Generator Calculator l Estimate Fuel Consumption, Runtime and Cost

Generator Fuel Consumption Calculator Widget
Actual load, not rated capacity, drives real fuel burn.
Cost Per Hour
$4.95
Fuel Used3.0 L/hr
8 Hour Cost$39.60
Load-adjusted
Formula runs instantly in your browser. No data is stored or sent anywhere.

Use this generator calculator to estimate generator fuel consumption, runtime, and operating cost based on generator size, fuel type, and load percentage. Whether you are planning backup power during an outage or sizing fuel requirements for a job site generator, this tool helps calculate how much fuel a generator uses per hour.

The power just went out, and you are standing next to your generator, trying to figure out how long that half tank of diesel is actually going to last you. Generators do not burn fuel at a steady rate, the way you might expect. A generator running at half its rated load does not simply burn half the fuel of a fully loaded one, and that gap catches a lot of people off guard right when they need accurate numbers the most.

We built this calculator so you can turn your generator’s size, fuel type, and load percentage into a real number, gallons or litres per hour, total runtime on your tank, and what that actually costs. Enter your generator’s kW rating, pick your fuel type, and tell us roughly how much load you are running, and the calculator handles the rest, no guessing, no spreadsheet.

Written by the FuelConsumptionCalc Research Team

Written by the FuelConsumptionCalc Research Team. Reviewed by the Power Equipment Research Team. Formulas and figures on this page are cross-checked against published generator manufacturer fuel consumption data and industry load-efficiency benchmarks.
Sources: fuel consumption benchmarks compared against published Cummins, Caterpillar, Kohler, and Generac fuel charts, specific fuel consumption (SFC) data for diesel, gasoline, propane, and natural gas generators, and industry load-efficiency research.

See our full methodology page for how we verify every figure on this site.
Last Updated: August 2026

Quick Answer

Fuel Consumption = Generator Size (kW) x Load Percentage x Fuel Rate Constant.
Example: 20 kW diesel generator at 75% load = approximately 1.5 gallons per hour

Load PercentageFuel Use vs Full Load
25% load30 to 40 percent of full-load rate
50% load60 to 70 percent of full-load rate
75% load85 to 90 percent of full-load rate
100% loadFull rated fuel rate

How This Generator Calculator Works

This calculator takes your generator’s size in kW, your fuel type, and your actual load percentage, then turns that into a real fuel consumption rate. Enter these three numbers and the calculator shows your hourly fuel burn immediately, accounting for the fact that generators do not burn fuel in a straight line with load the way a simple percentage multiplication would suggest.

Add your fuel price and tank size, and the calculator extends that intop your total runtime and what that runtime costs you. This gives you a realistic number to plan around during an outage or a job site rental, rather than a rough guess based on the generator’s full rated capacity.

Formula Used

The base calculation uses the standard generator fuel consumption formula:

Fuel Consumption (per hour) = Generator Size (kW) x Load Percentage x Fuel Rate Constant. Diesel generators average roughly 0.25 to 0.3 litres per kWh produced, commonly called the heat rate or specific fuel consumption. Gasoline generators run less efficiently per kWh than diesel, and propane and natural gas generators each have their own typical consumption rates.This base formula gets adjusted by a non-linear load curve, since fuel use does not scale perfectly straight with load percentage in real generators.

Example Calculation

Generator Size: 20 kW Fuel Type: Diesel Load: 75 percent

Step 1: Base Hourly Rate at Full Load:
20 kW x 0.28 L/kWh = 5.6 litres per hour at 100 percent load

Step 2: Apply the Non-Linear Load Adjustment:
At 75 percent load, real-world data shows roughly 85 to 90 percent of full-load consumption rather than a straight 75 percent, so the calculator uses approximately 87.5 percent here. 5.6 x 0.875 = 4.9 litres per hour

Step 3: Convert to Gallons:
4.9 / 3.78541 = approximately 1.3 gallons per hour This is the type of calculation the tool above runs the moment you click Calculate, using your own generator numbers instead of this example.

How to Use This Generator Calculator

Enter your generator’s kW size, select your fuel type, and enter your actual load percentage. Your fuel consumption rate, runtime, and cost appear instantly.

Enter Your Generator Size

Use your generator’s rated output in kW, found on the data plate or in the owner’s manual. If your generator is rated in kVA instead, multiply by the typical power factor of 0.8 to get an approximate kW figure for this calculator.

Select Your Fuel Type

Diesel, gasoline, propane, and natural gas generators each burn fuel at meaningfully different rates per kWh produced, so this selection changes your result directly. Diesel is generally the most fuel-efficient option of the four for sustained running.

Enter Your Load Percentage

Estimate the percentage of your generator’s rated capacity that your actual connected load uses, not the generator’s full capacity. A generator powering a few essentials during an outage might only be running at 25 to 40 percent load, while one running a full job site closer to capacity might sit at 70 to 80 percent.

Add Your Tank Size and Fuel Price

Enter your tank size and local fuel price to get your total runtime and cost on a full tank. For a one-time trip cost calculation on a vehicle rather than stationary equipment, our fuel cost calculator handles that separately.

kVA to kW Conversion for Generator Fuel Calculations

Many generators, especially larger standby and commercial units, are rated in kVA rather than kW, since kVA reflects total apparent power while kW reflects the real, usable power output this calculator needs. To convert, multiply your generator’s kVA rating by its power factor, typically around 0.8 for most generators, to get an approximate kW figure. A 100 kVA generator works out to roughly 100 x 0.8, or 80 kW, and a 500 kVA generator converts to roughly 400 kW. Check your generator’s nameplate for the exact power factor if precision matters, since some units use a slightly different figure.

Generator Fuel Consumption Formula Explained

Generator fuel consumption equals the generator’s kW size multiplied by load percentage, multiplied by a fuel rate constant specific to the fuel type, with a non-linear adjustment applied since generators do not burn fuel in a perfectly straight line with load.

The Basic Formula

Fuel Consumption (L/hr) = Generator Size (kW) x Load Percentage x Fuel Rate Constant. The fuel rate constant varies by fuel type, with diesel typically averaging 0.25 to 0.3 litres per kWh, the most efficient of the common options.

Why Fuel Use Is Not a Straight Percentage

A generator running at 50 percent load does not burn exactly half the fuel of the same generator at full load. Real-world data shows 25 percent load typically uses 30 to 40 percent of full-load consumption, 50 percent load uses 60 to 70 percent, and 75 percent load uses 85 to 90 percent. This happens because a generator engine has fixed internal friction and parasitic losses that stay roughly constant regardless of how much electrical load it is actually serving, so lower loads waste proportionally more fuel per unit of power produced. This is a similar underlying idea to how our boat fuel calculator treats cruising speed differently from wide open throttle; fuel efficiency is rarely a straight line against output.

Worked Example for a 20 kW Generator

A 20 kW diesel generator running at 75 percent load:

Full Load Rate: 20 kW x 0.28 L/kWh = 5.6 L/hr
Adjusted for 75% Load (87.5% of full-load rate): 5.6 x 0.875 = 4.9 L/hr
In Gallons: 4.9 / 3.78541 = approximately 1.3 gallons per hour

Fuel Consumption by Generator Size and Load

The table below shows approximate diesel fuel consumption in gallonsper hour for common generator sizes at different load percentages, calculated using the same non-linear load curve this calculator uses.

Generator Size25% Load50% Load75% Load100% Load
5 kW0.1 gal/hr0.2 gal/hr0.3 gal/hr0.4 gal/hr
10 kW0.3 gal/hr0.5 gal/hr0.6 gal/hr0.7 gal/hr
20 kW0.5 gal/hr1.0 gal/hr1.3 gal/hr1.5 gal/hr
30 kW0.8 gal/hr1.4 gal/hr1.9 gal/hr2.2 gal/hr
50 kW1.3 gal/hr2.4 gal/hr3.2 gal/hr3.7 gal/hr
75 kW1.9 gal/hr3.6 gal/hr4.9 gal/hr5.5 gal/hr
100 kW2.6 gal/hr4.8 gal/hr6.5 gal/hr7.4 gal/hr
150 kW3.9 gal/hr7.2 gal/hr9.7 gal/hr11.1 gal/hr
250 kW6.5 gal/hr12 gal/hr16.2 gal/hr18.5 gal/hr
500 kW12.9 gal/hr24 gal/hr32.4gal/hr37 gal/hr

These figures reflect diesel generators specifically, since diesel is the most commonly referenced fuel type for sizing charts. Gasoline, propane, and natural gas generators will show different absolute numbers using the same non-linear load pattern.

Running Watts vs Starting Watts

Running watts are the continuous power a connected load draws during normal operation, while starting watts, also called surge watts, are the temporary spike needed when motors, compressors, and pumps first switch on. This surge can run 2 to 5 times higher than the running watts figure for the same appliance, briefly, for a second or two.

This matters for fuel calculations because your generator needs enough headroom to handle that starting surge without stalling or shutting down, even though your actual fuel-burning load percentage for this calculator should reflect your running watts total, not the brief starting spike. Undersizing a generator to your running watts without accounting for starting watts is a common mistake that causes nuisance shutdowns under real-world conditions.

Standby vs Prime Power Fuel Consumption

Standby generators are rated for backup power during outages, designed to run intermittently and typically not recommended for continuous operation beyond a manufacturer-specified limit, often around 200 to 500 hours per year. Prime power generators, by contrast, are built and rated for continuous, unlimited running hours as a primary power source, common on remote job sites or in off-grid applications.

Prime-rated generators are generally engineered for better fuel efficiency and durability under sustained load compared to a standby unit pushed into similar continuous service, since standby units are not designed with that level of constant operation in mind. If you are calculating fuel consumption for a generator running many hours per day over an extended period, confirm whether your unit is rated for standby or prime power use, since running a standby generator at prime-power duty cycles can accelerate wear well beyond what this calculator’s fuel estimates alone would suggest.

What Affects Your Generator’s Fuel Consumption

Four factors shape your generator’s real fuel consumption the most.Load percentage has the largest single impact, while fuel type, altitude and temperature, and maintenance condition all compound on top of that baseline.

Load Percentage

Load percentage is the single biggest factor in fuel consumption per unit of power produced, with very light loads, under 30 percent, and maximum loads both reducing efficiency compared to the optimal 50 to 75 percent range most manufacturers recommend.

Fuel Type

Diesel generators are generally the most fuel-efficient option for sustained running, followed by natural gas, propane, and gasoline, roughly in that order. Gasoline generators are typically the least efficient for extended use, which matters more for long outages than short ones.

Altitude and Temperature

Higher altitude reduces air density, which decreases generator efficiency and increases fuel consumption for the same load percentage. Extreme temperatures, both hot and cold, can similarly reduce real-world efficiency compared to the conditions under which manufacturers’ fuel charts are typically tested. If you are comparing fuel figures across different unit systems for international equipment, our fuel unit converter handles gallons, litres, and other conversions directly.

Maintenance and Age

Dirty air filters, old engine oil, and worn injectors all increase fuel consumption above a well-maintained generator’s rated figures. Keeping up with routine maintenance helps your real-world fuel use stay closer to the manufacturer’s published numbers rather than drifting upward over time.

The Risk of Running a Generator Too Light

Running a generator at a sustained low load, generally under 30 percent, for extended periods can cause a condition called wet stacking, where unburned fuel and carbon buildup accumulate in the exhaust system because the engine never reaches a high enough operating temperature to burn fuel cleanly. Beyond the fuel efficiency cost, wet stacking can lead to real maintenance problems and reduced generator lifespan over time.

If you know your generator will spend most of an outage running well below 30 percent load, consider whether a smaller generator more closely matched to your actual needs would run more efficiently and reliably than an oversized unit running consistently light.

How to Improve Your Generator’s Fuel Efficiency

Size your generator to your actual expected load rather than buying significantly more capacity than you need, since running consistently in the 50 to 75 percent load range is both more fuel efficient and healthier for the engine than running consistently light. Our fuel cost calculator can help you compare running costs if you are deciding between equipment sizes.

Keep up with routine maintenance, including air filters, oil changes, and injector or carburetor service, since all of these directly affect real-world fuel consumption compared to a generator’s rated figures.

Turn off or unplug non-essential loads during an outage when fuel conservation matters most, since reducing your load percentage back toward the efficient range can meaningfully extend your runtime on a limited fuel supply. For predictable, scheduled use rather than emergency backup, diesel generally offers the best fuel efficiency of the common fuel types if you have a choice in equipment. If you run a generator regularly rather than only during occasional outages, our monthly fuel cost calculator helps you budget that recurring expense.

Methodology

Generator fuel consumption on this page is calculated using the formula:
Fuel Consumption = Generator Size (kW) x Load Percentage x Fuel Rate Constant, with a non-linear adjustment applied based on published real-world load efficiency data, since 25 percent load typically uses 30 to 40 percent of full-load consumption, 50 percent load uses 60 to 70 percent, and 75 percent load uses 85 to 90 percent rather than scaling in a straight line. Fuel rate constants are derived from generator manufacturer fuel consumption charts and published specific fuel consumption data by fuel type.

These figures reflect industry-published averages and are not a guarantee for any specific generator, fuel quality, or operating condition. Real-world fuel consumption depends on your specific generator’s efficiency, age, maintenance condition, altitude, and temperature, and your own logged fuel use will always be the most accurate figure for your equipment.

Frequently Asked Questions

It depends primarily on generator size, fuel type, and load percentage.A 20 kW diesel generator running at 75 percent load uses approximately1.3 gallons per hour, while the same generator at 25 percent load uses roughly 0.6 gallons per hour, not simply one third of the full-load rate, since fuel consumption does not scale in a perfectly straight line with load. This calculator runs that full calculation instantly once you enter your generator size, fuel type, and load percentage.

Most generators run most efficiently in the 50 to 75 percent load range, where the engine produces meaningful power output relative to its fixed internal losses without being pushed to its maximum capacity. Running consistently under 30 percent load wastes fuel proportionally and can risk wet stacking over time, while running at 100 percent load for extended periods also reduces efficiency and increases wear compared to the optimal middle range.

Divide your tank capacity by your calculated hourly fuel consumption rate to get your total runtime in hours. A 20 kW diesel generator burning approximately 1.3 gallons per hour at 75 percent load with a 20 gallon tank gives roughly 15.4 hours of runtime, though actual runtime will vary with your real load percentage over that period rather than a perfectly constant rate.

At 100 percent load, a 20 kW diesel generator typically burns approximately 1.5 gallons per hour. At a more common 50 percent load, that drops to roughly 1.0 gallon per hour, and at 25 percent load, roughly 0.6 gallons per hour, reflecting the non-linear relationship between load percentage and fuel consumption rather than a simple straight-line scaling.

Diesel is generally more fuel-efficient per kWh produced than gasoline, which usually makes it the cheaper option to run over an extended outage, even though diesel fuel itself sometimes costs more per gallon than gasoline depending on your region. For short, infrequent outages, the efficiency gap matters less and gasoline’s wider availability and lower equipment cost can make it the more practical overall choice.

Wet stacking is a condition where unburned fuel and carbon buildup accumulate in a diesel generator’s exhaust system because the engine has been run at a sustained low load, generally under 30 percent, and never reaches a high enough operating temperature to burn fuel completely and cleanly. Beyond wasting fuel, wet stacking can cause real maintenance issues and reduce the generator’s working lifespan if it continues unaddressed over many hours of light-load running.

Yes. Higher altitude reduces air density, which decreases a generator engine’s efficiency and can increase fuel consumption for the same electrical load percentage compared to sea level. This effect is most noticeable at significant elevations and is one of several environmental factors, alongside extreme temperatures, that can make real-world fuel use differ from a manufacturer’s standard-conditions fuel chart.

A 50 kW diesel generator burns approximately 2.4 gallons per hour at 50 percent load, or roughly 3.7 gallons per hour at full load, following the same non-linear load curve as smaller generators. Enter your own load percentage and fuel type into the calculator above for a figure specific to your actual operating conditions rather than a general benchmark.

A 100 kW diesel generator burns approximately 4.8 gallons per hour at 50 percent load, or roughly 7.4 gallons per hour at full load. As with smaller units, running at a lighter load uses proportionally more fuel per kWh produced than running in the 50 to 75 percent optimal range, which is worth factoring into fuel budgeting for larger standby or job site units.

Yes. Multiply your generator’s kVA rating by its power factor, typically around 0.8, to get an approximate kW figure, then enter that kW value into the calculator above along with your fuel type and load percentage. A 100 kVA generator converts to roughly 80 kW, and a 500 kVA generator converts to roughly 400 kW using this standard conversion.

Yes, generally. An aging generator with worn injectors, degraded compression, or accumulated carbon buildup typically burns more fuel per kWh produced than the same unit did when new, even at the identical load percentage. Regular maintenance, including air filters, oil changes, and injector or carburetor service, helps slow this drift and keeps real-world fuel use closer to the manufacturer’s original rated figures.

There is no single most efficient size in isolation, since efficiency depends on how well a generator’s size matches your actual load rather than the generator’s size alone. A generator running consistently in the 50 to 75 percent load range will be more fuel efficient per kWh produced than the same generator running at 20 percent load, so the more useful question is whether your generator is sized appropriately for your typical load rather than which kW rating is inherently best.

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Disclaimer

This calculator provides estimates only. Actual generator fuel consumption varies based on generator efficiency, age, maintenance condition, altitude, temperature, and actual load over time. See our full disclaimer page for more detail.