Last verified: October 1, 2026
If your house already has a natural gas line with enough capacity, natural gas is usually the better fuel: it costs less per hour and never runs out. Propane wins where there is no gas service, gives a little more output from the same generator and stores for years, but you need a tank big enough for a long outage and you pay more per hour of runtime.
Running cost per hour and per day
Using the manufacturer’s consumption figures for the Generac 22 kW at half load (221 cubic feet of natural gas or 2.53 gallons of propane per hour) and U.S. average residential prices from the EIA:
| Fuel | Price used | Cost per hour (half load) | Cost per day | Cost for a 3-day outage |
|---|---|---|---|---|
| Natural gas | $15 per 1,000 cu ft | About $3.30 | About $80 | About $240 |
| Propane | $2.67 per gallon | About $6.75 | About $162 | About $486 |
Prices: EIA residential natural gas (winter-to-spring 2026 national averages ranged from about $14 to $18 per thousand cubic feet) and residential propane ($2.67 per gallon in March 2026). Your local prices may be quite different, so plug them in.
Why propane costs more per hour
Per unit of energy, the two fuels are not far apart in price. A gallon of propane holds about 91,452 Btu and a cubic foot of natural gas about 1,036 Btu (EIA). At the prices above, that works out to roughly $2.90 per 100,000 Btu for propane against about $1.45 for natural gas. Propane’s residential delivery price is simply higher in most places.
Output: propane gives a little more power
Air-cooled standby generators are usually rated slightly higher on propane than on natural gas. Generac’s Guardian brochure, for example, lists its 22 kW model at 22 kW on propane and 19.5 kW on natural gas. If your load is close to the limit, size the generator for its natural gas rating.
How long a propane tank lasts
Propane tanks are normally filled to about 80% of their capacity. At 2.53 gallons per hour (22 kW at half load):
| Tank size | Usable propane (80%) | Approximate runtime |
|---|---|---|
| 100 lb cylinder (about 23.6 gal) | About 19 gal | About 7–8 hours |
| 250 gal | 200 gal | About 3.3 days |
| 500 gal | 400 gal | About 6.5 days |
| 1,000 gal | 800 gal | About 13 days |
Smaller generators and lighter loads stretch these numbers; heavy air conditioning use shortens them. If you also heat or cook with propane, that comes out of the same tank. Small cylinders are not a realistic fuel supply for a whole-house standby unit.
Natural gas: check the meter before you buy
A standby generator is one of the largest gas appliances a house can have. A 22 kW unit at full load uses 327 cubic feet per hour, more than a typical furnace and water heater combined. Many homes need a larger gas meter or a bigger supply line, which your gas utility and installer must approve. Ask about this at the quote stage, because a meter upgrade can add cost and weeks of waiting.
Reliability during disasters
- Natural gas is very rarely interrupted by storms, because the lines are underground. It can be shut off in earthquake zones or after severe flooding.
- Propane is stored on your property, so you are independent of the gas network, but deliveries can be delayed after a major disaster. Keep the tank topped up before hurricane or winter storm season.
Pros and cons at a glance
- Natural gas costs less to run and lasts as long as the utility supply does. It needs no storage, but it’s only available where there’s gas service, output is slightly lower, and you may face a meter upgrade and a gas line run.
- Propane costs more per hour and is limited by the size of your tank. It works anywhere, gives slightly more output and stores for years without going bad, but you need a tank (bought or leased), a pad, a regulator and a line.
Worked example: one week of outages per year
Suppose your home has about 7 days of generator running a year (several short outages plus one longer storm), with a 22 kW unit at half load.
On natural gas, 168 hours at 221 cubic feet per hour comes to about 37,100 cubic feet, roughly $557 a year at $15 per 1,000 cubic feet, or about $5,570 over ten years. On propane, the same 168 hours at 2.53 gallons per hour uses about 425 gallons, roughly $1,135 a year at $2.67 per gallon, or about $11,350 over ten years.
Fuel use figures are Generac’s half-load figures for the 22 kW, as in the first table; prices are national averages and yours may differ. Most homes run their generator far less than a week a year, so the real difference is usually smaller, but the example shows why natural gas wins where it’s available.
Switching fuels later
Many air-cooled standby generators can be converted between natural gas and propane by an authorized technician, according to the manufacturer’s instructions. That means you can start on propane today and switch to natural gas if service reaches your street later. Ask your installer whether the model you’re buying is field-convertible and what the conversion involves.
Safety notes for each fuel
- Propane is heavier than air and can collect in low spots; tanks must be placed according to code and the supplier’s rules, away from building openings.
- Natural gas is lighter than air. Both fuels are odorized: if you smell gas, leave the area and call your utility or supplier.
- Both require professional installation of the gas connection, a shut-off valve and leak testing.
Bottom line
- If you have natural gas and the meter can handle the load, choose natural gas.
- If you don’t, propane works well, but size the tank for your longest realistic outage (500 gallons is a common choice for whole-house units).
- Whichever fuel you choose, size the generator with our generator size calculator and compare it with the fuel cost in the tables above.