Last Updated on October 3, 2025
A lack of reliable power can spell disaster for many projects and facilities. From hospitals to construction sites, a diesel generator keeps business running and prevents costly delays or dangerous scenarios, such as spoiled food or malfunctioning medical equipment.
However, diesel generators only help if you have enough fuel to keep them running.
Consider these questions.
- What if you need the generator to power more equipment or systems than expected?
- What if your project or the power outage takes longer than expected?
- Do you know how much diesel fuel you’ll need?
In this article, you’ll learn how to estimate a generator’s fuel consumption so you can select the most efficient generator rental and correctly plan for the right amount of fuel.
Why You Need to Calculate Your Fuel Needs
Diesel generators can consume approximately 0.6–149 gallons of fuel per hour, depending on their size and how much they power.
This means you could easily spend thousands on fuel alone over the course of your project. This is especially true when you consider that the price of diesel fuel has ranged between $3.49–$4.71 per gallon from 2022 to 2025.
- Why Not Choose a Gas Generator? While gas generators generally use cheaper fuel, they are less efficient than diesel generators. This means they may not be able to power all your equipment reliably, requiring more or larger generators that can have greater fuel costs in the long run.
It’s important to calculate fuel consumption so you can select the right generator size and make sure you have enough fuel on hand to prevent costly delays or wasted budget. Having enough fuel is also critical during severe weather or other disasters because refueling stations may be closed, inaccessible or have a supply issue.
The Importance of Proper Generator Sizing
The term generator size refers to how many kilowatts (kW) a generator produces to power your equipment. The total kilowatts is also known as a generator’s “load.”
The ideal size is a generator from which you’ll use about 80% of its total load for most of the time it is in use.
Selecting the right generator size for your needs is the most important part of selecting a diesel generator.
A generator that is too small might:
- Cause multiple refueling delays, wasting time and budget.
- Provide unreliable power, which could lead to equipment malfunctions and safety issues.
- Damage the generator by causing it to overload and overheat, which is also a fire hazard.
A generator that is too large might:
- Cost more to rent or buy, wasting your budget.
- Waste fuel and damage the engine if unburned fuel causes wet stacking (a buildup of residue).
- Damage your equipment or cause other safety concerns.
About Generator Safety: To learn more about working with generators, read these OSHA Fact Sheets.
How to Calculate Diesel Generator Fuel Consumption
Use the steps below to help you determine what size generator you need and how much fuel you should plan for.
1. Estimate Your Total Fuel Requirements
To calculate the fuel requirements for your project, you need to know how much power your equipment will need on startup, while running and during surges in use.
- What Will Be Hooked Up to the Generator? List all the tools, systems and equipment that could possibly use the generator’s power. Don’t forget lights, security systems, computers and appliances. Make note of which items will use the generator continuously and which will only draw power occasionally.
- How Many Watts (W) or Kilowatts (kW) Do You Need? Look at manufacturer guides and equipment labels for how many watts each item in your list will use. Add all of it together to find the maximum kW you’ll need. (Reminder: 1 kilowatt = 1,000 watts.)
- Do You Need More Power for Startups or During Surges? Some appliances draw more power when they first start. Some equipment might only need to use the generator a few times. The generator will need to have enough load to supply power during startups and when extra items are added to the circuit.
- How Long Will You Use the Generator? How many hours per day will the generator be running and will it be running at the same capacity the whole time? Is an aging power grid or severe weather likely to extend your usage? When in doubt, overestimate how many hours or days you will use the generator.
- Add Up the Kilowatts and Run Time: Add up how many kilowatts are needed to start and run your equipment, and how many hours you plan to use the generator, then increase each number by about 20%. This gives you unused capacity to handle power spikes or unexpected events.
2. Consult the Diesel Generator Fuel Consumption Charts
Now you can select the right size of generator (one that produces more kilowatts than your maximum need) and estimate how much fuel it will use at any given point in your project.
The chart below offers general estimates for how many gallons per hour a diesel generator will use depending on its size and what percentage of its load is used.
IMPORTANT:
- This chart is for example purposes only. A specific model of generator may use more or less fuel than what’s shown here.
- Ask your equipment rental provider, consult the user’s manual or use a fuel consumption meter to find your model’s precise fuel consumption numbers.
- Running a generator for an extended period at less than 60% of a full load, may result in wet stacking (damaging residue, inefficient power and wasted fuel).
EXAMPLE: Estimating Fuel Consumption and Cost
While there’s not one universal diesel generator fuel consumption calculator, you can follow the steps below with the numbers on the char to estimate fuel consumption and cost.
- Find your generator’s size and the Gallons per Hour rate you’re likely to use.
- Multiply the Gallon per Hour rate by the number of hours you’ll use the generator at that capacity to get the total gallons you’ll need.
Example: A 100-kW generator running at full load for 24 hours straight is:
7.4 gal./hr. × 24 hours = 177.6 gallons of diesel fuel
- Multiply the total number of gallons by diesel’s current price per gallon.
Example: Continuing the example above, the total cost of fuel when diesel is $3.68 per gallon is:
177.6 gallons × $3.68 = $653.57
How Specific Models May Be Different
The chart below shows examples of how specific models may have slightly different fuel consumption rates than what was shown in the first chart. These are the fuel consumption rates of the skid-mounted and towable diesel generators offered at Herc Rentals.
3. Know What Affects Fuel Consumption and Plan Ahead
Now that you have an idea of how much fuel you need, take time to think through other factors that could impact your estimate. Several things can affect a diesel generator’s fuel consumption and efficiency, including:
- Generator Size and Load: The size of your generator will have a major impact on its fuel consumption and efficiency. A diesel generator becomes less fuel efficient when it’s oversized for what you need because the fuel can’t burn off correctly. Similarly, if the generator is overloaded for too long, the cooling fans and other systems will work harder and use more fuel.
- Run Time: If your generator only reaches 80% capacity for a few minutes each day, and the rest of the time it runs at 20% capacity, it becomes less fuel efficient.
- Maintenance and Condition of the Generator: If the generator does not receive quality maintenance and repairs, then it will wear out, reducing its efficiency. It is important to work with a reliable equipment provider and to monitor the condition of the pistons/cylinders, valves, seals and other moving parts.
- Environment: Both high and low temperatures can reduce fuel efficiency. High altitudes can also be a factor as low air density causes fuel inefficiency.
Find Fuel-Efficient Diesel Generators at Herc Rentals
When your project or facility needs a reliable diesel generator, contact the power generation experts at Herc Rentals. Our team of experts will help you find the right size of generator to power all your equipment safely, efficiently and continuously.
With Herc Rentals, you’ll also have access to the latest in generator technology and models for maximum operating and fuel efficiency. Our fleet of skid-mounted and towable diesel generators come in a variety of sizes, configurations and output capacities, perfect for any project. Plus, we can deliver them — and any accessory equipment you need, such as load banks — directly to your site, saving your team precious time and energy.
Explore Herc Rentals backup power generation solutions, or call us at 888-777-2700 to get powered up today.
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