Anyone that relies on propane in any capacity, either as a whole house fuel for heating or as a fuel source for standalone appliances like portable heaters, grills and so forth must become intimately acquainted with how long they can expect their propane to last under a given use schedule. Getting this part wrong can lead to running out of fuel at the worst possible time.
How long can propane last? A 20 lbs. canister fueling a stove with one burner on medium intensity will burn for around 15 hours. Simply divide the total amount of energy in a given quantity of LPG by the consumption rating of your appliances. On another note, the shelf life of propane is indefinite.
Your propane supply itself will never go bad or spoil in storage, so you need not worry about that – as long as your tanks are not leaking!
There is no need to guesstimate how long your propane supply will last once you know how to properly calculate it using a little bit of simple arithmetic. We will show you how in the remainder of this article.
Calculating Propane Energy Supply
You only need to determine a couple of simple figures before you can start assessing how long your propane supply will last under a given usage schedule.
These figures are the energy content of a given quantity of propane (which is pretty simple because these figures don’t change), and the consumption rating of your appliance, whatever it happens to be.
It is important that you use the same unit of measurement for each variable in order to get an accurate result, and the two common units of measurement regarding the energy content of liquid propane gas, or LPG, are BTUs, (British Thermal Units) or mega joules (abbreviated MJ).
Quantities are expressed either in metric or imperial units, with the expected liters, kilograms, gallons and pounds- once again depending on where you live.
One set or the other will be common depending on the country you live in, and since this article is written with American audiences in mind we will be using Imperial moon landing units the way the Founding Fathers intended, and that means we’ll be working with BTUs and pounds.
Great, so how many BTUs are in a pound of propane?
A pound of propane provides appx. 21,600 BTUs of energy (actually 21,594). There are a tad less than 4 ¼ lbs. of propane in a gallon, meaning a gallon of propane provides 91,502 BTUs. Knowing this, it is easy to calculate smaller or larger quantities through simple arithmetic.
Caution: Propane Capacity is often Approximated
Before you go betting your life on accurately determining how much propane you’ll need to last through the winter down to the hour, it is important to understand that propane capacities, especially for refillable containers, are usually approximated.
This is usually done as a safety measure because propane expands in hotter weather and contracts with cooler temperatures, and a topped off tank or canister can experience dangerous pressure spikes when the temperature changes.
The two most common canisters in use in the United States are the typical 20 lb. liquid propane gas canister as used by full size propane grills and large, whole house tanks that typically hold several hundred gallons at a time.
Both of these will usually only be filled to around 80% to 85% of its listed capacity, and you should never overfill past this measure!
Keep this figure in mind when calculating the “actual” numbers if your timetable is that tight.
Now, the only other thing we have to do before we break out our pad and pencil or calculator app is determine how much propane we have on hand, our actual quantity, and the consumption rating of whatever appliances or tools we will be running with the propane.
To keep things simple, let’s say we are working with a stove or grill utilizing adjustable burners, and a home furnace system. But your appliance could be anything, from a camp stove or propane lantern to a forklift that runs on the gas.
The burners we are dealing with have a variable output and that means their consumption is likewise variable. Run on low, consuming 21,600 BTUs/hr. or high at 43,200 BTUs/hr. The big furnace on the other hand uses a whopping 91,500 BTUs/hr.
Now, what kind of fuel tanks do we have on hand? Let us say for convenience sake we have a single 20 lb. tank of propane that actually holds 20 lb. of liquid propane gas in order to make our demonstration calculations easier. We also have a residential propane tank holding 200 gallons even.
Time to sharpen your pencils!
First, establish the total energy on hand supplied by the propane. If we multiply 20 (pounds) by 21,600 (appx. BTU per pound) we get 432,000, meaning that our little propane cylinder supplies 432,000 BTUs. How long will that run our grill / rangetop?
We mentioned earlier that it has four adjustable burners with variable consumption, so let us do some figuring:
• It could supply one burner on low for appx. 20 hours. That’s 432,000 divided by 1 (burner) times 21,600 (low) = 20. • It could run all four burners on low for appx. 5 hours. That’s 432,000 divided by 4 (burners) times 21,600 (low) = 5. • A single burner on high will burn for appx. 10 hours. 432,000 divided by 1 (burner) times 43,200 (high) = 10. • Again, four burners on high will run for appx. 2 ½ hours. 432,000 divided by 4 (burners) times 43,200 (high) = 2.5.
Simple enough, but how about that big furnace? The calculation is just the same only we are working with some bigger numbers. 200 gallons of propane yield 18,300,400 BTU’s (91,502 BTU’s/gal.) and since we know how much the furnace consumes in BTU’s per hour (91,500 in our example) we just plug in the data.
18,300,400 BTUs divided by 91,500 equals 200, rounded to the nearest whole digit. That is 200 hours of “up-time” for our furnace using our 200 gallon whole house tank.
Now, all we need to do is divide the total number of hours in up-time by 24 to see the total number of days in up-time we’ll have. In this case, 200 divided by 24 equals 8.33, so almost 8 ½ days if we run it continuously.
You should determine how many hours per day you actually need to run your furnace in order to beat typical seasonal weather, and then you will have the real number of “days” you can expect your fuel supply to last.
Long Term Storage
I’m happy to report that, compared with more traditional fossil fuels, propane will not spoil, go bad, get stale or lose effectiveness due to age. A 3-year-old tank of propane is going to be just as effective as a freshly bottled one.
The only consideration you should worry about when storing propane long-term is the integrity and upkeep of the tank.
The slightest, most infinitesimal leak will slowly rob you of your propane over time, and for this reason it is imperative that you keep your tanks inspected, maintained and up to date.
For smaller canisters this is usually a simple affair as you can just swap them out at your local hardware store or gas station much of the time.
For larger fixed tanks you should have them inspected annually at the minimum and never, ever defer maintenance, both for protecting your investment and for safety. I don’t need to tell you what will happen to your happy home if a 200 or 300 gallon tank of propane were to go up…
Determining how long your propane supply will last is a simple matter of arithmetic. By simply dividing the consumption of running appliances against the total energy provided by your supply of propane you can establish, in hours, the up-time based on what you use and the duration you use it.
You don’t have to worry about your propane supply going bad or deteriorating due to age but you should take care to stay on top of inspecting your storage containers so that leaks do not rob you of your precious supply.