Generator Fuel Options Reference

Overview

The fuel decision determines cost per kWh, run duration, refueling logistics, + storage hazards. For standby generators, fuel choice is typically natural gas or propane; for portable, also gasoline or diesel. This reference covers trade-offs.

Natural gas (most common standby)

Pros

• Continuous supply (utility pipeline; no refueling)
• Lower cost per BTU than propane
• Indefinite runtime
• No fuel storage on-site
• No fuel spills
• No fuel theft

Cons

• Subject to utility outage (rare, but earthquakes, gas line damage)
• Requires gas pipeline at property
• Slightly lower power density than gasoline/diesel
• Pressure drop concerns with high gas demand (verify meter sizing)

Best for:

• Suburban homes connected to natural gas
• Permanent installations
• Customers wanting "set it and forget it"

Cost (2025):

• Energy content: billed by the therm (100,000 BTU) or by CCF, roughly 1,030 BTU per cubic foot.
• Cost per BTU: the cheapest of the four fuels here in most US markets, often by a wide margin against propane.
• Consumption at full load: a 22 kW air-cooled unit pulls in the neighborhood of 300 cubic feet per hour at full load. Confirm the meter and the service can supply that on top of the furnace and water heater, or the generator will starve on a cold night.
• What to tell the customer: there is no tank, no delivery, and no refill scheduling. Run cost is the lowest available and it shows up on the gas bill they already get. The tradeoff is total dependence on the utility staying pressurized.

Propane (second most common standby)

Pros

• Higher power density than natural gas
• Can be stored on-site (no utility dependency)
• Works in rural areas
• Cleaner burn than gasoline/diesel
• Long shelf life (no degradation like gasoline)

Cons

• Tank required (storage + space)
• Refilling costs + scheduling
• Tank monitoring required
• Vapor pressure drops in extreme cold (under -10°F gradually)
• Initial tank cost

Best for:

• Rural / no natural gas service
• Customers wanting fuel independence

Tank sizing:

A propane tank fills to 80 percent of nominal volume per NFPA 58, so a 500 gallon tank carries 400 usable gallons, not 500. For a typical home + 22 kW generator: a 500-gallon tank gives roughly 5 days of runtime at full load. Adjust for actual load + outage duration concerns.

Cost (2025):

• Energy content: roughly 91,500 BTU per gallon of liquid propane.
• Cost per BTU: typically several times natural gas in the same market, and it moves with heating season demand. Propane is the expensive fuel of the two piped options.
• Consumption at full load: a 22 kW air-cooled unit burns in the neighborhood of 3.3 gallons per hour at full load, roughly half that at half load. Multiply by the outage hours the customer is planning for.
• Annual filling cost (if used): exercise cycles alone are minor, on the order of a few gallons a year for a weekly 10 to 20 minute run. The real fuel spend is outage runtime, so a customer in an area with a few multi-day outages a year should expect one significant fill per outage season, while a customer who rarely loses power may go years on a single fill.
• What to tell the customer: the tank fill is the visible cost, but the tank rental or purchase and the delivery minimum are what actually drive the yearly number. Have them price that with their propane supplier before you size the tank.

Gasoline (portable generators)

Pros

• Widely available (any gas station)
• Cheap fuel
• Easy to handle small quantities

Cons

• Limited shelf life (3-6 months without stabilizer; 1-2 years with)
• Flammable + explosive risk
• Theft concern
• Refueling required during outage
• Carburetor gumming if not maintained

Best for:

• Portable generators
• Short-duration outages
• Customers who can refuel during outage

Storage:

• Approved fuel cans (5 gallon max)
• Vented, cool location
• Away from ignition sources
• Fresh fuel only (stabilizer recommended)
• Code limits typical: 25 gallons in residential garage; varies by jurisdiction

Cost (2025):

• Energy content: roughly 114,000 BTU per gallon, more per gallon than propane.
• Cost per BTU: higher than natural gas, in the same neighborhood as propane, and it swings with pump prices.
• Consumption at full load: a 5 kW portable burns on the order of half a gallon per hour at full load. A 24 hour outage at heavy load is a multi-can day, and that is assuming stations are open and pumping, which after a wide-area outage they often are not.
• What to tell the customer: the fuel is cheap to buy and expensive to have. It goes stale, it has to be rotated, it is a fire and theft risk in the garage, and buying it during an outage means standing in line for it.

Diesel (commercial + large residential)

Pros

• Highest energy density (longest runtime per gallon)
• Long fuel storage life (with treatment)
• Reliable in cold weather
• Heavy-duty engine (longer lifespan)

Cons

• Most expensive fuel
• Smelly + noisy operation
• Particulate emissions higher
• Diesel engines more expensive to maintain
• Less common in residential

Best for:

• Large estates
• Mission-critical applications (medical equipment, server rooms)
• Locations without natural gas + budget for premium

Cost (2025):

• Energy content: roughly 138,000 BTU per gallon, the highest of the four. That is why diesel wins on runtime per gallon of on-site storage.
• Cost per BTU: fuel price per gallon runs high, but the energy density partly offsets it. On a per-kilowatt-hour-generated basis diesel is competitive with propane and well above natural gas.
• Consumption at full load: a 50 kW diesel set runs in the range of 3.5 to 4 gallons per hour at full load. Size the day tank and the fill schedule from the required runtime, not from the tank that came with the unit.
• What to tell the customer: the fuel spend is not the story. Storage tank cost, annual fuel polishing or treatment, and the higher engine maintenance are what make diesel a commercial decision. Where it earns its keep is long runtime with fuel entirely under the owner's control.

Dual-fuel options

Some generators accept multiple fuels:

• Tri-fuel (gasoline, propane, natural gas): portable + commercial
• Dual-fuel (gasoline + propane): common portable upgrade
• Convertible: natural gas + propane (standby with conversion kit)

Pros:

• Fuel flexibility
• Backup option if primary unavailable

Cons:

• More complex carburetion
• Slightly more expensive
• Different output ratings per fuel type

Fuel pressure requirements (gaseous)

Natural gas:

• Residential standbys land somewhere in the 5 to 11 inches water column band at the inlet, and the acceptable window is unit-specific. Take it off the data plate or the installation manual, not off another job
• Some commercial: 1-2 PSI with a regulator at the machine
• The number that matters is inlet pressure UNDER FULL LOAD, not at rest

Propane:

• Commonly 10 to 14 inches water column at the inlet, again unit-specific
• Second-stage regulator sets it. Confirm the regulator's rated capacity covers the generator's demand plus the house

Important: if pressure too low, generator runs lean (worse emissions, won't hit full power); too high, generator runs rich (carbon buildup, fouled spark plugs).

Fuel line sizing

There is no safe one-line rule of thumb here, and a table that gives you a pipe size off kW alone will undersize the run. Size it the way the code does:

1. Take the generator's fuel demand in CFH off the nameplate or spec sheet at FULL load. A 22 kW air-cooled unit is in the neighborhood of 300 CFH on natural gas.
2. Add every other appliance fed off the same section: furnace, water heater, range, dryer, pool heater. The generator is not the only thing running on the cold night it starts.
3. Measure the TOTAL DEVELOPED LENGTH to the farthest outlet, and add the equivalent length of the fittings.
4. Look the size up in the NFPA 54 capacity table for the actual delivery pressure and the allowed pressure drop. The tables differ completely between a low-pressure system (roughly 7 in WC at the meter, 0.5 in WC drop) and a 2 psi system with a regulator at the machine.

For scale on a low-pressure system: 3/4 in pipe carries well under 200 CFH at 25 ft, and 1 in carries only a bit over 200 CFH at 50 ft. That means a 300 CFH generator at any real distance from the meter is 1-1/4 in territory, not 3/4 in, unless the run is fed at 2 psi. Sizing that pipe by kW rule of thumb is exactly how a unit ends up dropping frequency under load on the first cold outage.

For propane, run the same method against the propane tables and the tank regulator's rated capacity.

Then verify: measure pressure AT the generator inlet with the unit at full load and every other gas appliance firing. Static pressure with nothing running tells you nothing.

Code + regulatory

Local codes:

• NFPA 54 (Natural Gas Code)
• NFPA 58 (Liquefied Petroleum Gas Code)
• IFGC (International Fuel Gas Code)
• Local amendments

Permits: gas permit + electrical permit + sometimes building permit

Inspection: pressure test + leak check + permit closeout

Tank placement (propane)

Code-required distances:

• For the common 125 to 500 gallon ASME tank, NFPA 58 sets 10 ft from the building and 10 ft from the line of adjoining property. This is the model code baseline, not a regional quirk
• Larger tanks carry larger separations. Look the size up in the NFPA 58 separation table rather than assuming 10 ft scales
• Keep it clear of ignition sources, and clear of the generator's own exhaust discharge

Local code and the propane supplier's own placement rules can be stricter. Verify before you dig.

Local code varies; verify before placement.

Refilling logistics (propane)

• Local propane supplier provides scheduled refills
• "Keep full" service: supplier monitors, refills when 30% remaining
• Customer pays per delivery or per gallon
• Some suppliers offer remote monitoring + automatic refill

When to recommend each fuel

Recommend natural gas:

• Utility service available
• Customer wants no fuel management
• Indefinite runtime priority

Recommend propane:

• Rural / no natural gas
• Customer wants fuel independence
• Outage frequency high

Recommend gasoline (portable only):

• Short-duration outages
• Customer comfortable with refueling
• Lower budget

Recommend diesel:

• Very large loads
• Mission-critical (medical, business)
• Large fuel storage acceptable

Fuel system maintenance

For each fuel:

• Natural gas: annual inspection of supply line + pressure regulator
• Propane: tank inspection annually; regulator every 10 years
• Gasoline: drain + replace fuel every 6 months OR use stabilizer
• Diesel: water + sediment removal; fuel polishing for long-storage

References

• NFPA 54, 58, 110
• IFGC (International Fuel Gas Code)
• Manufacturer fuel pressure specifications
• Manuall internal: Generator Sizing for Residential, Whole-House Standby Generator Installation SOP