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Generator fuel: natural gas vs propane vs diesel
How to pick standby fuel from the pipe in the yard, what propane tanks and diesel units actually add to the bid, and what HomeGuide's 2026 run-cost table implies for a multi-day outage.
The machine on the pad is only half the quote. Natural gas, liquid propane, and diesel change what you pay to install, what you pay when it runs, and whether the outage outlasts the tank. Most air-cooled home standbys are built as gas/propane units. Diesel shows up when the load is big, the site is commercial-ish, or someone wants a fuel they can store on the property without a utility.
If you have not sized the kilowatts yet, start with sizing a standby generator. This page is which fuel that engine drinks. If you are still choosing a battery, that fork is battery vs generator.
What does the install cost?
HomeGuide’s 2026 whole-house generator guide puts a typical standby at $6,000 to $11,000 installed, or $9,500 to $24,000 when the run is long or the unit is over 20 kW. Unit-only bands by fuel on that page:
- Natural gas: $3,000 to $6,000 (hooks to the existing line, no storage)
- Liquid propane: $2,000 to $6,000 (needs a tank)
- Diesel: $5,000 to $18,000 (more efficient on paper, you refuel it)
Labor is $3,000 to $5,000 in the common band, $5,000 to $12,000+ for bigger or farther jobs. Gas plumbing is extra distance: $15 to $50+ per linear foot. A precast pad is $150 to $400. Permits $80 to $450. Automatic transfer switch $600 to $2,500.
HomeGuide: whole house generator cost
Propane is where the bid grows a second invoice. HomeGuide’s generator page puts tank installation at $400 to $1,800. Their propane-tank guide is wider: $700 to $4,500 installed, with a typical 500-gallon above-ground around $1,500 and underground around $2,000. Renting a 500-gallon tank is $60 to $180 a year, usually tied to buying fuel from that supplier.
Diesel tanks are not in that HomeGuide propane table. Treat a diesel install as the higher unit price plus whatever the fire marshal wants for on-site fuel. If Bid A is $8,000 on gas and Bid B is $14,000 on propane, count the tank and the extra gas line before you assume B is greedy.
What does it cost to run for a day?
HomeGuide’s run-cost table (their current fuel prices, not yours) for a generator sitting there all day:
| Fuel | 7 kW | 15 kW | 20 kW | | Natural gas | $10–$20 | $22–$32 | $30–$40 | | Liquid propane | $60–$70 | $145–$165 | $150–$170 | | Diesel | $32–$40 | $90–$110 | $100–$120 |
A 15 to 20 kW standby in that same FAQ burns 133 to 330 cubic feet of natural gas per hour, or 3.16 to 3.45 gallons of propane. Whole-home they put at $30 to $170 per day depending on size and fuel. Manufacturers they cite will let a unit run up to about 500 hours continuous, roughly three weeks, which is a service interval, not a tank.
Those daily dollars move with the pump. EIA residential propane averaged $2.67 per gallon nationally in March 2026 (heating-season survey, excluding taxes). On-highway diesel was about $5.97 per gallon for the week ending September 7, 2026. Residential natural gas in January 2026 was $13.96 per thousand cubic feet. Summer gas rates look higher per unit because the customer charge is spread over almost no therms. Use a winter gas bill for outage math.
EIA: residential propane prices
A three-day ice storm on a 20 kW natural-gas unit at HomeGuide’s $30 to $40 a day is on the order of $90 to $120 in fuel. The same three days on propane at $150 to $170 a day is $450 to $510, and you still have to have that much in the tank before the trucks stop rolling.
When is natural gas the right call?
The meter is already on the house, the line can take another load, and your outages last days. Runtime is then “until the utility gas stays up.” You do not store fuel, you do not schedule a fill before a storm, and HomeGuide’s hourly table is the cheap column.
It fails when the same event that kills electric also drops gas pressure, or when a tree takes the meter. That is the argument for propane or a battery: the fuel is on your property. Ask the gas company how their system behaved in the last ice storm, not how it behaves in July.
Some dual-fuel air-cooled units make less power on natural gas than on propane. The sizing page already flags that derate. The bid should use the fuel you will actually burn.
When is propane the right call?
There is no gas main, or you do not trust the main in a long outage. Propane sits in a tank you own or lease. HomeGuide notes it lasts longer in storage and burns cleaner than some other fuels, and that it is more expensive and needs that tank.
Size the tank to the outage you actually get. HomeGuide’s 3.16 to 3.45 gallons an hour on a 15–20 kW unit means a 500-gallon tank (filled to the usual 80%, about 400 gallons usable) is on the order of five days if you ran it flat-out that whole time. Real outages are not flat-out. Still: if the last ice storm was a week and the trucks could not get down the road, 250 gallons is a short story.
Cold weather: vaporization can sag on a small, above-ground tank in a deep freeze. Underground tanks (HomeGuide’s 500-gallon underground around $1,900 typical in that guide) sit in more stable ground temperature. Flood plains are the exception, because a buried tank can shift.
Lease versus buy: lease is cheaper on day one and you are stuck buying fuel from that company. Buy is HomeGuide’s $1,500 neighborhood for a 500-gallon above-ground, and you can shop fills.
When is diesel the right call?
The load is past what a typical air-cooled gas/propane Guardian-class unit wants to do, or you already think like a shop: you want energy density in a tank, and you will treat the fuel. HomeGuide’s diesel unit band starts higher ($5,000 to $18,000) and their all-day run cost sits between gas and propane. You refuel it. You store it. You maintain it more.
Residential yards rarely need diesel. Noise, exhaust, and the fire code on the tank are why. If a dealer leads with diesel on a 22 kW house that already has a gas meter, ask why the gas unit does not cover the load list.
Diesel also gels in the cold if it sits untreated. Propane does not. Natural gas does not sit.
What should be on the quote?
- Fuel: NG, LP, diesel, or a conversion kit, and the kW on that fuel, not only the brochure peak.
- Gas line: length, size, who taps the meter, $15 to $50+ per foot as a sanity check.
- Propane: tank size, above or buried, own or lease, setbacks, who fills it the first time.
- Diesel: tank size, treatment plan, who is the AHJ on the tank.
- Transfer switch, pad, permits, as separate lines.
- A run-cost assumption: gallons or cubic feet per hour at half load, times your fuel price, not a national average from 2019.
- Clearances from doors and windows (HomeGuide: at least five feet from openings and flammables).
If Bid A is NG at $7,500 and Bid B is LP at $9,200, the $1,700 is often the tank. If Bid C is diesel at $16,000 on a 14 kW essential-load list, you are shopping a different product.
What goes wrong after you sign?
The unit is rated 22 kW on propane and they connected natural gas without rewriting the load list. The propane tank is 250 gallons and the last outage was eight days. The gas line is 80 feet of trench that was “included” until it was not. Diesel sits all summer and will not start in January. Someone put the exhaust under a bedroom window.
How do I decide in one afternoon?
- Is there a gas meter? If yes, price NG first, and ask what happens to gas pressure in your last long outage.
- If no meter, price propane with a tank sized to that outage, not to the grill.
- Diesel only if the kW or the site makes a liquid-cooled, tank-and-treat machine the actual product.
- Get two bids that name fuel, kW on that fuel, tank or line, and a gallons-or-cubic-feet-per-hour number you can multiply by last winter’s price.
The fuel is not a brand preference. It is whether the engine can still run on day four.