Short Answer 
A 100 lb propane tank contains approximately 2,157,600 BTU, equating to about 23.6 gallons of propane. With a consumption rate of 30,000 BTU per hour, the tank can last around 72 hours, or about three days, under constant usage.
Calculate BTU Capacity
A 100 lb propane tank has a capacity of approximately 2,157,600 BTU. To understand this, we first convert the weight of propane to gallons. Since propane weighs around 4.24 lbs per gallon, we can calculate that a 100 lb tank contains about 23.6 gallons, which when multiplied by the 91,500 BTU per gallon gives us the total BTU capacity.
Determine Usage Rate
The lifespan of the propane tank is influenced by the rate of consumption, which in this case is 30,000 BTU per hour. Knowing this rate helps us assess how quickly the propane will be consumed during usage, providing a clear picture of how often refills might be necessary under different usage scenarios.
Calculate Duration of Use
To find out how long the tank will last, we divide the total BTU capacity by the usage rate. Performing the calculation of 2,157,600 BTU divided by 30,000 BTU per hour results in approximately 72 hours of usage. This clearly indicates that under constant usage at the specified rate, the 100 lb propane tank is designed to last for about three days.