Natural Gas vs Propane for Standby Generators: Which Fuel Wins? (2026)
Every standby generator buyer answers the fuel question before anything else matters — because it determines the installation, the running cost, and what happens on day six of an outage. Most 14–26 kW home standby units burn either fuel (they’re field-convertible), so this is genuinely your choice. Here’s how to make it.
The 30-second version
| Natural gas | Propane (LP) | |
|---|---|---|
| Fuel cost | ~$2–$5/hr typical | ~$5–$10/hr typical |
| Supply | Unlimited while utility flows | Limited to your tank |
| Upfront | Gas line work ($500–$2,000+) | Tank + install ($1,500–$3,000+) |
| Power | Usually ~2 kW less than LP rating | Full nameplate rating |
| Maintenance | Minimal | Minimal + tank monitoring |
| Fails when | Gas utility fails | Tank runs dry / no delivery |
Choose natural gas if you have reliable utility service and want the cheapest, most hands-off ownership. Choose propane if you have no gas line, distrust utility resilience, or want fuel independence. Both are excellent; the wrong choice is the one you didn’t think through.
Cost per hour: the real numbers
Using published consumption figures for 20 kW-class units and typical residential rates:
Natural gas: ~160–230 cu-ft/hr at half load. At ~$1.00–$1.80/therm, that’s roughly $2–$4/hour. A 72-hour outage: ~$150–$290.
Propane: ~1.8–2.4 gal/hr at half load. At ~$2.50–$4.00/gallon, that’s roughly $5–$10/hour. A 72-hour outage: ~$400–$700.
Propane costs roughly 2× natural gas per hour for the same work, just about everywhere. Over a decade of occasional outages, the fuel-cost difference is hundreds, not thousands — meaningful, but small next to the $10,000–$15,000 installed project. Don’t let $300 of outage fuel decide a $12,000 purchase; let resilience decide it.
For the full per-model breakdown, see standby generator fuel consumption.
The power derate: propane makes more watts
This surprises buyers: the same generator is usually rated higher on propane. Examples from spec sheets:
- Kohler 20RESCL: 20 kW LP / 18 kW NG
- Briggs & Stratton 20kW: 20 kW LP / 18 kW NG
- Generac Guardian 22kW: 22 kW LP / 19.5 kW NG
- Generac Guardian 24kW: 24 kW LP / 21 kW NG
Propane packs more energy per unit of fuel-air mixture, so the engine makes more power. The practical consequence: size against the fuel you’ll burn. A buyer whose load calc says “20 kW” and who’ll burn natural gas needs a 22 kW-class unit (19.5–21 kW on NG), not a 20. This is the single most common fuel-related sizing error — our sizing guide and kW calculator both account for it.
Reliability in real outages
Natural gas is wonderfully boring — until it isn’t. Utility gas is extraordinarily reliable day-to-day, but it has failure modes: earthquakes, floods washing out mains, freeze-offs at wellheads (Texas, February 2021 — the canonical example), and utility shutoffs. When the gas fails, your generator is a very expensive lawn ornament, and there’s nothing you can do about it.
Propane is independence you can see: the tank in your yard is fuel no utility can take away. It stores indefinitely without degrading, works in extreme cold (with proper tank sizing — vaporization drops as the tank empties and temperatures fall), and you can top it off before storm season. Its failure mode is running dry — which is entirely within your control if you sized the tank right and keep it filled.
The resilience ranking, honestly: propane with a properly sized tank > natural gas > propane with a marginal tank. A 250-gallon tank in hurricane country is false confidence; a 1,000-gallon tank is a fortress.
Tank sizing, concretely
Propane tanks fill to 80%, so usable gallons ≈ 80% of nominal. At ~2–2.5 gal/hr (20 kW at half load):
- 250 gal → ~200 usable → 3–4 days. Adequate for areas where outages rarely exceed 48 hours. Marginal everywhere else.
- 500 gal → ~400 usable → 6–8 days. The suburban sweet spot. Covers the vast majority of US outage scenarios.
- 1,000 gal → ~800 usable → 12–16 days. Rural, last-on-the-feeder, or “never again” buyers.
Underground vs above-ground is mostly aesthetics and local code — underground costs more to install but disappears into the landscaping and is protected from weather and tampering. Either way, own the tank if you can: leased tanks lock you to one supplier’s pricing; owned tanks let you shop propane like heating oil.
Installation differences
Natural gas needs a plumber to run a dedicated gas line from the meter to the generator — sized for both distance and load (a 20 kW unit wants ~280 cu-ft/hr at full tilt). Long runs need bigger pipe; too-small pipe causes pressure drop, rough running, and fault codes. And critically: confirm the meter’s capacity with the utility. Many standard residential meters can’t feed a generator plus the furnace simultaneously — upsizes are routine but must happen before install day.
Propane needs the tank placed per code (10+ feet from the house and ignition sources, with local variations), a regulator set, and a buried or above-ground line to the generator. No utility coordination, no meter — but the tank placement, trenching, and first fill are all on your project’s bill. Get the installation cost estimator for your scenario.
Cold weather: propane’s quiet advantage
In extreme cold, propane vaporization slows — a nearly-empty tank in single digits can’t feed a 20 kW unit at full load. The fixes are straightforward: keep the tank reasonably full in winter (never let it drop below ~30% in cold months), and size the tank so “low” still vaporizes enough. Natural gas has the opposite cold-weather story: it’s fine until the utility has problems, at which point entire regions go dark simultaneously (again, Texas 2021). For cold-climate buyers who choose propane, the rule is simple: a bigger tank is cold-weather reliability.
Can you switch later?
Yes — mechanically. Generac, Kohler, Cummins, and Briggs 20 kW-class units are all field-convertible: a regulator and orifice change, usually under an hour for a tech. But the infrastructure isn’t convertible: the gas line or the tank is the expensive part, and you’d be paying for fuel infrastructure twice. There are legitimate reasons to convert (moved from city to rural, gas service became unreliable), but “I’ll decide later” is an expensive plan. Decide now.
What about diesel?
The third fuel rarely makes sense for homes, but buyers ask. Diesel standby units (mostly 25 kW and up, liquid-cooled) offer the best fuel efficiency and the longest engine life — but diesel fuel degrades in 6–12 months without treatment, needs polishing for long storage, and the units cost substantially more. Diesel is the right answer for commercial sites and off-grid homesteads with big tanks and fuel rotation discipline. For a suburban home choosing between NG and LP, diesel is a distraction — the air-cooled gas units on this site (best standby generators) are cheaper to buy, cheaper to maintain, and better matched to residential duty cycles.
What installers actually recommend
Ask ten installers and you’ll hear a consistent pattern: they recommend whatever fuel the house already has, unless there’s a resilience reason not to. The logic is practical, not ideological — the cheapest reliable fuel is the infrastructure you don’t have to build. An existing natural gas service with a confirmed-adequate meter beats a new propane tank on cost and simplicity nearly every time.
The exceptions installers flag: homes at the end of rural gas lines with pressure problems, areas with a history of utility gas curtailments, and buyers who’ve personally lived through a gas outage. Those buyers get steered to propane, usually with a bigger tank than they first imagined. When your installer recommends a fuel, ask why — the good ones have a reason grounded in your street, not a brand preference.
The decision framework
- No natural gas line at the street? Propane. Done.
- Gas line available, reliable utility, suburban outages of hours-to-days? Natural gas. Cheapest to run, zero fuel logistics.
- Gas available but you’ve lived through a utility gas failure, or outages run a week+? Propane with a 500+ gallon owned tank. Pay the fuel premium for independence.
- Rural, well water, long feeder lines? Propane, 1,000 gallons if the budget allows. You’re already independent on water; be independent on fuel too.
Then size the unit for the fuel you chose (sizing guide), compare the actual models (best standby generators 2026), and run the fuel math for your rates (fuel consumption guide).