Fuel Type: The Second Most Important Decision
After sizing, fuel type is the most consequential decision when selecting a commercial generator. Each fuel has distinct advantages and constraints that make it better suited for specific applications. There is no universally "best" fuel — the right choice depends on your facility's infrastructure, location, and criticality requirements.
Diesel Generators
Advantages
- Highest energy density: Diesel fuel contains approximately 137,000 BTUs per gallon vs 91,500 for gasoline and 84,300 for propane. This means more power per gallon and longer runtime from smaller fuel tanks.
- No utility dependency: Diesel generators run from an on-site fuel tank, independent of gas utility lines. If a natural disaster disrupts gas service, diesel generators continue operating.
- Best for prime power: Diesel engines are engineered for continuous operation at high loads. They're the standard choice for prime power applications where the generator runs 24/7 as the primary power source.
- Longest service life: Diesel engines typically last 20,000–30,000 hours before major overhaul, compared to 10,000–15,000 hours for natural gas engines.
Disadvantages
- Higher fuel cost per kWh: Diesel fuel costs more than natural gas on a per-kWh basis, particularly in areas with low natural gas rates.
- Fuel storage requirements: On-site diesel tanks require permitting, secondary containment, and periodic fuel treatment to prevent algae growth and water contamination.
- Higher maintenance: Diesel generators require more frequent oil changes, filter replacements, and fuel system maintenance than gas units.
- Emissions: Diesel generators produce more particulate emissions and NOx than natural gas units. Newer Tier 4 Final diesel engines address this but at higher cost.
Natural Gas Generators
Advantages
- Continuous fuel supply: Connected to the utility gas line, natural gas generators never run out of fuel — no tanks to refill, no fuel deliveries to schedule.
- Lower running cost: Natural gas is typically the cheapest fuel per kWh, especially for facilities that already have gas service.
- Cleaner emissions: Natural gas burns cleaner than diesel, with significantly lower particulate emissions. Easier to meet environmental regulations.
- Lower maintenance: Natural gas engines require less frequent oil changes and don't have fuel filter contamination issues.
Disadvantages
- Utility dependent: If the gas utility line is damaged (earthquake, construction accident, or prolonged disaster), the generator cannot operate.
- Lower energy density: Natural gas generators are typically 10–15% less fuel-efficient than comparable diesel units at the same load.
- Gas line sizing: Large commercial generators require high-volume gas service. If your existing gas line is undersized, upgrading it adds significant cost.
Propane (LP Gas) Generators
Advantages
- No utility dependency: Like diesel, propane is stored on-site — but in a cleaner, safer tank system that requires less maintenance than diesel storage.
- Clean burning: Propane burns very cleanly with minimal carbon buildup in the engine. This means less maintenance and longer engine life compared to diesel.
- Excellent for rural locations: Properties without natural gas service can use propane as a readily available alternative.
- Long shelf life: Unlike diesel (which degrades over time) and gasoline (which goes stale in months), propane stored in a sealed tank lasts indefinitely.
Disadvantages
- Tank installation required: An above-ground or underground propane tank requires space, permitting, and periodic refilling.
- Limited runtime: A 500-gallon tank powering a 22 kW generator at 50% load lasts approximately 4–5 days. Extended outages require scheduled fuel deliveries.
- Fuel cost variability: Propane prices fluctuate seasonally and regionally, making cost prediction less reliable than natural gas.
Dual Fuel Generators
Dual fuel generators can run on either natural gas or propane, providing maximum flexibility. They're ideal for facilities that want the convenience of natural gas with the backup security of an on-site propane tank. If the gas utility fails, the generator switches to propane and continues operating.
Decision Matrix by Application
- Data centers and hospitals: Diesel (reliability, utility independence, prime power capability)
- Commercial office buildings: Natural gas (low cost, clean, continuous supply)
- Rural commercial facilities: Propane or dual fuel (no gas service available)
- Construction sites: Diesel (portable, high energy density, no utility connection needed)
- Residential standby: Natural gas or dual fuel (lowest cost, automatic operation)
Browse by Fuel Type
GeneratorProDirect carries generators in all fuel types from Generac, Kohler, Cummins, and Briggs & Stratton — all shipping tax-free in 45+ states. Browse all generators and filter by fuel type.
