How Long Will A 20 lb Propane Tank Last At 50,000 BTU? (6.31h to 8.41h)

A 20 lb propane tank is used for all sorts of propane-powered heaters. This includes patio heaters, generators, propane grills, and so on. Something like a propane heater can produce 50,000 BTU of heating output. Based on the propane BTU content, we can calculate how long will a 20 lb propane tank last at 50,000 BTU.

To adequately estimate how long will a single filling of a 20 lb propane tank last at 50,000 BTU, we have to determine 2 things:

1. How many BTUs are in a 20 lb propane tank? We know that 1 gallon of propane contains 91,452 BTU. We’ll calculate how many BTUs are in a 20 lb propane tank.
2. What is the efficiency of burning propane? Not 100% of propane’s BTU content is turned into heating. Propane patio heaters, for example, have 75-95% propane burning efficiency (AFUE value from 75 AFUE to 95 AFUE). We need to take this efficiency into account if we are to accurately estimate how long will a 20 lb propane tank last.

When we figure these two things out, we can simply estimate how long will a propane tank last. Example: If 20 lb of propane (accounted for efficiency) contains 50,000 BTU, it will last 1h powering a 50,000 BTU heater.

In general (you can see full calculations and details below), 20 lb propane tank at 50,000 BTU will last between 6.31 hours and 8.41 hours. The most efficient propane patio heaters can last about 8 hours on a 20 lb propane reservoir.

Let’s start with calculating how much energy does a 20 lb propane tank hold:

How Many BTUs Are In A 20 lb Propane Tank?

It’s impossible to know how long will a 20 lb propane tank last at 50,000 BTU if we don’t know how many BTUs are in that tank. Luckily, the US Energy Information Administration lists the BTU content of propane:

1 gallon of propane = 91,452 BTU (Source: EIA)

If we transform 20 lb to gallons, we get this result: 20 lb of propane is equal to 4.6 gallons of propane.

Now we know that we have 4.6 gallons of propane and each gallon can produce 91,452 BTU. Let’s calculate how many BTUs are in a 20 lb propane tank like this:

20 lb propane tank BTU content = 4.6 gallons × 91,452 BTU per gallon = 420,679 BTU

As you can see, 20 lb propane tank contains BTUs of heating energy.

Lifespan Of 20 lb Propane Tank At 50,000 BTU Output

If burning propane would be a 100% efficient process, we can simply calculate how long will a 20 lb tank last. We know that such a tank contains 420,679 BTU and that a heater burns 50,000 BTU worth of propane every hour.

Here is how we can calculate the maximum time a 20 lb tank can last at 50,000 BTU output (100% efficiency):

Time = 420,679 BTU / 50,000 BTU per hour = 8.41 hours

That means that in ideal conditions, a 20 lb propane tank at 50,000 BTU lasts for 8.41 hours (8 hours and 16 minutes).

However, the conditions – energy efficiency of burning propane – are not perfect (100%). Propane burns, depending on the heater, with 75% to 95% efficiency. That means that the time how long will a 20 lb propane tank last at 50,000 BTU is reduced by 5% to 25%.

Depending on the propane-burning efficiency, denoted by the AFUE rating of specific heaters, you can see how long will 20 lb propane tank last:

The best outdoor patio heaters, for example, have about 90% propane-burning efficiency. For such 50,000 BTU heaters, a 20 lb propane tank will last between 7 and 8 hours. That is to say that a 20 lb propane tank can last a whole evening, even if it has to power a 50,000 BTU patio heater.

We hope this helps. If you have any questions regarding how long do 20 lb propane tanks last, you can pose a question in the comments below and we’ll try to help you out.