How Long Will A Generator Run On 5 Gallons Of Gas? (Calculator + Chart)

Generators stop working when they run out of gas. That’s why we looked into how long a generator will run on 5 gallons of gas.

Here’s the short answer:

A generator on 5 gallons of gas will last anywhere from 53 minutes to 290 hours and 38 minutes (for a 30kW propane generator at 100% load to a 500-watt gasoline generator at 25% load, respectively). The exact time primarily depends on generator size, and secondarily on the choice of fuel (gasoline vs. propane).

That’s quite a range. How to figure out how long will 5 gallons of gas last in a generator exactly? Well, we need to calculate a bit, based on several factors.

To help you out, we have prepared a ‘5-Gallon Tank Generator Running Hours Calculator’ where you can insert the max. running wattage of your generator, and choice of fuel (gasoline vs. propane), and it will calculate how many hours such a generator lasts on a 5-gallon gas tank.

Below the calculator, you will also find a calculated chart of how long does a 5-gallon tank lasts for 500W to 30,000W gasoline and propane generators.

Here is what the calculator looks like (you can find it further on):

The 3 most important factors that determine how long will 5 gallons of gas last in a generator are:

1. Size of the generator. A bigger 10,000W generator will run out of gas much faster than a small 2,000W generator.
2. Generator load. If you run a generator on 100% output running wattage, it will burn through the 5 gallons of gas faster than if you would run it on 50% running wattage.
3. Choice of fuel: Gasoline vs Propane. 1 gallon of propane can generate up to 91,500 BTUs. 1 gallon of gasoline can generate up to 124,000 BTUs. That means that a gasoline generator will run 36% longer than a propane generator (if we presume the same energy efficiency).

Key Note: We also have to take up generator efficiency into account. Gas generators are not nearly 100% efficient. Rather, as per the generator efficiency and cost analysis here, we can squeeze only about 20% of the total energy out of gas or propane. According to Wikipedia, car engines have about 28% efficiency for comparison.

To adequately calculate the running time of a generator on a 5-gallon gas tank, we need to first determine the energy content of both gasoline and propane:

Energy Content Of Gasoline And Propane

Generators run on either gasoline or propane.

5 gallons of gasoline contain 620,000 BTU of energy. If we convert BTUs to watts (1 BTU = 0.293 Watts), we can get up to 181,660 watts out of 5 gallons of gasoline. If we apply the 20% generator efficiency, we see that 5 gallons of gas give us 36,332 watts of useful electricity.

Example: That’s enough to run a 1,000W generator for 36 hours and 20 minutes at 100% load, or 145 hours and 20 minutes at 25% load (quarter load). If you have a bigger 10,000W generator, 5 gallons of gasoline can run at 100% load for only 3 hours and 38 minutes, or it can run at 25% load for 14 hours and 38 minutes.

5 gallons of propane contain 457,500 BTU of energy. That’s 134,048 watts of electrical energy. We know that propane generators are only about 20% effective, giving us 26,810 watts of actually useful electricity.

Example: With a 5-gallon propane tank, we can run a 10,000W generator for 13 hours and 24 minutes. A smaller 2,000W generator can use that energy to run for 67 hours and 1 minute; almost 3 days.

Standard generators do give 100% load all the time. Inverter generators, on the other hand, adjust the power output to the power draw; they will last longer on a 5-gallon of gas (25% load, 50% load, sometimes even 75% load).

Here is the 5-gallon tank generator calculator. Just pick gas or propane, specified generator wattage, load percentage, and you will get an estimate of how many hours will a generator run on a 5-gallon tank:

5-Gallon Tank Generator Running Hours Calculator

Here is how this calculator works:

Let’s say you have an 8,000-watt gasoline generator with a 5-gallon tank. How long will this gasoline generator run continuously on 100% load? Just pick the ‘Gas (Gasoline)’ generator, set the generator wattage to ‘8000’, and the load percentage to ‘100’. Here is the result:

An 8,000-watt gasoline generator will run continuously for 4.54 hours at 100% load.

Using this calculator, we can calculate exactly how long will a 5-gallon tank last for different generator sizes:

Calculated Table For Different Generator Sizes (5-Gallon Tank)

Now you can just use this chart to check how long will your generator run continuously on the 5-gallon tank at 100% load.

Example: How long will a 22kW gasoline generator run at 100% load on 5-gallon tank? Just check this chart for the result: It will run for 1.65 hours (that’s 1 hour and 39 minutes).

Hopefully, you can now adequately estimate how long will a 5-gallon tank run a generator.