How long can I store gasoline for a generator?

Untreated gasoline begins degrading within 30 to 60 days. Ethanol-blended gasoline (E10, the standard at most pumps) absorbs moisture and degrades faster than ethanol-free fuel. A quality fuel stabilizer such as STA-BIL or Pri-G extends usable life to roughly 12 months in a sealed, properly stored container. Even with stabilizer, rotate your stored gasoline by using it in vehicles, mowers, or other equipment and replacing it with fresh fuel at least once a year.

What size propane tank do I need for a standby generator?

Tank size depends on the generator capacity and the run time you want between deliveries. A common residential standby generator (22 kW) consumes roughly 2 to 3 gallons of propane per hour at half load. A 500-gallon tank at 80% fill holds 400 usable gallons, providing roughly 130 to 200 hours of run time at moderate loading. For a portable generator running 8 to 12 hours per day, a standard 20-pound grill cylinder provides roughly 5 to 8 hours of run time depending on load, and a 100-pound cylinder provides roughly 25 to 40 hours.

Is natural gas reliable enough for a standby generator?

Natural gas is convenient because it eliminates on-site refueling, but it depends on the utility gas distribution system remaining functional. Gas mains use compressors and regulators that require electricity. During extended regional outages, earthquake damage, or major infrastructure failures, gas pressure can drop or stop. Natural gas is reliable for typical short outages in areas with modern gas infrastructure. For properties in earthquake zones, hurricane corridors, or areas with aging gas systems, propane provides fuel independence because the supply is stored on your property.

Why do generators produce less power on propane than gasoline?

Propane has a lower energy density per unit volume than gasoline. A gallon of propane contains roughly 91,500 BTU compared to roughly 120,000 BTU for a gallon of gasoline. This means a dual-fuel generator typically produces 85 to 90 percent of its gasoline-rated output when running on propane. Natural gas produces even less, typically 80 to 85 percent of the gasoline rating. Always size your generator based on the output rating for the fuel you plan to use, not the gasoline peak number.

Should I get a dual-fuel generator?

For household preparedness, dual-fuel (gasoline plus propane) is the most resilient portable configuration. Gasoline gives you full rated output and is available at any gas station during normal times. Propane stores indefinitely in sealed cylinders without degrading, making it the better long-term emergency fuel. Having both options means that when gas stations lose power during an outage or gasoline supplies run short, your propane cylinders are still full. The cost premium for dual-fuel is typically $50 to $150 over a gasoline-only model.

Generator Fuel: Gasoline, Propane, Natural Gas, and Diesel Storage and Planning

THE REALITY

The generator works. The fuel runs out.

Most generator failures during outages are not mechanical. The engine starts, the transfer switch transfers, the lights come on. Then the fuel runs out. The gasoline in the shed has turned to varnish. The propane tank is at 15% because nobody checked it since last winter. The gas station down the road has no power to run its pumps.

Fuel planning is the least exciting part of generator ownership and the part most likely to determine whether the generator actually gets you through an extended outage. The type of fuel, the amount stored, the storage method, the rotation schedule, and the consumption rate under your actual loads are all decisions you make before the power goes out.

This guide covers each fuel type in depth: its storage characteristics, its practical limits, the consumption math for multi-day planning, and the safety rules that apply. It ends with the case for dual-fuel capability, which changes the calculus for portable generator preparedness more than any single feature.

GASOLINE

Gasoline. Available, powerful, perishable.

Gasoline produces the highest rated output on most portable generators. It is available at any gas station during normal times. It is familiar, and most households already handle it for vehicles and lawn equipment. These advantages make gasoline the default generator fuel.

The problem is storage life. Gasoline is a perishable fuel. It degrades, and when it degrades, it damages engines.

How gasoline degrades

Gasoline begins oxidizing as soon as it leaves the refinery. Lighter volatile compounds evaporate, leaving heavier residues behind. The fuel darkens, develops a sour smell, and eventually forms gums and varnish that coat carburetors, fuel lines, and injectors. Ethanol-blended gasoline (E10, which is standard at most U.S. pumps) adds a second problem: ethanol is hygroscopic, meaning it absorbs moisture from the air. Given enough time and humidity, the ethanol-water mixture separates from the gasoline and settles to the bottom of the tank, a process called phase separation.

Untreated gasoline stored in a vented container in a warm environment can begin degrading in as little as 30 days. In a sealed, temperature-stable container with low humidity exposure, untreated gasoline may remain usable for 60 to 90 days. Neither timeline is long enough for emergency fuel that might sit unused for a year.

Stabilizer and rotation

A quality fuel stabilizer (STA-BIL, Pri-G, or equivalent) added at the time of purchase slows oxidation and extends usable storage life to roughly 12 months in a sealed container. Some stabilizer products claim longer. Treat the 12-month figure as the practical planning horizon and rotate before it expires.

Rotation means using the stored fuel and replacing it with fresh fuel on a regular cycle. The simplest method: fill your approved fuel containers at the gas station, add stabilizer immediately, date the container with a marker, and use the oldest fuel first in your vehicle, mower, or other gasoline equipment. Replace it with fresh stabilized fuel. This keeps your emergency supply perpetually fresh without wasting any gasoline.

Ethanol-free gasoline (sometimes sold as recreational fuel or marine fuel) avoids the ethanol moisture problem and generally stores longer than E10. It costs more per gallon and is not available everywhere. If you can find it, it is the better choice for long-term generator fuel storage. It still needs stabilizer.

Safe gasoline storage

Store only in approved containers (UL-listed or ASTM-compliant fuel cans, typically red for gasoline). Never use glass, unapproved plastic, or improvised containers.

Store away from the house, in a well-ventilated, covered location such as a detached shed or garage. Not inside the home, not in a basement, not near ignition sources or water heaters. NFPA 30A provides the fire code framework for small-quantity fuel storage.

Keep containers sealed when not in use. Exposure to air accelerates degradation and increases vapor hazard. Leave a small expansion space at the top of the container.

Check local fire codes for maximum storage quantities. Many jurisdictions limit residential gasoline storage to 25 gallons outside an attached garage and less inside one. Some homeowner insurance policies have fuel storage exclusions.

Never refuel a running or hot generator. Shut it down, let it cool for several minutes, then refuel outdoors. Gasoline vapor is heavier than air and can flow along the ground to an ignition source.

How much gasoline to store

A typical 3,500 to 7,500-watt portable generator consumes roughly 0.5 to 1.0 gallon per hour at half load, depending on engine size and efficiency. At 0.75 gallons per hour running 10 hours per day, you need 7.5 gallons per day. Three days requires about 22 gallons. Five days requires about 37 gallons.

Those numbers assume the generator runs 10 hours per day, not 24. Most households can run the generator during daylight hours to keep the refrigerator cold, charge devices, and run the well pump, then shut down overnight. This approach cuts fuel consumption by more than half compared to continuous operation and extends your supply proportionally.

PROPANE

Propane. The fuel that waits.

Propane does not degrade. A properly maintained steel or composite cylinder holds usable fuel indefinitely. There is no stabilizer to add, no rotation schedule to track, no varnish forming in the carburetor. When the power goes out three years after you filled the tank, the propane works exactly as it did on the day you filled it.

That storage stability is propane's primary advantage for preparedness. It is the set-it-and-verify-it fuel. The tradeoffs are lower energy density than gasoline (roughly 85 to 90% of the gasoline output rating on most dual-fuel generators), the need for a properly sized tank and regulator, and the logistics of delivery for larger tanks.

Portable propane: cylinders and bottles

For portable generators, propane typically comes in standard cylinders:

Cylinder size	Propane capacity	Approximate run time
1-pound camping cylinder	1 lb (0.24 gal)	1 to 2 hours at low load
20-pound grill cylinder	4.7 gal (at 80% fill)	5 to 10 hours
30-pound cylinder	7 gal	8 to 15 hours
40-pound cylinder	9.4 gal	10 to 20 hours
100-pound cylinder	23.6 gal	25 to 45 hours

Run times are approximate ranges at half load for a typical 3,000 to 5,500-watt dual-fuel generator. Actual consumption depends on engine size, load, and ambient temperature. Cold weather reduces tank vaporization rate and effective output.

For a three-day outage running a portable generator 10 hours per day, you need roughly 15 to 25 gallons of propane. That is three to five 20-pound grill cylinders, or one 100-pound cylinder. Keeping three or four full 20-pound cylinders on hand is a practical baseline for portable generator preparedness. They store safely outdoors, exchange readily at hardware stores and gas stations, and integrate with your grilling supply so rotation happens naturally.

Stationary propane: tanks for standby generators

A standby generator on propane connects to a dedicated stationary tank, typically 120 to 1,000 gallons for residential use. The tank is sized for the generator's consumption rate and the number of days you want to run without a delivery.

Tank size	Usable gallons (80% fill)	Run time at half load (22 kW)
120 gallon	96 gal	32 to 48 hours
250 gallon	200 gal	65 to 100 hours
500 gallon	400 gal	130 to 200 hours
1,000 gallon	800 gal	260 to 400 hours

Consumption estimate: 2 to 3 gallons per hour for a 22 kW generator at approximately 50% load. Actual rates depend on load, ambient temperature, and generator efficiency. Consult the manufacturer's fuel consumption table for your specific model.

A 500-gallon tank is the most common residential standby size. It provides roughly five to eight days of continuous operation for a typical home generator, or two to three weeks of intermittent use (running 10 to 12 hours per day). If your property already has a propane tank for heating, the generator can often share the supply, but the tank and delivery schedule must account for both the heating load and the generator load during an outage.

Cold weather and propane vaporization

Propane must vaporize (change from liquid to gas) before the generator can burn it. Vaporization rate depends on the liquid surface area inside the tank and the ambient temperature. In very cold weather, a small cylinder may not vaporize fast enough to feed a large generator at full load. This is one reason standby generators use large stationary tanks rather than portable cylinders: more liquid surface area means more consistent vaporization in cold conditions. If you run a portable generator on propane in winter, keep a spare cylinder ready and swap before the first one freezes up under heavy draw.

NATURAL GAS

Natural gas. Convenient, with a dependency.

A natural-gas standby generator draws fuel from the utility gas main. No tank to fill. No delivery to schedule. No fuel to rotate. As long as the gas system is operating, the fuel supply is effectively unlimited. This is the most convenient arrangement and the most common fuel choice for suburban standby generators.

The dependency is structural. A natural-gas generator relies on two utilities: the electric grid (which has already failed, or the generator would not be running) and the gas distribution system (which must stay functional for the generator to keep running). These are separate systems, but they are not independent. Gas distribution infrastructure includes compressor stations, pressure regulators, and monitoring equipment that require electricity. During a short local outage, the gas system almost always continues because the broader grid is still feeding the gas infrastructure. During a regional disaster that takes down large sections of the grid, gas pressure can drop or fail.

Natural gas is also the lowest-output fuel for most generators. A unit rated at 22 kW on propane may produce only 19 to 20 kW on natural gas. This is because natural gas has lower energy content per cubic foot than propane per gallon, and the fuel delivery is limited by the gas meter and service line pressure. Before installing a natural-gas standby generator, the installer must verify that the existing gas service can deliver adequate volume and pressure at the generator's full-load consumption rate, on top of normal household gas use.

When natural gas is the right choice

You have reliable utility gas service. Your outages are typically short (hours to a day or two). Your area has modern gas infrastructure unlikely to fail during the same events that take down the electrical grid. You do not want to manage a propane tank and delivery schedule. You live in a suburban or urban area where on-site propane storage may be restricted or impractical.

When propane is the better choice

You live in an area without utility gas service. You are in an earthquake zone, hurricane corridor, or wildfire area where infrastructure damage is likely. You want fuel independence. You plan for multi-day or multi-week outages. You prioritize self-reliance over convenience. You already have a propane tank for heating or cooking.

DIESEL

Diesel. The commercial workhorse.

Diesel generators dominate commercial, industrial, agricultural, and institutional standby power. The engines are mechanically simpler, more efficient under sustained heavy load, and built for long service life. Diesel fuel has higher energy density than gasoline, and diesel generators typically consume less fuel per kilowatt-hour generated than gasoline equivalents.

For residential use, diesel is uncommon. The units are heavier, louder, more expensive, and the exhaust is more noticeable than gaseous-fuel generators. Diesel storage also requires more management than propane: the fuel absorbs moisture, develops microbial growth in the water layer, and degrades over time without treatment. On-site diesel storage tanks require periodic fuel testing, water draining, and fuel polishing (filtering) to maintain fuel quality.

Diesel makes practical sense for three situations:

Farms with existing diesel infrastructure. If the property already has a diesel tank for tractors and equipment, a diesel generator shares the supply and the existing delivery relationship.

Large sustained loads. Properties requiring 30 kW or more with extended continuous operation. Diesel engines handle sustained heavy loading more efficiently than gasoline or gaseous-fuel engines at this scale.

Commercial and institutional buildings. Hospitals, data centers, water treatment facilities, and commercial buildings where code-required emergency power typically specifies diesel systems with on-site fuel storage.

Diesel storage management

Diesel fuel stored in a tank with a headspace of air will accumulate moisture through condensation. Water at the bottom of the tank grows bacteria and fungi (microbial contamination) that produce sludge, clog filters, and degrade fuel quality. Managing diesel storage means testing fuel quality annually, draining accumulated water, using biocide additives when needed, and running fuel through a polishing filter periodically. This is routine for farms and commercial operations with diesel experience, but it is a meaningful commitment for a residential property. Propane avoids all of these concerns.

THE MATH

Fuel consumption math for multi-day outages

The question is not "how long will the generator run?" It is "how many days can I sustain this, and what does the fuel plan look like?"

Two variables control the answer: how much fuel the generator burns per hour, and how many hours per day you run it.

Generator size	Gasoline (gal/hr at 50% load)	Propane (gal/hr at 50% load)
2,000 to 3,500 W inverter	0.2 to 0.4	0.3 to 0.6
5,000 to 7,500 W portable	0.5 to 0.8	0.7 to 1.2
7,500 to 12,000 W portable	0.7 to 1.2	1.0 to 1.6
14 to 22 kW standby	n/a (gaseous fuel)	1.5 to 3.0

These are general ranges at approximately 50% rated load. Actual consumption depends on engine efficiency, load level, altitude, temperature, and the specific generator model. Check the manufacturer's specifications for your unit.

The shift-running strategy

Running a generator 24 hours a day through a multi-day outage is expensive in fuel and hard on the engine. Most households can operate on a shift schedule: run the generator 8 to 12 hours during the day, shut down overnight, and rely on the thermal mass of the refrigerator and freezer to hold temperature. A modern refrigerator keeps food safe for 4 to 6 hours with the door closed. A full chest freezer holds temperature for 24 to 48 hours unopened.

Shift-running cuts fuel consumption roughly in half and doubles the number of days your fuel supply covers. A 20-gallon gasoline reserve that lasts 25 to 30 hours of continuous operation stretches to 3 to 4 days of shift-running. Three 20-pound propane cylinders that cover 15 to 24 hours of continuous operation cover 2 to 4 days on a shift schedule.

Pairing a generator with a battery power station improves this further. Run the generator during the day to power the house and charge the battery station. The battery station carries overnight loads (CPAP, phone chargers, router, a few lights) silently and without fuel, extending the quiet hours and stretching the fuel supply.

THE RECOMMENDATION

Why dual-fuel changes the preparedness math

A dual-fuel portable generator runs on gasoline or propane, selected by a fuel switch or valve on the unit. The cost premium over a gasoline-only generator in the same wattage class is typically $50 to $150. For household preparedness, this small premium buys a meaningful capability gain.

Gasoline gives you full rated output and is available at any functioning gas station. It is the right choice for day-to-day generator use and short outages where you can refuel easily. But gasoline depends on gas stations having power to run their pumps. After a major storm, when the power is out across the county and lines form at the few open stations, gasoline availability becomes uncertain.

Propane gives you independence from the supply chain. Your cylinders were filled last month or last year, and they are still full and usable. You do not need a gas station. You do not need a delivery truck. The fuel is on your property, ready. The output is lower (85 to 90% of gasoline rating), but you have it when you need it.

Dual-fuel lets you start on propane (immediately available, no trip to the gas station in a storm), then switch to gasoline if gasoline becomes available and you want the extra output. Or start on gasoline and switch to propane when the gasoline runs out. The two fuels cover each other's weaknesses. Gasoline covers propane's lower output. Propane covers gasoline's short storage life and supply-chain vulnerability.

Tri-fuel: adding natural gas

Tri-fuel generators add natural gas as a third option. If your property has utility gas service, this gives you a third fuel source that requires no storage or refueling as long as the gas system is operational. Natural gas produces the lowest output of the three fuels (typically 80 to 85% of the gasoline rating), but it is effectively unlimited while the gas main is functioning. Tri-fuel is worth the additional premium if you have gas service and want maximum flexibility. Size the generator from the natural-gas rating if that is your primary planned fuel.

SAFETY

Fuel safety rules that apply to every fuel type

Every fuel-burning generator produces carbon monoxide

Gasoline, propane, natural gas, diesel. All combustion produces CO. Operate outdoors only, at least 20 feet from any door, window, or vent, with exhaust directed away from the building. Never in a garage, shed, carport, or enclosed porch, even with the door open. Install CO alarms on every level of the home. The CPSC reports that generator-related CO poisoning kills dozens of people every year in the United States.

Never refuel a running or hot generator

Shut the generator down and let it cool for several minutes before adding gasoline. Gasoline vapor is heavier than air and can travel along the ground to a hot exhaust pipe or spark. Propane cylinders can be connected or disconnected with the generator off, following the manufacturer's connection procedure.

Store fuel properly and legally

Use approved containers for gasoline (red, UL-listed). Store them in a well-ventilated location away from the home and ignition sources. Propane cylinders store upright outdoors. Check local fire codes for quantity limits. Never store gasoline inside the home, in a basement, or in an attached living space.

Size the fuel connection to the generator

For standby generators on propane or natural gas, the fuel delivery system (tank, regulator, piping, meter) must deliver adequate pressure and volume at the generator's full-load consumption rate. An undersized gas line or regulator can cause low fuel pressure under heavy load, leading to poor performance, overheating, or shutdown. This is professional installation territory. The installer verifies fuel delivery as part of the commissioning process.

Full generator safety guide

Generator fuel. The plan behind the plan.