How Long Do Generators Last? (Running Hours, Not Lifespan)

I’ve read quite a lot of articles about how long generators last. They usually say up to 25 years or 30,000 hours. But that’s not really what I’m asking when asking how long does a generator last (I’m not interested in generator lifespan). With this question, we are usually interested in how many hours can a generator run continuously on a full tank.

Example: How long does a 10kW generator last on a tank of gas? If the fuel tank capacity is the standard 5 gallons (of diesel, for example), I want to hear that it will run for X hours when running at 100% load, and for X hours when running at quarter load (25%).

By the way, if we presume 20% diesel efficiency, this 10kW generator will last for 4.03 hours (4 hours and 2 minutes) on 100% load, and 16.11 hours (16 hours and 7 minutes) on 25% load. You can check all these numbers in the 1st, 2nd, 3rd, and 4th charts further on.

With a bit of math, we can fully answer how long do generators last on a tank of gas (continuous usage, the type of usage you would need in the event of a power outage). We are going to go through these 3 key resources you can freely use:

1. How to calculate how long does a generator last? This will depend on generator size (wattage), type of fuel (diesel, gasoline, kerosene, liquid propane or LPG), fuel tank capacity, and load; this is everything we need. We are going to look at how to use the Generator Running Time Formula to estimate how many hours can a generator run continuously on a full tank.
2. Generator Running Time Calculator. With the formula, you can calculate the continuous running hours yourself by hand. It’s a bit of calculations, and that’s why we made it easier by programming a calculator. You insert generator wattage, fuel type, tank capacity, and load, and the calculator will tell you how long will a generator run continuously before you need to refill the tank.
3. Generator Running Time Chart. We have calculated continuous running hours on a full tank for diesel, gasoline, kerosene, and liquid propane tanks with sizes ranging from 500W to 30,000W on 25% and 100% (full) load. All the results you need are summarized in 4 charts further on, one for each fuel type.

First off, let’s look at how to calculate how long can you continuously run a generator on a full tank:

How To Calculate How Long Do Generators Last?

Alright, let’s start with our example from above: We have a 22kW diesel generator with a 5-gallon tank. We presume 20% generator efficiency (we did the analysis of generator efficiencies and cost per kWh here, most have between 17% and 23% efficiency). How long will it run?

First of all, we know that all of our energy is contained in that 5 gallons of diesel. 1 gallon of diesel contains, according to the US EIA, 137,381 BTU of energy. Here is all the available energy:

Generator Available Energy = 5 Gallons × 137,381 BTU/Gallon = 686,905 BTU

Alright, a generator cannot extract all of that 686,905 BTU of energy out of diesel. Most generators have about 20% efficiency. So, we don’t have 686,905 BTU of energy available; we only have 1/5 of that, or 137,381 BTU, in a 5-gallon diesel generator tank.

Now, we can convert these useful BTUs into electricity (kWh). We can use this BTU to kWh converter, or just calculate manually by knowing that 1 kWh of electricity is equal to 3,412 BTU. Here is how many kWh we can get out of these 5 gallons of diesel (20% efficiency):

Generator Available kWh = 137,381 BTU / (3,412 BTU/kWh) = 40,26 kWh

Alright, now we are getting somewhere. A generator with a 5-gallon diesel tank contains enough diesel to produce 40,26 kWh of electricity (at 20% efficiency). It becomes quite simple from here on. Example: If you have a 20kW diesel generator, it will run for a bit more than 2 hours (in 1 hour it produces 20 kWh of electricity).

Note: We always use running wattage here, not peak wattage. Peak wattage is just for starting the generator; when producing electricity, it will produce running wattage. Example: Westinghouse 12,500 Watt Dual Fuel Home Backup Generator has a 12,500-watt peak wattage and 9,500-watt running wattage. We use the 9,500-watt or 9.5kW wattage for these calculations.

Now, we have a 22kW diesel generator running at 100% load with a 5-gallon diesel tank. How long will it last on a single full tank? Here is how we calculate that:

22kW Diesel Generator Running Time =  (5 Gallons × 137,381 BTU/Gallon × 0.20 (Efficiency) / 3,412 BTU/kWh) / (22 kW × 1 (Full Load)) = 1.83 Hours 

As we can see, a 22kW diesel generator on a 5-gallon tank will run for 1.83 hours (1 hour and 50 minutes) at full load. We can write a full general generator running hours formula like this:

Generator Running Hours = (Tank Capacity (Gallons) × Fuel Energy Content Per Gallon × Efficiency / 3,412 BTU/kWh) / (Generator Wattage (in kW) × Load)

Clearly, this calculation is a bit complex. That’s why we simplified it by creating an automatic calculator. You just insert all these variables, and the calculator will tell how long will a generator last on a full tank.

Below the calculator, you will also find 4 charts (for diesel, gasoline, kerosene, and liquid propane). Each of them specifies how long will little 500 watts to very large whole-house 30kW running wattage generator run on 25% and 100% load on a 5-gallon tank and 20% efficiency. Let’s look at the calculator here:

How Long Will A Generator Last Calculator

Alright, here is how this calculator works:

Let’s say have a Champion Power Equipment 100900 2000-watt with 1600-watt running wattage generator. It has a 1.1-gallon gasoline tank. You want to know how long will it last at quarter load (25%) on this tank. To calculate this, slide the 1st wattage slider to ‘1600’, pick ‘Gasoline’, slide the 2nd tank capacity slider to ‘1.1’, and slide the 3rd load percentage slider to ’25’.

We get the estimated result: This Champion generator will last for 19.38 hours on a full tank at quarter load.

This is just one example. We have calculated the running times for diesel, gasoline, liquid propane, and kerosene generators (from 500W to 30,000W running wattage) at both 25% and 100% loads and summarized them in 4 neat charts you can consult. In all cases, we standardized the 20% efficiency and 5-gallon tank capacity. Here are all 4 charts, starting with the diesel generators:

Note: All the results are calculated for a 5-gallon tank, with 20% efficiency.

How Long Do Diesel Generators Last? (1st Chart)

As we can see, diesel generators run for up to about 15 hours on 100% load and up to about 60 hours on 25% load.

The smaller diesel generators like 500W generators can last up to 322.11 hours on a 5-gallon tank, but, in practice, these small generators have small 0.8-gallon or even smaller tanks; use the calculator above to determine the realistic continuous running hours estimate. Similarly, big 20kW+ generators can have bigger tanks; 7-gallon, 8-gallon, 10-gallon, or even more.

Diesel is one of the energy richest fuel sources. 1 gallon of diesel can produce 137,381 BTU at 100% efficiency. That’s why diesel generators last the longest, and that’s also a part of the reason why they are so popular.

Let’s look at the gasoline generators that don’t last as long since gasoline has 12.5% less energy content than diesel:

How Long Do Gasoline Generators Last? (2nd Chart)

Here we see that gasoline generators don’t last as long as diesel generators. The running hours in the chart above are for a 5-gallon tank, and 20% efficiency; smaller gasoline generators will have a below 5-gallon tank and bigger units will have a larger than a 5-gallon tank.

1 gallon of gasoline has an energy content of 120,238 BTU. That is less than diesel and kerosene energy content, but more than liquid propane energy content.

In fact, liquid propane has only 91,452 BTU per gallon energy content. That’s why liquid propane generators with the same tank capacity last a shorter amount of time. Here is the full chart for liquid propane (LPG) generators:

How Long Do Liquid Propane Generators Last? (3rd Chart)

The 5-gallon tank here is not all that appropriate (we’ll explain why) but it illustrates the running hours for a generator well.

As we said before, LPG is the least energy-rich generator fuel source. Here are examples of same-size generators and running hours using different generators (at 100% load):

• A 10kW diesel generator will last for 4.03 hours.
• A 10kW gasoline generator will last for 3.52 hours.
• A 10kW kerosene generator will last for 3.87 hours.
• A 10kW propane generator will last for 2.68 hours.

This might seem like a weakness of propane generators, but it’s really not. Why? Well, with diesel, gasoline, and kerosene, we usually have canisters or bottles of this. With liquid propane, however, many homeowners have 100-gallon, 250-gallon, 500-gallon, or even 1000-gallon propane tanks in the backyard. You can always refill your propane generators because you have a 100+ gallon supply right there.

Alright, last but not least, let’s look at kerosene generators and how long they last:

How Long Do Kerosene Generators Last? (4th Chart)

As you can see, kerosene generators last longer than gasoline or propane generators on the same-size tank, but not as long as diesel generators.

With these calculations, including the calculator and 4 charts, we hope that you now have a good understanding of how long generators last. If you have any questions or would like for us to do some math for you, you can use the comments below, and we will help you out.