How Much Propane Does a Tankless Water Heater Use? Let’s Do the Actual Math

How much propane does a tankless water heater use? Depends who you ask, and that’s the problem. I pulled up eight pages on this and got answers ranging from 15 gallons a month to 45 gallons a month for the same size household, with running costs from $30 to $107.

That’s a four times spread on a question with a definite answer. So I sat down and worked it out properly, and it turns out most of those numbers are wrong in the same direction for the same reason.

If you’re on a meter and a monthly bill, being out by three times is annoying. If you’re hauling cylinders up a dirt road in February, it’s the difference between two deliveries a year and five.

The formula, because this is just arithmetic

No mystery here. Water takes a fixed amount of energy to heat and propane holds a fixed amount of energy.

A gallon of water weighs 8.33 pounds, and one BTU raises one pound of water by one degree Fahrenheit. A gallon of propane holds 91,452 BTU according to the EIA’s conversion figures, which everyone rounds to 91,500 and so will I.

Propane gallons per day = (hot water gallons × temperature rise × 8.33) ÷ (91,500 × efficiency)

Temperature rise is your target temperature minus whatever comes out of the ground. Efficiency is the unit’s UEF rating, usually somewhere between 0.82 and 0.96 depending on whether it’s condensing.

That’s the whole thing. Three numbers you can look up and one you can measure.

Where the published numbers go wrong

Here’s what I found when I actually checked one.

One site walks through a family of four using 60 gallons of hot water a day, a 65 degree rise, and a 0.93 efficiency unit. They publish the answer as 1.27 gallons a day, 38 gallons a month, $107 a month.

Run their own inputs through their own formula. 60 × 65 × 8.33 = 32,487 BTU a day. Divide by 91,500 × 0.93 and you get 0.38 gallons a day. About 11.5 a month.

They’re out by more than three times. Same formula, same numbers, different answer.

The other common claim is that a 40,000 BTU unit burns about 1.5 gallons a day. Check that one too. A gallon and a half is roughly 137,000 BTU, and at 40,000 BTU an hour that means the burner ran for three and a half hours. Nobody’s tankless heater runs for three and a half hours a day. Sixty gallons of hot water at a couple of gallons a minute is under half an hour of actual burn time.

That’s the error, and it’s everywhere. People take the full fire burn rate off the spec sheet and multiply it by a runtime that belongs to a tank heater, not a tankless one. The whole point of tankless is that it only runs when the tap is open.

What it actually costs

Running the formula at a 70 degree rise with a 0.90 efficiency unit:

Household	Hot water a day	Propane a day	Propane a month	At $3 a gallon
1 to 2 people	30 gallons	0.21 gal	about 6 gal	$19
3 to 4 people	60 gallons	0.42 gal	about 13 gal	$38
5 to 6 people	90 gallons	0.64 gal	about 19 gal	$57

I’ll be straight about what these are. That’s my arithmetic from published constants, not metered data from a house. The inputs are all checkable and the maths is above, so run your own numbers with your own household and your own propane price.

For what it’s worth, the only published figure that came close was a propane dealer quoting about $30 a month for a family of four. The dealer who actually delivers the stuff was the one who had it roughly right, which tells you something.

Winter changes it more than household size does

Same house, same habits, same heater. Only the ground temperature changes.

At a 50 degree rise in August, that family of four burns 0.30 gallons a day. At an 80 degree rise in January, 0.49.

That’s 63 percent more propane in winter, from nothing but colder incoming water. Which is why people start googling whether their heater is broken every February. It isn’t. It’s doing more work because the water arriving is colder, and a tankless unit can’t hide that the way a tank in a warm basement can.

Size your delivery schedule on the January number, not the annual average.

What this means if you’re on cylinders

Here’s where it stops being trivia.

A 100 lb cylinder holds about 23.6 gallons of propane. That family of four, on water heating alone, empties one in under two months. Add a range and a fridge and you’re refilling monthly.

Which is fine if a truck comes to you. Less fine if you’re loading cylinders into a pickup and driving forty minutes each way in the snow.

And there’s a harder limit underneath that, which is the thing I’d most want you to know before you buy a unit.

Your cylinders might not keep up, and it has nothing to do with how much is in them

Propane has to boil out of the liquid before it reaches the burner, and how fast it can do that depends on the temperature outside and how much liquid is left.

A full 100 lb cylinder delivers around 167,000 BTU an hour at 20 degrees. At zero degrees, the same full cylinder manages about 113,000. The working rule of thumb is roughly 75,000 BTU per cylinder.

Now look at what a whole house tankless wants at full fire. 150,000 to 199,000 BTU an hour.

So a unit that runs beautifully in September can starve in January, and it won’t look broken. It’ll just go lukewarm halfway through a shower. And it gets worse as the cylinder empties, because less liquid means less surface area to boil from, which means the problem shows up exactly when you’re waiting on a delivery.

If you’re running a whole house unit off cylinders, manifold enough of them to cover full fire at your coldest expected temperature, or get a proper tank. I went through the rest of this trade off in the tankless vs tank water heater guide, including why an electric tankless is simply off the table off grid.

Altitude, if you’re up high

One source puts the derate at roughly 4 percent for every 1,000 feet above 2,000 feet, which would mean a unit in Denver burning about 12 percent more propane than the same one at sea level.

I only found that figure in one place so I’d treat it as a rough guide rather than gospel, but the direction is right and thin air is a real thing. If you’re at elevation, check the manufacturer’s high altitude specification, because most units need a conversion kit above a certain height anyway.

Does tankless actually use less than a tank?

Yes, though less dramatically than the marketing suggests.

The Department of Energy puts tankless at 24 to 34 percent more efficient for households using 41 gallons a day or less, dropping to 8 to 14 percent for heavy users. One analysis puts the propane reduction at 20 to 40 percent for the same household.

The reason is standby loss. A tank keeps 40 or 50 gallons hot around the clock whether you’re home or not, so it fires to maintain temperature even while you’re at work. Tankless doesn’t. That’s a bigger deal on a seasonal cabin than in a full time house, because a tank sitting hot in an empty building is burning propane for nobody.

Worth thinking about your propane load as a whole rather than one appliance at a time, especially if you’re also running an absorption fridge. I’ve broken that one down in the propane refrigerator guide.

The only number you can trust is your own

Everything above is arithmetic. Useful arithmetic, but still a model.

The real number comes from measuring, and measuring propane is annoying because the tank is outside, the gauge is a percentage rather than a quantity, and nobody walks out in the rain to check it.

Which is why tank level sensors exist. They magnet onto the base of the tank, read the level ultrasonically and send it to your phone, with long range versions for tanks well away from the house. Cheap, and they turn a guess into a number.

If you fit one, do this. Note the level, run a normal month, note it again. That single figure beats every estimate on the internet including mine, because it’s your house, your water temperature, your showers and your unit.

Frequently asked questions

How much propane does a tankless water heater use? For a household of three or four, roughly 0.4 gallons a day at a 70 degree temperature rise. One or two people is closer to 0.2, and five or six around 0.6. Cold incoming water pushes all of those up.

How much propane will a tankless water heater use per month? About 6 gallons for one or two people, 13 for three or four, and 19 for five or six, at typical winter temperatures. At $3 a gallon that’s $19, $38 and $57.

How much propane does a tankless water heater use per hour? At full fire, a whole house unit burns roughly 1.5 to 2.2 gallons an hour. But that’s the wrong question, because it almost never runs at full fire and it almost never runs for a full hour. What matters is total runtime, which is usually under half an hour a day.

Can you run a tankless water heater on propane? Yes, and most off-grid installs do. Just check the unit is configured for propane rather than natural gas, since they’re not interchangeable without a conversion kit, and check your supply can vaporize enough on the coldest day.

How long will a 100 lb propane tank last on a tankless water heater? Around two months for a family of four on water heating alone. Less if you’re also cooking or running a fridge on it, and less again in winter.

Why is my propane usage suddenly higher? Usually the season. Colder groundwater means a bigger temperature rise means more gas, and 60 percent more in January than August is normal rather than a fault.

Where to go from here

Work out your own number with the formula rather than trusting anyone’s table, mine included. You need three things: how much hot water your household uses daily, your coldest incoming water temperature, and your unit’s efficiency rating.

Then check your supply can actually deliver at full fire in January, because that’s the failure that looks like a broken heater and isn’t.

How much propane does a tankless water heater use in your house? Only your own meter knows, and the formula gets you close enough to plan around. I break down every line item, with three real world quote scenarios, in my tankless water heater cost guide.

And if you want to stop guessing entirely, measure it for a month. It’s the only honest number there is.

Next:

• Tankless vs Tank Water Heater for which one actually suits an off-grid build
• How Does a Propane Refrigerator Work for the other appliance quietly eating your tank
• Off-Grid Cost Calculator for putting water heating next to everything else you’re running