Propane tankless water heaters are the true propane guzzlers (still about 41% more economic than conventional tank water heaters). You will hardly find a propane-power appliance that uses more propane than these on-demand heaters. How much propane does a tankless water heater use exactly?
The quantity of propane (and corresponding dollar expenditure) depends on 3 key factors. These are:
- Size of a tankless propane tank. An 11 GPM 199,000 BTU propane tankless heater will obviously use more propane than a 7 GPM 75,000 BTU unit, for example.
- Hot water generation. A propane tankless water heater that runs for 1h/day will spend more than the same unit running for 20 min/day.
- Energy efficiency. Propane on-demand hot water heaters are usually highly efficient. Propane burning efficiency (denoted by the EF factor) starts at 80%. The most energy-efficient propane tankless heaters have up to 99% energy efficiency (0.99 EF); these units use the least amount of propane for the highest hot water output.
Depending on these 3 factors, we can calculate how much propane does a tankless propane heater use. Here is the summarized outtake that illustrates propane use (we presume 100% output, 100% energy efficiency, and $2.41 average national price per gallon of propane):
- Smallest 53,000 BTU unit (low end): 0.58 gallons of propane per running hour. That’s a $1.40/hour running cost.
- Biggest 199,000 BTU unit (high end): 2.18 gallons of propane per running hour. That’s a $5.25/hour running cost.
If you look at Energy Star labels, the standardized annual propane tankless heater usage is set at 200 gallons. That’s about $482 per year. There is a wide range of how much it actually costs to run a propane tankless water heater per year.
According to our calculations, here are the low-end and high-end costs:
- Low-end (53,000 BTU unit, 20 min/day, 0.99 EF): 71.22 gallons/year. That’s about $171.64 per year.
- High-end (199,000 BTU unit, 60 min/day, 0.80 EF): 992.80 gallons/year. That’s about $2,392.65 per year.
That doesn’t tell us a lot about how much propane does our own propane tankless water heater use, right?
That’s because some households have small units, others have bigger units, some households use these propane on-demand water heaters for 1 hour per day while others use them for just 20 minutes per day.
To help everybody adequately estimate the propane usage of their own unit, we have created a Tankless Propane Usage calculator. You insert the size of your unit in BTU, energy efficiency, and how roughly how many minutes per day you run your unit. You will find the calculator further on.
Here’s what the calculation for a 95,000 BTU (9.8 GPM), 0.95 EF propane tankless heater used 40 min/day looks like:
On top of that, we have calculated how much propane do different sized propane tankless heaters use per year, if you use them 20 minutes/day, 40 minutes/day, and 60 minutes/day (results summarized in the table after the calculation). That will give you a ballpark figure of how many gallons of propane does your tankless propane use per year.
Let’s first see how everybody can calculate how much propane does the instantaneous water heater use:
How To Calculate Propane Usage For Tankless Water Heater?
Water is notoriously hard to heat up. That is due to high specific heat (4.19 kJ/kg°C). Propane tankless heaters can heat more than 10+ GPM of water in less than 15 seconds. That requires a huge amount of energy; all of that energy is provided by burning propane.
To calculate how much propane these on-demand heaters burn through to heat up water, we need to know what is the energy content of propane.
The US Energy Information Administration helps us out here with this energy content: 1 gallon of propane = 91,452 BTU. That means that burning a gallon of propane produces 91,452 BTU (at 100% efficiency).
Now, we have different sizes of tankless propane water heaters. If we take a look at Rinnai units (Rinnai produces the best propane tankless heaters; you can check them out here), we see they offer units ranging from 5.3 GPM (these units produce 53,000 BTU heating output at 100% capacity) to 11 GPM (these units produce 199,000 BTU heating output at 100% capacity).
Let’s take the biggest residential propane tankless heater as an example. That’s an 11 GPM Rinnai RU199iP; this ‘199’ in the model number tells you that this propane tankless heater generates 199,000 BTU/hr heating output when operating at 100% capacity.
We know that such a unit will produce 199,000 BTU of heating output by burning propane. We also know that burning 1 gallon of propane will produce 91,452 BTU of heat.
With this, we can calculate how much propane does such a big tankless propane heater use per hour. Here’s the calculation:
Propane Used In 1 Hour = 199,000 BTU (Heater Capacity) / 91,452 BTU per gallon = 2.18 Gallons of Propane
That means that the biggest propane tankless water heater will burn 2.18 gallons of propane per hour (at 100% output). We can also calculate how much does running such a propane heater costs per hour simply by multiplying the propane usage and propane cost per gallon ($2.41 is the natural average). Here’s how we do that:
Running Cost Per Hour = 2.18 gall/h (Propane usage) × $2.41/gall (Propane cost) = $5.25/h
You can redo these calculations pretty much for any tankless propane water heater. Of course, we also have to take into account that propane doesn’t burn at 100% efficiency. Burning efficiency depends on the EF (Efficiency Factor) of the individual heater.
Now, all this might sound a bit complex. That’s why we simplified the whole thing with an easy-to-use propane usage calculator and a summarized table below the calculator.
You can freely use it here:
Tankless Propane Water Heater Usage Calculator
To calculate how much propane your tankless propane heater is estimated to burn through in a year, you just insert the size of the unit (in BTU), energy efficiency (it’s usually around 95%), and how long per day are you likely to use the unit at 100% capacity.
This last one – running minutes per day – is what the majority of homeowners struggle with the most. A standard household will use the tankless propane heater (at 100% capacity) for about 40 minutes per day.
Here’s the calculator. You can play around with the numbers a bit as well:
As you can see, the amount of propane the tankless propane water heater uses depends on the size, efficiency, and how much per day you use it.
To make things even simpler, we have calculated some values for the most common sizes of propane tankless heaters, and gathered them in this table:
How Much Propane Do Various Tankless Propane Water Heaters Use Per Year (Table)
Here we use average 95% efficiency for all calculations:
|Propane Tankless Unit Size (in BTU):||Yearly Propane Usage For 20 Min/Day:||Yearly Propane Usage For 40 Min/Day:||Yearly Propane Usage For 60 Min/Day:|
|53,000 BTU (5.3 GPM)||74.22 Gallons/Year||148.44 Gallons/Year||222.66 Gallons/Year|
|65,000 BTU (6.5 GPM)||91.03 Gallons/Year||182.05 Gallons/Year||273.08 Gallons/Year|
|75,000 BTU (7.5 GPM)||105.03 Gallons/Year||210.06 Gallons/Year||315.09 Gallons/Year|
|94,000 BTU (9.8 GPM)||131.64 Gallons/Year||263.28 Gallons/Year||394.92 Gallons/Year|
|100,000 BTU (10.2 GPM)||140.04 Gallons/Year||280.08 Gallons/Year||420.12 Gallons/Year|
|130,000 BTU (10.4 GPM)||182.05 Gallons/Year||364.11 Gallons/Year||546.16 Gallons/Year|
|160,000 BTU (10.7 GPM)||224.07 Gallons/Year||448.13 Gallons/Year||672.20 Gallons/Year|
|199,000 BTU (11 GPM)||278.68 Gallons/Year||557.36 Gallons/Year||836.04 Gallons/Year|
Hopefully, now you have a better understanding of how much propane do on-demand propane water heaters use. Another useful article covers the topic of what size on-demand heater you need; you can check it out here.
If you have any questions about your specific unit, you can use the comments for that and we’ll try to help you out.