savings possible over time.
What’s surprising to many is the BTU rating. Gas tankless units can reach as high as 199,000 BTUs, compared to just 30,000–50,000 BTUs for a standard tank geyser. They do burn more fuel per minute, but only while in use.
With a propane-powered tankless heater, you’ll likely notice your gas consumption dropping because it only runs when needed. The key is choosing the right size unit so you get endless hot water on demand—without unnecessary waste.
Is Propane Or Natural Gas Better For Tankless Water Heaters?
Both propane and natural gas pair well with tankless water heaters, but I usually lean toward gas because it has some clear advantages. For one, it burns HOTTER than natural gas, which makes it especially efficient for tankless systems that need to deliver quick, on-demand heat. Over the years, I’ve installed propane tankless units not just in city homes, but also in off-grid cabins and rural houses, and they’ve always provided steady, reliable performance.
Now, you might point out that natural gas is cheaper—and that’s true. Per BTU, it often costs less and is very convenient if you already have a utility connection.
But propane wins on FLEXIBILITY. You don’t need to be tied to a gas line, so it works in more locations. It’s widely available, easy to store, and can power other appliances too—things like dryers, stoves, or even backup generators. In colder regions, where incoming water is icy and demands more heating power, I’ve found gas tankless units to hold up really well.
On top of that, propane burns CLEANER than natural gas, which makes it a more eco-friendly choice if you’re mindful of your home’s carbon footprint. And when your system is sized properly, you get the best of both worlds: unlimited hot water on demand, great efficiency, and the freedom to set it up wherever it fits your lifestyle.
Pro Tips To Minimize Propane Consumption
If you’re using a gas tankless water heater, there are a few smart ways to cut down on propane use without giving up comfort –
Pick a tankless unit that matches your peak demand, not those rare worst-case scenarios. An oversized system wastes fuel, while an undersized one works overtime and ends up using more propane.
Keep the hot water temperature around 120°F instead of 140°F. This eases the strain on your system and lowers gas consumption.
If you have hard water, consider installing a water softener. It prevents sediment buildup inside the geyser and keeps propane usage in check.
Use cold water for laundry and rinsing dishes whenever you can instead of relying on hot water.
Don’t skip professional maintenance! I flush mine once a year and tell my clients to do the same, at least annually, to keep the burner efficient and bills under control.
Despite propane being a cleaner and more efficient fuel than natural gas, I’ve noticed many homeowners hesitate to choose it—mainly because of the higher upfront cost. True, it can be more expensive per gallon, but in the long run, how much you actually use with a tankless water heater depends on smart sizing, mindful usage, and, of course, regular maintenance.
From my own experience at home and with clients, the difference really shows over time—not just in fuel savings, but also in comfort and reliability.
Use your gas tankless water heater wisely, and it will reward you with endless hot showers and noticeably lower bills for years to come.
