Prepping

So, How Long Does Propane Last?

Propane has much to commend it as a fuel source, especially for the purposes of disaster readiness and SHTF preparation. Propane is efficient and adaptable, capable of fueling all kinds of appliances and tools, large or small, and even vehicles that are adapted to run on it.

Propane is great, but only as long as your fuel supply lasts, like any other. What you need to know is how long your propane supply will endure.

A typical 20 lbs. tank of propane fueling the average grill will last 18 to 20 hours of continuous use, a stove will last 5 – 10 hours, and a furnace will run for almost 100 hours.

Of course, how long a given supply of propane will last is determined by how much fuel your tools, appliances and vehicles consume in use. Calculating how long your supply will last is simply done by dividing energy consumption against total energy supply.

Propane is a great survival fuel, but like anything else you must be sure that the quantity you have will last you through the anticipated event. Don’t worry, we won’t be doing any advanced math and the process is quite simple. I will walk you through it below.

First Things First: What’s the Storage Life of Propane?

Probably the best attribute of propane as a survival fuel is its incomparably long storage life. Kept in a properly maintained and protected pressure vessel, liquid propane fuel will not degrade in any meaningful way for at least 30 years. That is incredible! No other fuel in common usage even comes close.

Gasoline is typically good for only between 3 and 6 months, perhaps 12 months on the outside if pure gasoline is mixed with a stabilizer additive. Diesel is usually good for about a year and a half in storage, but it must be treated to prevent the accumulation of algae growth which can badly clog a fuel system.

For generators, vehicles and tools you can rest assured knowing that your propane will be ready, waiting and able to do the job when called upon with no need to rotate it or mess with it in an attempt to prevent spoilage.

How Much Energy Does a Unit of Propane Provide?

A given quantity of propane will provide a certain amount of energy, and this unit of energy is called a BTU, short for British Thermal Unit.

We don’t need to get into the history of that unit of measure to understand it and do what we need to do. All you need to know is that the various appliances and tools fueled by propane measure their consumption in BTUs per hour. More on that later.

Now, the typical output and consumption ratios of the BTUs provided by propane are usually done by the pound and by the hour.

A single pound of liquid propane fuel provides 21,594 BTUs. However, we can make our work and our lives a little easier if we round that off to a neat figure, so for the remainder of our calculations we will say it provides 21,600 BTUs.

In North America, particularly the United States, liquid propane is often sold by the gallon and the next thing we need to figure out is how many pounds of propane are in a gallon.

Once again rounding just a little bit, we come up with a figure of four and a quarter pounds in a gallon. If we calculate the number of pounds in a gallon by the number of BTUs in a pound, we come up with a figure of 91,500 BTUs in every gallon of propane.

Great, Now to Figure out How Many BTUs worth of Propane You Have

If you have a supply of propane fuel on hand, you will probably have it in one of two typical vessels for commercial consumption. The first is what is known as a barbecue cylinder, or barbecue tank.

These ubiquitous cylinders are the ones you hook up to your outdoor propane grill to fire up the family cookout. These cylinders are rated to hold 20 lb of propane, but you should note that they are usually sold with 17 or 18 lb of propane fuel in them.

The reason why is that propane fuel expands and contracts with temperature changes, and a topped off cylinder that warms up could experience a dangerous over pressure event.

The next common vessel is a fixed, large tank designed to feed a home that uses propane or an installed standby generator. Tanks of this type vary greatly in capacity, but at the minimum hold dozens and dozens of pounds and often hundreds of pounds of propane.

By whatever means, determine how much propane your container actually contains. This could be done using a gauge or using a scale that is calibrated for the purpose.

Next, all you’ll need to do is multiply the pounds of propane you have by 21,600 to determine the total BTUs of energy you have on hand.

If you are determining how many gallons of propane you have, all you’ll need to do then is divide that figure by four and a quarter to convert it to pounds and then multiply pounds as normal above.

With that done, we are ready to calculate our appliances and other devices’ rate of consumption against our total energy supply to determine the uptime available to us.

Now Determine How Many BTUs / Hour Your Devices and Appliances Require

Pretty much every propane fueled appliance, tool and vehicle will have a listed BTU rating. This rating is better expressed as BTUs consumed per hour of operation.

Don’t make the mistake of thinking that this is necessarily its output, especially regarding something like a propane heater. Remember, the BTUs per hour figure is its rate of fuel consumption!

This information might not necessarily be listed on the device or appliance and might not be even conveniently shown on its packaging. If in doubt, simply consult the manual or even contact the manufacturer.

In the case of devices with variable controls, like a stove top or grill for instance, the burners will usually be rated at a certain level of output, or will show various rates of consumption for differing outputs, high, medium, low, etc.

Once you know how many BTUs per hour your device consumes, all we need to do then is divide this figure against our total energy supply to see how many hours worth of operation we have. Don’t forget to factor in the running of multiple burners or heater coils as appropriate depending on the appliance in question.

Examples of Propane Consumption Calculation

Still confused? Don’t worry, I will provide you with some examples below so you can see exactly how to reach these determinations when calculating uptime.

For our purposes, I will be using two of the most common propane appliances, a propane camp stove top and a propane fueled furnace.

For purposes of this example, the camp stove has two burners, each with a high and low setting. The low setting of each burner consumes 21,600 BTU’s/Hr., while the high setting of each consumes 43,200 BTU’s/Hr.

For the furnace, it is a simple affair with one setting that adequately heats our house and chugs down a whopping 75,000 BTU’s/Hr. Yikes!

Now, how about our propane supply? To keep it simple, say we have a barbecue cylinder that has 18 lbs. of propane in it, and a large, on-site tank that currently holds 80 gallons of propane. What’s our total energy supply, in BTUs?

For the barbeque cylinder, we come up with a figure of 388,800 BTU’s. That’s 18 pounds times 21,600, the number of BTU’s in a pound of propane.

For the larger tank, we come up with 7,320,000 BTU’s. Whoa! How? Remember, there are appx. 4 ¼ lbs. of propane in every gallon, so we must multiple the number of gallons in the tank by 4.25 to yield the number of pounds, then multiply that figure by 21,600 as before to find the total BTU’s.

Then all we must do is divide the BTU’s/Hr. for each of our appliances (on a given setting in the case of the stove) by the total supply of each container of propane.

Stove:

Burners on high: 86,400 BTU’s/Hr. divided into 388,800 BTU’s/Cylinder equals 4 ½ hours.

Burners on low: 43,200 BTU’s/Hr. divided into 388,800 BTU’s/Cylinder equals 9 hours.

Furnace:

Single setting of 75,000 BTU’s/Hr. divided into 7,320,000 BTU’s/Tank equals 97.6 hours.

That’s all there is to it!

Conclusion

Propane will last a very long time in storage, easily 30 years or more so long as the vessel is in good shape and not allowed to degrade.

How long your propane supply will last in use is completely dependent on how much fuel your devices or appliances consume and how often or how long you use them.

Calculating the total “uptime” that your propane supply can provide is a simple calculation so long as you know how many pounds of propane you have and how much energy a given device consumes by the hour.

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