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.
Interesting, but the formula doesn’t adjusting for GPM water flowthrough.
The lowest GPM described above is about 5GPM, but I’m looking at 3.7GPM which is quite common for smaller tankless heaters.
How will that impact consumption
I’m looking at 97k BTU, 3.7 GPM, 15 minutes a day (one or two person, mainly for shower only) and I don’t know the efficiency but I imagine a typical 95% for propane.
Hello Herb, it’s true, you need adequately sized pipes in order to allow for sufficient water flow. Well, a smaller tankless heater will use less propane if you compare energy expenditure at a 100% output rate. The relationship here is linear; you will need 26% less propane when running a 3.7 GPM unit vs 5 GPM unit at 100% output. Hope this helps.
We have an Rinnai R85, suddenly no power. We do have 120 voltage at unit terminals and both fuses good.
We ran out of propane as well.
Would that have any impact, ie: if no fuel flows, power disabled??
Hi Bobby, gas tankless water heater not working after the power outage is usually associated with a pilot light. In many cases, the pilot light has gone out and you have to reignite it. Obviously, you will need propane as fuel as well.
I am considering using a tankless water heater to heat my pool.
The temperature is 56 and I would like to have it at 80
I can use a sump pump for inlet pressure
The pool is 13000 gallons
How long would it take.
How much propane gas would I need.
I live in California and all my family /grandkids are coming for Thanksgiving.
Please help.
Hi Ken, alright, this is going to be a very interesting calculation. First off, we need to know what size tankless propane heater you have. Let’s presume you have the strongest residential 199,000 BTU Rinnai RU199iN. At 57-degree input water temperature, it has a 6.0 GPM specified flow. That means that it can adequately heat up 6 gallons of water per minute; that’s 360 gallon of hot water per hour.
Usually, these max. flow are specs are given for increasing the temperature of water up to 130-140 degrees. You need a 80-degree water; that means that, ideally, the tankless heater can provide 3 times as much water if it has to heat it from 56 to 80 degrees (not 56 to 140 degrees). So, practically, the biggest residential heater can heat up to 80 degrees about 1000 gallons of water per hour.
To fill a 13,000-gallon pull at this 1000 GPH (1000 gallons per hour) flow, you would need to run it for 13 hours. Luckily for the grandkids, that’s not all that long actually. Even at a specified 6.0 GPM flow rate (bringing water temp to 140 degrees), you would need to run it at 100% output for 36 hours (this is the max. time for a boiling pool). In short, you can do that in time for Thanksgiving.
Now, propage usage and cost. The 199,000 BTU heater will produce 199,000 BTU every hour. If you run it for 13 hours, that’s 2,587,000 BTU in total. 1 gallon of propane can produce 91,500 BTU; you will need 28.3 gallons of propane. If the cost of propane is your area is about $2.50 per gallon, that’s about $70.75 for the full pool.
Even if you go via specs and run it for 36 hours (bringing the temperature to 140-degrees, instead of 80-degrees), it will consume 78.4 gallons of propane, costing $196.00.
All in all, this is doable if you have a 199k BTU tankless propane heater. If you have a smaller heater, just share the specs in the comment below, and we can calculate how long will it take for your tankless water heater to fill a pool. The cost of propane will stay the same (about $70).
Hope some of this helps. 🙂
Thank you for this great post. I think it would’ve been useful to also include a comparison with tank included water heaters. We currently are looking at options to change ours and considering electric, but if a tankless unit is less costly to run of course we’d prefer that.
Our current tank is indoors and power-vented, which we thought was very cool and wondered if that can also be done with a tankless unit?
Is there a way to tie in the old water heater to use as a storage for hot water to recirculate back into the supply line for the tankless? (Since it’d take less energy for water that’s already warm) I’m thinking “sure, recirculating valve” but at what point in the system?
Last question, I promise – how is performance affected with hard water? We’re in the mountains and our water is pretty hard. Thanks!
Hi Cris, calculating the exact running cost difference between standard water heaters and tankless water heaters is quite complex (lots of variables). Here is what the DOE has to say about this “For homes that use 41 gallons or less of hot water daily, demand water heaters can be 24%–34% more energy efficient than conventional storage tank water heaters. They can be 8%–14% more energy efficient for homes that use a lot of hot water — around 86 gallons per day.”
Power venting is cool, agreed. With a tankless heater, you can have power venting or direct vented. So, power vented tankless water heaters do exist.
Tankless units can also have recirculating pumps. Here is a whole article about that. A recirculating tankless unit makes sure that you have a small amount of hot water prepared. I see the sense of using the old storage water heater here, but when you go tankless, you usually go tankless all the way (including the recirculating, and the storage heater goes into the bin).
Uh, that hard water will be a problem. Tankless units are not immune to hard water; you can still run it, but you run the risk of significantly reducing its natural lifespan (especially if you have over 180 PPM water hardness). In these scenarios, getting a whole-house water softener is a valid option. Hope this helps.