Here’s a secret that deflates half the forum arguments on this topic: for most homeowners, the natural gas vs propane decision was made by their address. Homes with gas service overwhelmingly should use it; homes without it are propane homes by default, since trucking in a gas line isn’t on the menu. The genuine decision zone is narrower than the debate suggests — but inside it (dual-fuel unit owners, new rural builds, tri-fuel shoppers, and anyone weighing a propane tank against a gas-line upgrade), the differences are real and quantifiable.
So let’s quantify them: energy content and what it does to output, cost per kilowatt-hour delivered, the storage question that decides emergencies, the cold-weather wrinkle, and the honest verdict by situation. Spoiler shaped like a thesis: natural gas wins on economics and logistics; propane wins on independence and storage — and the best setups increasingly refuse to choose [→ /portable-generators/best-dual-fuel-generators/].
The Physics That Drives Everything
Two fuels, two energy densities, and most of the comparison falls out of this table:
| Property | Natural gas | Propane (LP) |
|---|---|---|
| Energy content | ~1,000 BTU per cubic foot | ~91,500 BTU per gallon (~21,600 BTU/lb) |
| Delivery | Utility pipeline — continuous | Tank on your property — finite |
| Generator output vs gasoline rating | ~10–20% derate (fuel-dependent, unit-specific) | ~10% derate |
| Storage life | N/A — never stored | Indefinite — doesn’t degrade |
| Cold-weather behavior | Unaffected at the appliance | Vaporization slows in deep cold (small tanks especially) |
| Runs out mid-outage? | Effectively never* | When the tank empties |
*Utility gas interruptions during catastrophic events are possible but rare — the asterisk propane partisans correctly cite, honestly sized: it’s a tail risk, not a planning baseline.
Three practical consequences:
- Both fuels derate output versus gasoline ratings — propane mildly, natural gas a bit more — which is why every sizing exercise on this site says to use the derated figure for your actual fuel [→ /standby-generators/22kw-vs-24kw-vs-26kw-generac/]. Between the two, propane’s higher energy density gives it the small output edge (a fact tri-fuel owners notice on the selector switch).
- Natural gas converts “fuel supply” from a noun you manage into infrastructure you rent — no tanks, no deliveries, no gauge anxiety at day five [→ /portable-generators/how-long-run-generator-continuously/].
- Propane converts “fuel supply” into something you own outright — unaffected by pipeline events, utility territory, or the delivery truck’s schedule, and immortal in storage. Independence has a shape, and it’s a 500-gallon tank.
Cost to Run: The Math
Fuel pricing varies by region and season, so here’s the worked comparison at illustrative prices — swap in your local numbers (your gas bill’s per-therm rate; your propane supplier’s per-gallon price):
A 22 kW-class standby at a realistic mixed outage load (~50%):
| Natural gas | Propane | |
|---|---|---|
| Approx. consumption at ~50% load | ~150 – 190 ft³/hr (~1.5–1.9 therms) | ~1.7 – 2.2 gal/hr |
| Illustrative fuel price | $1.20 – $1.60/therm | $2.50 – $3.50/gal |
| Cost per hour | ≈ $1.80 – $3.00 | ≈ $4.25 – $7.70 |
| Cost per day (duty-mixed) | ≈ $20 – $60 | ≈ $50 – $150 |
| Cost of a 10-day outage | ≈ $200 – $600 | ≈ $500 – $1,500 |
Ranges reflect typical residential rates; propane prices swing hard by season, region, and contract (leased-tank customers often pay their supplier’s captive rate — a real line item in the ownership math). Rerun with your actual rates; the ratio is the durable lesson.
The durable lesson: at typical US residential pricing, natural gas runs a generator at roughly one-third to one-half the fuel cost of propane for the same watts. Over a multi-day event — let alone a generator’s 15–25 year life of self-tests and outages [→ /standby-generators/how-long-do-whole-house-generators-last/] — that ratio is real money. It’s also, quietly, the economic half of why tri-fuel portables with a natural-gas quick-connect are this site’s favorite configuration [→ /installation-costs/generator-installation-cost/].
And the propane rebuttal, fairly stated: the comparison assumes gas service exists at your address at typical rates. Where it doesn’t, propane isn’t the expensive option — it’s the only piped-fuel option, and its true comparison is against gasoline logistics, which it wins on storage life alone [→ /fuel-running-costs/how-to-store-gasoline-generator/].
The Storage Question (Where Propane Runs Up the Score)
Cost-per-hour is one axis; having fuel at all is the other:
- Natural gas stores nothing and needs nothing stored — its supply chain is the utility’s problem, which is a luxury 51 weeks a year and a single point of dependence in week 52.
- Propane stores forever: the 500-gallon tank filled two years ago is at full strength tonight — no stabilizer, no rotation, no degradation [→ /fuel-running-costs/propane-tank-size-standby-generator/]. For emergency equipment specifically — machines that must run perfectly after months of idleness — that immortality is propane’s killer feature, and the entire reason dual-fuel owners keep propane as the stored reserve even when they exercise on gasoline [→ /portable-generators/best-dual-fuel-generators/].
- The propane logistics, honestly: finite supply means tank math before the storm (a 500-gallon tank ≈ 5–8 days of substantial standby use; a 20-lb portable tank ≈ an evening on a big unit [→ /fuel-running-costs/how-much-gas-does-a-generator-use/]), delivery scheduling in rural winters, and tank costs — $1,800–$3,500 purchased, or leased with captive-supplier pricing attached.
- The cold-weather wrinkle: propane’s vaporization slows in deep cold — small tanks under heavy draw can struggle to feed a hungry generator exactly during the ice-storm scenario. Mitigations exist (bigger tanks, higher fill levels, proper regulators); awareness is the free one.
The Verdict, By Situation
Home with natural gas service, buying standby → Natural gas, and it’s not close: one-third-to-half the running cost, unlimited runtime, zero storage duties. Do the supply verification our install guide insists on [→ /installation-costs/natural-gas-line-sizing-standby-generators/], size against the NG-derated output, and enjoy infrastructure.
Rural home, no gas service → Propane, by default and with pride: size the tank for your outage horizon (500 gallons is the serious-storm standard), own rather than lease if the math allows, keep it above half before storm season, and know your cold-weather margins.
Dual-fuel portable owner → Both, by role: gasoline for max output and exercise runs, propane as the immortal stored reserve — the strategy from our dual-fuel guide, unchanged [→ /portable-generators/best-dual-fuel-generators/].
Tri-fuel shopper with gas at the house → The endgame: natural-gas quick-connect for unlimited cheap runtime, propane bottles as the pipeline-event backup, gasoline for portability — every fuel doing what it’s best at [→ /portable-generators/connect-portable-generator-to-house/].
The edge case — gas available but far: where connecting gas means $2,000+ of line work, a propane tank sometimes beats it on install cost… and then loses it back in running cost over a decade. Run the 10-year math, not the install-day math [→ /standby-generators/whole-house-generator-cost/].
Frequently Asked Questions
Is it cheaper to run a generator on natural gas or propane? Natural gas, typically by a factor of 2–3x at US residential rates — roughly $20–$60/day versus $50–$150/day for a standby at mixed load. Propane’s counterweights are independence and indefinite storage, not running cost.
Does a generator produce less power on natural gas or propane? Both derate versus gasoline ratings; natural gas derates more (~10–20%, unit-specific) than propane (~10%), because propane carries more energy per unit. Size against the derated figure for whichever fuel you’ll actually run.
How long will a 500-gallon propane tank run a generator? Roughly 5–8 days of substantial use for a 22 kW-class standby at mixed load (~1.7–2.2 gal/hr at half load) — and usefully longer with duty-cycling. It’s the tank size hurricane-country propane homes standardize on for a reason.
Can a natural gas generator run out of fuel? Practically no — the pipeline supplies indefinitely, which is the fuel’s defining luxury. The honest asterisk: utility gas interruptions during catastrophic events are possible but rare; homes wanting a hedge keep a propane-capable unit or dual-fuel option.
Is propane bad for generators? The opposite of bad: it burns cleaner than gasoline (less carbon fouling, longer oil life by some measures) and never varnishes a carburetor in storage. Its costs are price per BTU, finite tanks, and cold-weather vaporization limits — logistics, not engine harm.
Should I convert my generator from propane to natural gas? If your home has gas service and your unit supports both (most standby units configure at install; tri-fuel portables switch freely), running NG for economy while retaining LP capability is the strong default — the conversion question is really a supply-verification and installer question [→ /installation-costs/natural-gas-line-sizing-standby-generators/].