Propane is a versatile, efficient and all-around good choice of fuel for prepping. Propane is just as suitable for running your furnace, hot water heater and stove at home as it is firing up your grill, keeping your workshop or campsite warm with a portable heater or even operating certain kinds of heavy equipment like forklifts. But the fuel is only good as long as it lasts.
So about how long will your propane supply last in use? You simply divide your energy consumption against your energy supply. You convert the amount of propane you have (in gallons or pounds) to BTUs, then divide that to your device’s energy consumption BTUs / hour.
Your logistical situation is made somewhat simpler by the fact that propane will not spoil or lose its volatility unlike gasoline and other petroleum products.
You don’t need to be afraid of the math on this one, and you certainly don’t need a degree to figure it out. This is an essential part of prepping if propane is a big part of your plan. Keep reading to get all the details.
How Do I Figure Out My Total Energy Supply?
Finding the figures you need to calculate how long your propane supply will last against your demand is a pretty straightforward affair. First we need to determine how much energy is in a given quantity of propane, easy enough since that information is readily available, and then we need to determine how much energy our appliances consume over a given period of time.
Like any formula where you are trying to solve for a given quantity, you must ensure that you solve using the same units of measurement if you don’t convert. I know I’m starting to lose some of you with that very sentence, but please let me assure you that this is quite simple.
In America, the only units of measure and energy we are concerned with 99% of the time when dealing with propane are your usual pounds and gallons (concerning quantity) and BTUs (concerning energy). BTU stands for British Thermal Unit, but I’ll spare you the minutiae associated with that measure.
For our purposes a pound of propane will furnish so many BTUs worth of energy. A larger quantity, such as a gallon, will contain so many pounds of propane, making calculating how much more or less a given quantity has a simple matter of multiplication or division.
How Many BTUs are Provided by Propane?
The first figure we need is how much energy is provided by a pound of propane. The answer is that a pound of propane provides 21,594 BTUs. Since we would prefer to work with nice, tidy numbers to make our close approximation simpler to calculate we will round it off and say that a pound of propane contains 21,600 BTUs worth of energy.
The next unit of measure concerning propane that is commonly used in America is the gallon, and a single gallon of propane is not quite 4 ¼ lbs. Again, a very near approximation is fine for our purposes. If we run the numbers on that and round it off very neatly we find that a gallon of propane provides 91,500 BTUs worth of energy.
These units of measure are convenient because the most common sources of propane for consumers in America are 20 lb. cylinders (of the type you typically hook up to your outdoor grill) and large, fixed tanks that supply entire homes with propane (if the home is not hooked up to public propane supply). These large, fixed tanks often hold hundreds of gallons of propane at a time.
I must mention here that propane capacity even in commercially filled tanks and cylinders is often approximate. For instance your 20 lb. cylinder there will not hold 20 whole pounds of propane, instead holding typically 18 lbs.
This is because the fillers know that propane contracts when it is cold and expands when it is hot and if they filled your cylinder all the way up to the brim it could create a dangerous overpressure event when the tank warmed up.
How Much Propane Does My Appliance Use?
The vast majority of propane fueled appliances will have a BTU rating typically expressed as BTU’s/hour. A common mistake made concerning propane appliances is that this is how many BTU’s they emit when they are running, or rather how much heat they crank out.
This is a mistake, as this figure actually tells us about how much propane the device will consume when it is operating.
In the case of something like a stove or grill you’ll need to consult the specifications or the manual and learn if that is peak consumption with all burners going or nominal consumption at a certain setting or certain number of burners.
Most appliances and devices will provide you with a BTUs per hour rating for each burner at a certain level of output.
Once we know how many BTUs our appliance consumes per hour on a given setting it is a simple matter to divide that rating against our total BTU supply provided by the propane.
If we are running multiple appliances or a single appliance with multiple burners we will simply multiply the BTU consumption before dividing. Easy peasy lemon squeezy.
Example Supply Calculations
To walk you through the steps of solving your propane consumption formula let us say that we are dealing with a common propane grill and a typical propane furnace installed in a small house. There are always many variables for the BTU consumption rating of an appliance but we are going to keep it super simple for your first time.
Let us say that our grill is a no-fuss, no-muss two burner affair, and that each burner has a high and low setting. We will say for our purposes that the low setting on each burner consumes 21,600 BTU’s an hour, while the high setting consumes 43,200 BTU’s an hour, or twice as much.
Let us also say that the small furnace installed in our house guzzles BTU’s at 75,000 an hour. That is a lot more than our grill!
Now, what about our propane supply? That’s easy: we have for this example a single 20 lb. cylinder of propane for the grill (that actually contains 18 lb of propane) and a 100 gallon tank on our property that supplies the furnace. Because that 100 gallon tank is only 80% full, we have 80 gallons in that one.
So, how many BTU’s does each tank supply?
- Cylinder: 388,800 (18 pounds x 21,600 BTU per pound)
- Tank: 7,320,000 (80 gallons x 91,500 BTU per gallon)
Nice. Now all we need to do is divide the consumption rating of our appliances against our supply and that will reveal to us how long we can operate those appliances and at what level for what length of time. Let’s unpack it just below.
- Grill, both burners on low: 9 hours (two burners at 21,600 BTU each = 43,200 BTU. 388,800/43,200 = 9)
- Grill, both burners on high: 4.5 hours (two burners at 43,200 BTU each = 86,400 BTU. 388,800/86,400 = 4.5)
Easy enough right? How about the furnace? Arbitrarily let’s say it has a fixed rate of consumption, so no adjustments for output needed.
- Furnace: 97.6 hours (7,320,000 / 75,000 BTU = 97.6)
In the case of the furnace you could divide 97.6 hours by 24 to get your runtime in full days (4.06; call it 4 days of constant on) or by whatever interval of hours you typically run your furnace for the season to determine your “practical” days of runtime.
Remember: Propane Does Not Go Bad
One really good feature about propane as mentioned previously is it does not spoil, go bad or otherwise fail due to a long time sitting in storage. In stark contrast to gasoline and diesel.
The only concern with storing propane long term is that you’ll need to stay on top of the canisters and tanks to ensure they are not leaking. So long as they aren’t leaking, you won’t have any problems pulling your propane out of mothballs.
Conclusion
All that is required to figure out how long your personal supply of propane will last is an easy formula that divides the consumption of your appliances or other propane-fueled devices against the total energy furnished by the amount of propane you have on hand.
If calculated correctly, you will know in a flash how many hours of runtime, approximately, you may expect from your propane supply.