Calculating How Long Your BBQ Propane Tank Will Last
Cooking food over a hot open flame is one of my favorite summer time activities. Although I prefer the flavor of a charcoal grill, nothing can beat the convenience and control of grilling with propane. Whether it's a perfectly seared tuna or a flawlessly cooked steak, a propane grill can give you the precise control necessary to properly prepare it.
However, there are a few downsides to this method of grilling. Besides having a diminished flavor, propane tanks tend to run out of fuel at the worst possible moment. To fix this problem, I initially bought a propane tank gauge. I had hoped that it would tell me when the tank was getting low so that I could go get a refill just before it ran out. Unfortunately, this did not work too well for me. The gauges that I tried were either highly inaccurate or actually reduced the amount of gas flowing to the burners. After scratching my head for a moment, I thought to myself, "Wouldn't it be nice to know long the tank would last under normal cooking conditions?" If I knew this information, I could calculate about how many meals I could prepare before my grill ran out of fuel. So that's exactly what I did. In this article, I will derive a formula which you can use to determine how long your propane tank will last under normal cooking conditions.
A Bit of Information About Propane
Propane is a very lightweight hydrocarbon with a chemical formula of C3H8. It is a by-product of the crude oil and natural gas refining processes. At room temperature, propane is a gas. The gas is compressed into a liquid and stored in a variety of sizes of steel containers. Propane has a heating value between 91,200 & 92,000 BTU per gallon (depending on your source). A heating value of 91,600 BTU/gal (21,600 BTU/lb) is the commonly accepted value presented by the American Gas Association. Since it is also the midpoint of the range of values found in my research, this is the number you should use to calculate your propane usage time.
A Bit of Information About Grills
Your estimated total cooking time for a single propane tank will also depend on the type and size of grill that you have. There are literally hundreds of types and configurations of propane grills that you can choose from. All grills should have a maximum heating value reported in their manual. If you don't have the manual, you can estimate that each burner produces about 10,000 BTU/hr of heat. A typical grill might have a maximum total heat output of 20,000-40,000 BTU/Hour (2-4 Burners). Some small grills may only produce 10,000 BTU/hour (1 Burner), while larger ones may have an output of 80,000 BTU/Hour (8 Burners).
Deriving this formula is rather simple. Let 'T' equal the grilling time, 'P' equal the amount of propane you have, 'H' equal the heating value of the propane, and 'B' equal the maximum heat output of your grill.
First, multiply the amount of propane you have by its heating value: PH
Next, divide that by your grill's maximum heat output to get the minimum grilling time: Tmin = PH/B
However, most people don't cook at the maximum grill setting for an extended period of time. In fact, propane usage during typical cooking conditions is normally around 50-75% of the maximum heat output. If you cook with the all the burners set on medium, your output would be around 75% of the maximum. If you use less burners, your actual output will also change. This is where I introduce an efficiency factor, 'k,' to adjust the time calculation for this variation.
With the introduction of the 'k' factor, the final formula becomes:
The 'k' factor effectively reduces your grills total output in the computation. Use the chart at right to determine the 'k' value for your typical grilling scheme. The derivation of the various 'k' factors is beyond the scope of this article.
Once you select a 'k' factor based on your typical grill's usage, you can compute the total estimated grilling time, T. Divide this time by the average time it takes to cook a meal to estimate the number of meals you can grill. Now all you have to do is keep track of how many times you use the grill to know how much fuel you have left.
For example, if you have a six burner grill and only cook with one burner on the lowest setting, you are effectively generating only 10% (0.10) of the maximum heat output of that grill. However, if you crank that same burner to its maximum setting, you will only be generating about 17% of the maximum heat that could be generated by the grill.
John just bought a new propane tank (15lbs of propane) and wants to know how long this will last him. John's grill has a maximum heat output of 30,000 BTU with 3 burners. A typical meal takes John 30 minutes to complete using medium heat.
Step 1: Define the Variables:
Amount of Propane: P = 15 lbs
Heating Value of Propane: H = 21,600 BTU/LB
Maximum Heat Output of Grill: B = 30,000 BTU/hr
'k' Factor: 0.74 (All Burners, Medium Heat)
Step 2: Solve for Propane Usage Time:
T = (15lbs)(21600 BTU/LB)/(0.74)(30,000 BTU/hr) = 14.6 Hours
Step 3: Now compute how many meals John can make with his grilling arrangement:
Meals = 14.6 hrs / 0.5 hrs = 29.18 meals. Therefore, John could cook once a day for a month or about once per week for 7 months.
Quickly Find Out How Much Propane You Have Left
Let's say that you want to know how much propane you have left in the tank connected to your grill and you don't have a pressure gauge with you so what do you do? It's simple actually, all you need to do is weigh the propane tank and then subtract the Tare Weight (marked as a number following "TW" on the top of the tank) from the weight shown on the scale. A standard barbecue propane tank can hold 20lbs of propane and even if you have 1 or 2 pounds of fuel left, that can be enough to prepare just one more meal.
Should I Refill or Exchange My Tank?
This is a common question these days. If you want convenience (and you'll pay for it) then go for a tank exchange. Some retailers even have automated machines that can exchange a tank for you without even having to interact with an employee.
However, if you want to save lots of money, always have your tank refilled. Not only will you pay less per pound of propane, you won't lose what's left in the tank when you exchange it. Also, if you get your tanks refilled, you will often actually get more propane than if you just exchanged the tank. Many tank exchanges don't even give you a completely full tank (which would contain 20lbs of propane) but instead leave the tank filled to 15-18lbs. Most people never check and don't even realize that they aren't getting a full tank!