Generator Natural Gas + Propane Conversion Reference

Why this reference exists

Most modern standby generators run on natural gas (NG) or propane (LP). Converting between fuel types - or correctly sizing fuel supply at install - affects performance, longevity, + customer satisfaction. Wrong fuel hookup = generator can't carry load at peak; customer fury during the very outage they bought the generator for. This is the working framework.

Fuel choice at install

Natural gas:

• Pulled from utility supply line
• Unlimited duration during outage (utility usually independent of electric)
• Slightly higher operating cost per BTU than propane
• Standard residential 0.5 psi (sometimes 2 psi for larger generators)
• Most common for whole-home generators

Propane:

• Bottle (20-100 lb) or tank (250-1,000 gallon)
• Finite duration (tank size limits)
• Slightly more BTU per cubic foot than NG (denser fuel)
• Higher pressure typical (2-11 psi at burner)
• Common when no NG available OR customer prefers tank ownership

Diesel (rare residential):

• Diesel tank or in-fuel-tank
• Long shelf life (with stabilizer)
• More expensive fuel
• Higher fuel storage required
• Specific to certain commercial / large residential

For most: NG if available; LP if not.

Why conversion sometimes needed

At install time:

• Generator ships LP-ready; customer has NG → convert
• Generator ships NG-ready; customer has LP → convert
• Customer relocates; new fuel source

Mid-life conversion:

• New gas service to property (NG available where wasn't before)
• LP tank decommissioned (customer switches to electric or different fuel)
• Cost-of-fuel comparison

How conversion works (generator side)

Most modern residential generators are convertible:

• Manufacturer ships kit OR conversion is field-settable
• Carburetor or fuel injection has adjustment screw / jet swap
• Some have fuel mode switch (NG / LP)
• Manufacturer manual specifies procedure

Conversion process:

1. Disconnect existing fuel supply
2. Adjust internal carburetor or jet per manual
3. Reconnect to new fuel source with appropriate pressure regulator
4. Run + adjust idle mixture
5. Verify operation under load

Time: 1-3 hours typical.

Cost: a modest labor charge plus a smaller parts cost (if needed).

Fuel supply sizing

Generator BTU consumption per kW output:

• NG: ~10-15 cf/hr per kW (10K-15K BTU per kW)
• LP: ~10-15 cf/hr per kW (equivalent BTU)

For typical 20 kW residential generator:

• Continuous BTU demand: 200K-300K BTU/hr at full load
• LP gallon: ~91,500 BTU
• 250-gallon tank: ~23M BTU available
• Runtime at full load: 75-115 hours (3-5 days)
• 500-gallon tank: 150-230 hours (6-10 days)
• 1000-gallon tank: 12-20 days

Natural gas: utility supply, unlimited within meter capacity.

Per NFPA 54 (NG) or NFPA 58 (LP), gas line sized to deliver demand.

Fuel line sizing

For NG at 0.5 psi residential:

• 1/2" pipe up to 50 ft: ~150K BTU (small generator only)
• 3/4" pipe up to 50 ft: ~400K BTU (most residential generators OK)
• 1" pipe up to 100 ft: ~800K BTU (large)
• 1.25" pipe up to 100 ft: ~1.5M BTU (commercial-sized)

Verify per NFPA 54 Table 6.2.

For LP at 2 psi:

• 1/2" pipe up to 100 ft: ~200K BTU (small)
• 3/4" pipe up to 100 ft: ~600K BTU (most residential)
• 1" pipe up to 200 ft: 1.2M+ BTU

Higher pressure = more BTU per pipe size.

Common pitfalls

Undersized gas line:

• Generator can't reach rated output
• Sputters or shuts down under load
• Customer reports "generator weak"
• Fix: upsize pipe

Wrong pressure:

• Generator expects 2 psi LP; gets 11 psi: damages valve
• Or expects 11 psi; gets 2 psi: insufficient flow
• Verify per manufacturer; adjust regulator

Inverted fuel mode:

• LP generator run on NG: weak, poor combustion
• NG generator run on LP: rich mixture, soot, poor combustion
• Conversion needed

LP tank too small for outage duration:

• 250 gallons + 20 kW generator at high load: 3-5 days
• Long outage (week+): runs out
• Customer-side education

Customer talking points

For new install:

1. "Generator on [NG / LP]; fuel line sized for full output."
2. "NG: unlimited during outage. LP: tank-limited; [tank size] gives [X days] full load."
3. "Conversion if you switch fuels later: a modest labor charge + parts."

For LP tank size selection:

1. "Family with average use, 20 kW generator: 250 gallons = 3 days, 500 gallons = 6 days."
2. "Tank purchase runs a solid low-four-figure sum + install."

Tank ownership: lease vs buy

Lease tank from gas company:

• Limited to one supplier
• Per-gallon cost set by supplier (often higher)
• 5-10 year contract typical
• Customer can't shop fuel suppliers

Buy tank:

• A solid low-four-figure sum upfront
• Choose any LP supplier
• Negotiate price annually
• Customer responsibility for maintenance
• Faster payback if customer comparison-shops

For most: lease if low LP usage; buy if heavy use (generator + heat + water heat) + customer wants flexibility.

Conversion cost summary

Service + maintenance

Annual service includes:

• Fuel pressure verification
• Combustion test
• Air filter
• Carburetor / fuel injection check
• Tank inspection (LP)
• Pressure regulator check
• Customer-side preventive

A modest flat fee per visit.

For multi-customer routes, this recurring service adds up to a meaningful annual revenue stream per 100 customers on the route.

Permits + safety

Always required for new gas line:

• Gas permit
• Inspection at completion
• Pressure test documentation
• Customer notification of permit closure

Safety:

• Combustible gas detector during work
• No ignition source during work
• Customer + pets out of area
• CO detector verified at completion
• Customer briefed on shutoff location

Code references

• NFPA 54 (National Fuel Gas Code) - NG
• NFPA 58 (Liquefied Petroleum Gas Code) - LP
• NFPA 37 (Stationary Combustion Engines)
• Manufacturer-specific install + conversion manuals

When to refer

Some scenarios outside generator-tech scope:

• New utility NG service: utility coordinates
• LP tank install requires propane company (some require licensed installer)
• High-pressure LP system (over 10 psi): specialty
• Commercial-scale (above 30 kW residential equivalent): commercial gas

For most residential generator work: stay within plumber's gas license scope (typically all residential gas piping).

References

• NFPA 54 + 58 (Fuel Gas Codes)
• Manufacturer install + conversion manuals (Generac, Kohler, Briggs, Champion, Westinghouse)
• AHJ + utility coordination requirements
• Manuall internal: Backup Generator Sizing and Installation Reference, Gas Line Installation and Service Reference