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:

**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.**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:

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## 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 Ã— 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:

Propane Burning Efficiency: |
Burning Time Of 20 lb Tank at 50k BTU: |

100% | 8.41 hours |

95% | 7.99 hours |

90% |
7.57 hours |

85% | 7.15 hours |

80% | 6.73 hours |

75% | 6.31 hours |

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.