The most common fuel types for portable generators are gasoline, propane (liquid petroleum gas, or LPG), and natural gas (NG). Gasoline offers the highest energy density and maximum peak wattage, but it degrades within 3 to 6 months and easily clogs carburetors. Propane produces roughly 10% to 15% less power per gallon equivalent, but it stores indefinitely without degrading or gumming fuel jets. Natural gas provides an uninterrupted utility supply but reduces engine power output by 15% to 20% and eliminates generator mobility.
Quick Fuel Comparison: At-a-Glance
| Fuel Type | Energy Content | Shelf Life | Carburetor Maintenance | Output Loss vs. Gas | Portability & Sourcing |
|---|---|---|---|---|---|
| Gasoline | ~114,000 BTU/gal | 3–6 months (1–2 yrs stabilized) | High (evaporation varnish, jets clog) | Baseline (0%) | High; standard jerrycans at gas stations |
| Propane (LPG) | ~91,500 BTU/gal | Indefinite (tank lifespan limits) | None (enters engine as clean vapor) | -10% to -15% | Moderate; 20 lb to 100 lb pressurized tanks |
| Natural Gas (NG) | ~1,000 BTU/cu ft | Continuous (piped utility grid) | None (clean gaseous fuel) | -15% to -20% | Fixed; tied to household gas plumbing line |
| Diesel | ~138,500 BTU/gal | 12–24 months (with biocides) | Low (injectors require clean fuel filters) | +10% efficiency | Moderate; heavy portable units, bulk storage |
1. Energy Density and Operating Efficiency
A generator engine converts thermal energy (measured in British Thermal Units, or BTUs) into mechanical rotation, which the alternator turns into electrical power. The fuel’s energy density dictates how much power the engine can generate per unit of fuel consumed.
ENERGY CONTENT COMPARISON (BTU per Gallon Equivalent)
Gasoline: ████████████████████████ (~114,000 BTU)
Propane: ███████████████████ (~91,500 BTU) [~20% less energy/vol]
Diesel: ████████████████████████████ (~138,500 BTU) [+21% energy/vol]
Gasoline: Maximum Wattage, Fast Burn
Because gasoline contains approximately 114,000 BTUs per gallon, portable engines produce their full advertised surge and running wattages on gas. However, small carbureted generator engines typically achieve only 15% to 20% thermal efficiency. A standard 7,500-watt generator running under a 50% electrical load burns roughly 0.6 to 0.75 gallons of gasoline per hour. To track exact burn rates across load levels, consult GPH Charts: Generator Fuel Consumption Calculations.
Propane: Clean Burning with Moderate Wattage Derating
Liquid propane expands into a dry vapor before entering the engine’s intake. Because a gallon of liquid propane delivers around 91,500 BTUs, the engine ingests less potential heat energy per intake stroke.
- The Wattage Penalty: An engine rated for 9,000 peak / 7,500 running watts on gasoline typically derates to around 8,100 peak / 6,750 running watts on propane. You must account for this drop when sizing household loads.
- Consumption: A standard 20-pound barbecue tank contains roughly 4.7 gallons of propane. At a 50% electrical load on a mid-sized generator, that tank will empty in about 6 to 9 hours of continuous runtime. Read more in Shelf Life & Efficiency: Propane Generator Systems.
Natural Gas: High Convenience, Lowest Volumetric Output
Natural gas delivered to homes through municipal utility pipes is mostly methane, supplying about 1,000 BTUs per cubic foot. In addition to delivering lower volumetric energy than vaporized propane, natural gas enters the carburetor at very low pressure (typically 5 to 7 inches of water column). This lower density starves the engine of peak charge air, resulting in a 15% to 20% wattage derating compared to gasoline.
2. Fuel Storage, Shelf Life, and Fuel System Wear
How a fuel behaves during months or years of storage is often more important for emergency backup than raw thermal efficiency.
FUEL SYSTEM DEGRADATION OVER TIME
[Untreated Gasoline] ──(90 Days)──► [Phase Separation & Gum] ──► [Clogged Carburetor Jets]
[Propane / NG] ──(Years)───► [Zero Chemical Change] ──► [Clean Engine Restart]
Gasoline Storage Vulnerabilities
Modern pump gasoline contains up to 10% or 15% ethanol ($E10$ or $E15$). Ethanol is hygroscopic: it actively pulls moisture out of the ambient air.
- Phase Separation: Over 3 to 6 months, water absorbed by ethanol drops out of suspension, settling at the bottom of the fuel tank as an ethanol-water slurry that ruins engine components.
- Evaporation and Gumming: Light chemical fractions evaporate from the carburetor bowl, leaving thick varnishes that plug the tiny brass pilot jets (often 0.015 to 0.025 inches in diameter). When this happens, the engine will either hunt, sputter, or refuse to idle without the choke on.
- Mitigation: Untreated gasoline must be rotated every 90 days. Using high-grade fuel stabilizers can extend storage life to 12 to 24 months, provided the stabilizer is mixed immediately upon purchase. For treatment protocols, review Fuel Stabilization: Gasoline Generators for Home Backup and Carburetor Protection: Best Generator Fuel Stabilizers.
Propane and Natural Gas Storage Advantages
- Indefinite Storage: Propane stored in sealed, heavy-gauge steel cylinders does not oxidize, absorb moisture, or break down over time. A tank filled a decade ago delivers the exact same combustion quality today.
- Zero Carburetor Maintenance: Because gaseous fuels leave no residue, fuel bowls cannot gum up, and float needles cannot stick. Dual-fuel generators run on propane can sit idle for two years and fire up on the first pull.
3. Cold Weather Dynamics and Operational Hazards
Ambient temperature influences how fuels move from storage containers into the engine:
PROPANE VAPORIZATION IN COLD WEATHER
[Warm Ambient Temp (70°F)] ──► Normal internal pressure (~120 PSI) ──► Steady vapor feed
[Freezing Temp (-10°F)] ──► Low internal pressure (~30 PSI) ──► Engine starves of fuel
- Propane Tank Freeze-Up: Liquid propane boils into vapor by drawing heat from the metal tank walls. At low ambient temperatures (below 20°F / -7°C), rapid fuel draw by a large generator cools the tank faster than ambient air can warm it. Frost forms on the outside of the cylinder, internal tank pressure drops, and the generator starves for fuel even though the tank is half full. To counter this, large generators run on propane in winter conditions require larger 100-pound tanks or multi-tank manifolds to increase vaporization surface area.
- Gasoline in Cold Weather: Gasoline atomizes poorly in freezing temperatures, requiring richer air-fuel mixtures (closing the carburetor choke) to pull sufficient fuel vapor into the combustion chamber for starting.
4. Selecting the Right Fuel Strategy for Your Backup System
For most homes, the optimal setup is a dual-fuel or tri-fuel generator. This allows you to choose your fuel based on the situation:
- Keep Propane for Inactive Readiness: Store two to four 20-pound or 30-pound propane tanks to ensure the generator can start instantly during sudden outages without draining, rotating, or stabilizing fuel.
- Use Gasoline for High-Demand Peak Run Periods: If an extended outage strikes and you need peak continuous wattage to run a 240V well pump or air conditioner, switch to gasoline to capitalize on its higher energy density.
- Plumb Natural Gas for Infinite Runtime: If natural gas is available at your home, installing an exterior quick-connect fitting provides an uninterrupted fuel supply that eliminates refueling runs during severe winter storms.
To evaluate multi-fuel generators or conversion options, see Fuel Flexibility: Dual Fuel & Tri-Fuel Generators and check runtime management in Consumption Rates: Portable Generator Runtime Expectations.