Generator Fuel Consumption: Diesel vs Natural Gas vs Propane
A 100kW diesel generator burns 7 gallons/hour at full load. Natural gas: 1,000 cubic ft/hr. Propane: 10 gal/hr. Complete consumption tables and cost comparison.
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Open Calculator →Fuel is the single largest operating cost for any generator that runs more than a few hundred hours per year. A 500 kW diesel generator at 75% load burns roughly 27 gallons per hour. At $3.50 per gallon, that is $94.50 every hour — $2,268 per day — $826,000 per year if running continuously. Choosing the right fuel type and properly sizing your fuel storage can save tens of thousands of dollars annually. Getting it wrong can leave you out of fuel during the outage that matters most.
Fuel Consumption Rates by Generator Size
Fuel consumption depends on three variables: generator size (kW), load factor (percentage of rated capacity being used), and fuel type. Here are real-world consumption rates at common load levels:
Diesel Generators
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 30 kW | 1.6 GPH | 2.3 GPH | 3.2 GPH | 4.1 GPH |
| 100 kW | 3.6 GPH | 5.4 GPH | 7.4 GPH | 9.8 GPH |
| 200 kW | 6.4 GPH | 10.1 GPH | 14.3 GPH | 18.5 GPH |
| 500 kW | 13.7 GPH | 20.8 GPH | 27.1 GPH | 36.5 GPH |
| 1,000 kW | 24.6 GPH | 39.7 GPH | 55.3 GPH | 71.1 GPH |
| 2,000 kW | 48.4 GPH | 78.0 GPH | 109.5 GPH | 141.2 GPH |
Natural Gas Generators
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 30 kW | 120 CFH | 176 CFH | 235 CFH | 301 CFH |
| 100 kW | 325 CFH | 500 CFH | 675 CFH | 875 CFH |
| 200 kW | 595 CFH | 940 CFH | 1,285 CFH | 1,650 CFH |
| 500 kW | 1,300 CFH | 2,100 CFH | 2,900 CFH | 3,750 CFH |
| 1,000 kW | 2,475 CFH | 4,000 CFH | 5,600 CFH | 7,250 CFH |
CFH = cubic feet per hour
Propane (LP Gas) Generators
| Generator Size | 25% Load | 50% Load | 75% Load | 100% Load |
|---|---|---|---|---|
| 30 kW | 2.4 GPH | 3.6 GPH | 4.9 GPH | 6.3 GPH |
| 100 kW | 6.8 GPH | 10.5 GPH | 14.1 GPH | 18.3 GPH |
| 200 kW | 12.3 GPH | 19.6 GPH | 26.9 GPH | 34.5 GPH |
| 500 kW | 27.0 GPH | 43.5 GPH | 59.8 GPH | 77.0 GPH |
GPH = gallons per hour (liquid propane)
Cost Comparison: Diesel vs Natural Gas vs Propane
Raw fuel consumption does not tell the whole story. What matters is the cost per kWh generated — and that depends on both the consumption rate and the local fuel price.
| Metric | Diesel | Natural Gas | Propane |
|---|---|---|---|
| Fuel price (2026 avg) | $3.30-$4.20/gal | $8-$14/MCF | $2.50-$3.50/gal |
| Energy content | 138,500 BTU/gal | 1,020 BTU/CF | 91,500 BTU/gal |
| Efficiency (electrical) | 35-42% | 28-38% | 25-32% |
| Cost per kWh (at 75% load) | $0.18-$0.28 | $0.12-$0.22 | $0.22-$0.35 |
| 500 kW unit, 24hr runtime | $1,700-$2,700 | $1,150-$2,100 | $2,100-$3,350 |
| 500 kW unit, annual (2,000 hrs) | $142,000-$224,000 | $96,000-$176,000 | $175,000-$280,000 |
Natural gas is consistently the cheapest fuel on a per-kWh basis, primarily because pipeline gas prices have remained low relative to liquid fuels. However, natural gas generators have lower electrical efficiency and are not available in the largest sizes (typically maxing out at 1,500-2,000 kW per unit versus 3,000+ kW for diesel). They also require a pipeline connection — which may not survive the same disaster that took out utility power.
Runtime Calculations and Tank Sizing
For diesel and propane generators, runtime is limited by your fuel storage capacity. The formula is simple:
Runtime (hours) = Tank capacity (gallons) / Consumption rate (GPH)
Diesel Tank Sizing Examples
| Generator | 75% Load GPH | 500 Gal Tank | 1,000 Gal Tank | 2,000 Gal Tank |
|---|---|---|---|---|
| 100 kW | 7.4 | 68 hours (2.8 days) | 135 hours (5.6 days) | 270 hours (11.3 days) |
| 200 kW | 14.3 | 35 hours (1.5 days) | 70 hours (2.9 days) | 140 hours (5.8 days) |
| 500 kW | 27.1 | 18 hours (0.8 days) | 37 hours (1.5 days) | 74 hours (3.1 days) |
| 1,000 kW | 55.3 | 9 hours | 18 hours (0.8 days) | 36 hours (1.5 days) |
Most building codes and standards (like NFPA 110 for healthcare facilities) specify minimum runtime requirements. Hospitals must have enough fuel for 96 hours. Data centers targeting Tier III uptime typically store 48-72 hours of fuel on-site with contracted refueling within 24 hours. Sizing the tank to your actual needs prevents both the cost of an oversized tank and the risk of running dry.
The Refueling Factor
During a widespread outage (hurricane, ice storm, earthquake), fuel delivery may be delayed 24-72 hours or more. Every facility relying on generator power needs the same fuel at the same time, and tanker trucks cannot be everywhere. Your tank should hold enough fuel to ride out the delivery delay, not just the expected outage duration.
A practical rule: store at least 150% of your minimum required runtime in fuel capacity. If you need 48 hours of runtime, tank for 72 hours. The cost of a larger tank ($3,000-$8,000 more for the next size up) is trivial compared to the cost of a generator sitting idle because the fuel ran out.
Environmental and Regulatory Considerations
Emissions regulations increasingly affect generator fuel choices, especially in states like California, New York, and Massachusetts:
- Diesel: Tier 4 Final emissions standards require expensive aftertreatment systems (DPF, SCR) that add $20,000-$60,000 to generator cost. Some jurisdictions limit diesel generator runtime for air quality reasons.
- Natural gas: Significantly lower NOx and particulate emissions than diesel. Easier to permit in urban areas and near sensitive receptors. Some states offer permitting advantages for natural gas generators.
- Propane: Burns cleaner than diesel but less efficiently. No Tier 4 aftertreatment needed. Good option where natural gas pipelines are not available but cleaner emissions are required.
- Bi-fuel (diesel/natural gas): Emerging option that uses natural gas as the primary fuel with diesel pilot injection. Reduces fuel cost by 30-50% and emissions by 30-60% compared to straight diesel.
Fuel Storage Costs
| Tank Type | Capacity | Installed Cost |
|---|---|---|
| Sub-base (under generator) | 150-1,000 gal | $3,000-$12,000 |
| Above-ground (single wall) | 500-10,000 gal | $4,000-$25,000 |
| Above-ground (double wall) | 500-10,000 gal | $6,000-$40,000 |
| Underground | 1,000-20,000 gal | $15,000-$60,000+ |
| Propane (above-ground) | 500-2,000 gal | $3,000-$10,000 |
Double-wall tanks are required in most jurisdictions for diesel storage near buildings. Underground tanks require environmental permitting and ongoing leak detection monitoring — add $2,000-$5,000/year in compliance costs.
Making the Right Choice
- Standby use, under 500 hrs/year: Diesel is the default. Fuel cost is minimal, and diesel generators have the highest power density, smallest footprint, and best reliability track record.
- Prime use, pipeline gas available: Natural gas wins on operating cost. The lower efficiency is more than offset by the lower fuel price, and you never run out of fuel (unless the pipeline goes down).
- Remote sites, no pipeline: Diesel or propane. Diesel for large loads, propane for smaller installations where cleaner emissions or longer storage life matters (diesel degrades in storage; propane does not).
- Emissions-sensitive locations: Natural gas or bi-fuel. Worth the higher equipment cost to avoid permitting headaches and community opposition.
Use our generator sizing calculator to estimate fuel consumption, tank sizing, and annual fuel costs for your specific generator application. Select your fuel type, generator size, and expected load factor to get detailed runtime and cost projections.