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How Long can a Generator Run? [Basic Calculations]


A generator, whether portable or standby, will operate and give you energy as long as it has enough fuel. If your generator is hooked up to a natural gas connection, you’ll have virtually unlimited fuel from the utility company. Other types of generators will fail when the pistons, camshaft, and other vital parts are worn out, as well as lubricating fluids run out.

The runtime of any generator can be easily found in the owner’s manual or specifications sheet. Here is an example:

The following four fuels are used by the majority of generators:

  • Gasoline. Gasoline-powered (or petrol) generators are usually less expensive to buy than any other type, which makes them a very popular choice. However, gasoline-powered generators must be refueled frequently during an emergency, and when everyone else is also trying to get some gas… well, you got my point…
  • Diesel. Although diesel generators burn fuel slowly (which makes them cost-effective), they have environmental drawbacks. Diesel fuel emits more pollutants than other fuels, which may make it difficult to meet EPA guidelines.
  • Natural gas. Natural gas-powered generators burn a lot cleaner than diesel or gasoline, although if you experience a power outage, gas lines could be affected as well. The gas grid is powered by electricity which helps to maintain the pressure throughout the system. A series of compressors and pumping stations keeps the pressure up.
  • Propane. Propane is one of the most adaptable fuels! It is suitable for all kinds of uses, from portable to standby. The best part? During an emergency, a fuel delivery service can actually get to you and refill the tank!

How long can a gasoline generator run?

Gasoline is a popular and easy-to-find fuel, but it is highly combustible and dangerous to store in big quantities. Gasoline costs are high, and it’s the least efficient way to power a generator, at least for lengthy periods of time.

Because gasoline is more convenient, it’s a very popular choice with portable generators. If you have a 5,000-watt generator (for example), expect it to burn:

18 gallons of fuel in a 24-hour period

For long-term usage, gasoline-powered generators are considered to be too costly, and this is why inverter-generators are becoming increasingly popular. The majority of gasoline inverter-generators have an eco (or economy mode) that adjusts engine speed (and hence fuel usage) in response to load demands.

These are the most fuel-efficient generators available. One tank of gas, which might be as little as 1-2 gallons, will usually last 8-10 hours. As a result, running two inverter generators, each of which is typically 2K-4K watts, is usually more efficient than running a bigger 5K+ watt open-frame generator.


The problems with these types of generators are that you should never refill a generator when it is running. Even while it may seem tempting to just add extra petrol to the tank, doing so is exceedingly risky.

Hot generators and fumes from gasoline are not good companions! The gasoline that you’re adding might ignite, causing the generator or the gasoline tank that you’re holding to burst into flames without warning.

To solve this problem, all you must do is to let your generator cool down before replenishing the gasoline. But, there is another option (for portable generator):

An extended fuel kit!

An extended fuel kit (paid link), that I actually found on Amazon, with an external pipe is the best method to run a portable generator for days or even weeks. You may securely replenish the secondary gas container while your generator continues to function.

For standby generators, there is an option that is called a “day-tank”, which extends its runtime substantially. When a sub-base fuel tank’s physical size restriction isn’t adequate, a day-tank may be a great solution!

It’s an immediate source of fuel that comes from a bigger fuel storage tank. It might be a stand-alone tank positioned near the generator, or a sub-base tank with facilities for usage as a day tank. A day-tank is often intended to hold a certain quantity of gasoline that gets automatically refilled via pumps and controllers.

How long will a diesel generator run?

Diesel generators are not as popular as gasoline generators, but those that do are more efficient. Diesel fuel is less combustible and less expensive in comparison to other fuels.

These types of generators are commonly used on construction sites and manufacturing facilities.

As an example, a 20kw generator will use about 1.6 gallons of fuel per hour. Here is a full breakdown (loads are in gal/hr):

Size (kW) 1/4 Load
1/2 Load
3/4 Load ) Full Load
20 0.6 0.9 1.3 1.6
30 1.3 1.8 2.4 2.9
40 1.6 2.3 3.2 4.0
60 1.8 2.9 3.8 4.8
75 2.4 3.4 4.6 6.1
100 2.6 4.1 5.8 7.4
125 3.1 5.0 7.1 9.1
135 3.3 5.4 7.6 9.8
150 3.6 5.9 8.4 10.9
175 4.1 6.8 9.7 12.7
200 4.7 7.7 11.0 14.4
230 5.3 8.8 12.5 16.6
250 5.7 9.5 13.6 18.0
300 6.8 11.3 16.1 21.5
350 7.9 13.1 18.7 25.1
400 8.9 14.9 21.3 28.6
500 11.0 18.5 26.4 35.7
600 13.2 22.0 31.5 42.8
750 16.3 27.4 39.3 53.4
1000 21.6 36.4 52.1 71.1
1250 26.9 45.3 65.0 88.8
1500 32.2 54.3 77.8 106.5
1750 37.5 63.2 90.7 124.2
2000 42.8 72.2 103.5 141.9
2250 48.1 81.1 116.4 159.6

Table source.

** Please note that these numbers are approximate and your individual case may be different.

How long will a generator run on gas?

Natural gas is usually a preferred choice of fuel since it does not become stale and burns very clean. Many residences are already equipped with a gas line, which makes natural gas a very convenient option to power your generator.

Natural gas, like propane, is simple to store and maybe supplied directly to your home. As an example, a 20kw generator would require around 289 cubic feet of gas at full load.

Here is a full breakdown (loads are in ft3/hr):

Size (kW) 1/4 Load
1/2 Load
3/4 Load
Full Load
20 157 188 247 289
30 202 260 348 416
40 246 333 449 543
60 334 479 652 798
75 400 588 803 990
100 510 771 1056 1308
125 621 953 1308 1627
135 665 1026 1409 1754
150 731 1135 1561 1946
175 841 1317 1813 2264
200 952 1500 2066 2583
230 1084 1718 2369 2965
250 1172 1864 2571 3220
300 1393 2229 3076 3857
350 1614 2593 3581 4495
400 1834 2958 4086 5132
500 2276 3687 5096 6407
600 2717 4416 6107 7681
750 3379 5509 7622 9593
1000 4482 7332 10147 12780

Table source.

** Please note that these numbers are approximate and every individual case is different. Your natural gas generator will run as long as gas is being delivered to it.

How long will a generator run on propane?

Propane is one of the least harmful fuels to the environment. Propane has a long shelf life, burns cleanly, and can be stored in small or big tanks. Delivery straight to your house is offered as well, making it even more simple and convenient.

Before we get started, you need to know that the propane fuel tank cannot be filled up fully (you need to allow space for expansion and contraction). This means, for example, that a 500-gallon tank may only be filled with 400 gallons of gas.

Propane burns faster than natural gas and since we are restricted on size, we need to crunch some numbers to figure out how the generator runtime on the desired size of a cylinder.


First, we need to determine how much horsepower does your load requires. The formula goes as follows:

(watts / 1000) x 2 = Horsepower

This means that a 2-horsepower generator can provide you with around 1000 watts of power.


What is the BTU capacity of a generator? Under full load, an engine of a generator that runs at 3600 rpm will burn around 10K BTU per hour (per horsepower). We can use this formula to calculate:

horsepower x 10,000 = BTU

This means that 10,000 BTU per hour of fuel is required to create 1 horsepower of mechanical energy.


Propane has a 92,000 BTU per gallon fuel value. You can use the following formula to figure out how many gallons of the propane it equals:

BTU / 92000 = propane in gallons

Here is what we have done (our example):

  • Horsepower: (1,850 watts / 1,000) x 2 = 3.7 horsepower
  • BTU: 3.7 x 10,000 = 37,000 BTU
  • Propane: 37,000 BTU / 92,000 = 0.40 gallons of propane per hour

There is also a formula that combines all these steps into one:

 (watts/3,000) x 0.65 = propane in gallons per hour

Let’s continue with our example:

  • (1,850 watts/3,000) x 0.65 = 0.40 gallons per hour of propane

Usage time

Now, let’s get to the actual usage and use the following table for reference:

20-pound propane cylinder 4.76 gallons
30-pound propane cylinder 7.14 gallons
40-pound propane cylinder 9.4 gallons

The weight of 0.65 gallons of propane is 2.73 pounds, which means if your propane cylinder is 20 pounds, it will hold 4.76 gallons (20/2.73 x 0.65) as mentioned in the table above.

If we continue with our example, which gave us 0.40 gallons per hour of needed fuel based on our usage (1,850 watts), we:

  • 20lb tank. Divide 4.76 by our hourly usage, which is 0.40 and we get 11 hours of runtime.
  • 30lb tank. Almost 18 hours of runtime.
  • 40lb tank. About 23.5 hours of runtime.

These numbers are all at FULL LOAD, which you probably will not do since you don’t want your generator to be out of service prematurely. You can use the following information (taken from another source) to calculate consumption for different load types:

Watts HP Full Load 75% Load 50% Load
1850 3.5 35,000 26,250 17,500
4000 8 80,000 60,000 40,000
5000 10 100,000 75,000 50,000
7500 15.5 155,000 116,250 77,500
8000 16 160,000 120,000 80,000
10,000 20 200,000 150,000 100,000
12,000 24 240,000 180,000 120,000

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