Key takeaways
- A portable generator often runs roughly 6 to 12 hours per tank at half load. See how load, tank size and fuel type change runtime, with a worked example.
Most portable generators run somewhere between 6 and 12 hours on a full tank when they carry about half of their rated load, but the real number swings widely with tank size, electrical load and fuel type. The only trustworthy figure for a specific unit comes from its manual, adjusted for what you actually plan to plug in. This guide explains where manufacturer runtime numbers come from, how to adjust them, and how to plan fuel for an outage without guessing.
Why the number on the box is only a starting point
Runtime ratings printed in spec sheets are usually measured at a stated load, very often 50 percent of the rated output. That is a convenient benchmark, not a promise. If your house draws less than half the rating, the engine burns less fuel and the tank lasts longer. If you pull close to the maximum, the same tank can empty in a fraction of the advertised time.
Because the benchmark is chosen by the manufacturer, two generators with similar tanks can list very different hours. Always read the footnote that explains the load level, and check whether the figure refers to gasoline, propane or both.
The three factors that set your runtime
Runtime is a simple relationship: fuel available divided by fuel burned per hour. Three things control that ratio.
- Tank capacity. Portable gasoline units commonly carry anywhere from about one gallon on small models to several gallons on larger open-frame generators. A bigger tank buys time but also adds weight and fuel to store.
- Electrical load. Fuel consumption climbs as you add appliances. Starting surges from motors, such as a refrigerator compressor or a well pump, briefly spike the draw.
- Engine design. Conventional engines run at a fixed speed regardless of demand. Inverter generators usually throttle up and down, which often saves fuel at light loads.
Altitude, temperature, engine condition and the age of the fuel can also nudge the result, so treat any calculated number as an estimate.
Gasoline, propane and dual fuel compared
The fuel itself changes both runtime and convenience. The table below summarizes widely known general differences; confirm specifics in the manual of the model you are considering.
| Fuel | Typical storage | Runtime tendency | Main trade-off |
|---|---|---|---|
| Gasoline | Approved cans, rotated regularly | Often the longest per gallon | Degrades over months without stabilizer |
| Propane | Sealed cylinders | Often somewhat shorter per gallon-equivalent | Output may be rated lower than on gasoline |
| Dual fuel | Either of the above | Depends on the fuel in use | Higher purchase price, more flexibility |
Dual fuel models appeal to people who want gasoline for long runs and propane as a backup that sits in a garage for years without spoiling.
A worked example with stated assumptions
These numbers are hypothetical and are here only to show the arithmetic. Suppose a generator has a 6-gallon tank and a manual that lists 10 hours of runtime at 50 percent load. That implies roughly 0.6 gallons per hour at that load. Now assume you plan to run a refrigerator, a few LED lights, a router and phone chargers, which together draw well under half of the generator’s rating.
If the manual or fuel curve suggests about 0.4 gallons per hour at that lighter load, then 6 gallons divided by 0.4 gives about 15 hours. If you instead add a space heater and a microwave and the draw climbs near the rating, consumption might reach 0.9 gallons per hour, and the same tank would last closer to 6 to 7 hours. The tank did not change; the load did.
Notice that the planning number you need is not hours per tank but gallons per day. Running 12 hours daily at 0.4 gallons per hour needs about 4.8 gallons, which is a useful figure for deciding how much fuel to store.
How to estimate runtime for your own generator
- Find the tank capacity and the rated runtime, including the load percentage it was measured at, in the owner’s manual or specification sheet.
- Divide capacity by runtime to get approximate gallons per hour at that load.
- List every appliance you intend to power and add up the running watts from each nameplate or manual.
- Compare that total with the generator’s rated output to see roughly what percentage of load you will carry.
- Scale the fuel burn up or down from the benchmark, remembering that the relationship is not perfectly linear and that starting surges add extra.
- Multiply the hourly burn by the hours per day you expect to run, then by the number of days, and add a margin.
Inverter versus conventional engines
A conventional open-frame generator usually holds its engine near a constant speed so that the output frequency stays steady, which means it burns a fairly similar amount of fuel whether the load is light or heavy, with only a modest drop at low demand. Inverter generators convert the output electronically, so the engine can idle down when demand is small. That is why many inverter models quote long runtimes at one quarter load.
The trade-off is cost and, often, lower total output for a given size. If your plan is to power a refrigerator, lights and chargers overnight, an inverter design may stretch a small tank a long way. If you need to run a well pump or an air conditioner, check the starting wattage requirements first, then compare runtime at that heavier load rather than at the flattering quarter-load figure.
Planning fuel for a multi-day outage
A single tank rarely covers an outage that lasts days. Decide whether you will run continuously or in scheduled blocks. Many households run the generator for a few hours at a time to keep food cold and recharge devices, which stretches fuel considerably compared with running around the clock.
Build a buffer of at least one extra day beyond your expected need, since refueling trips can be hard when roads or stations are affected. Remember that stored gasoline has limits on how much you can safely keep at home, so check local rules and use only approved containers.
Safe refueling and storage
Treat fuel handling as a safety task first. Turn the engine off and let it cool before opening the cap, refuel outdoors, and keep sparks, flames and cigarettes away. Never run a generator in a garage, basement, or near windows and doors, because exhaust contains carbon monoxide, a colorless and odorless gas that can build up quickly. Follow the manual for placement distance from the house, and install battery-powered carbon monoxide alarms indoors. If you are unsure about connecting a generator to household wiring, hire a licensed electrician rather than improvising.
Common mistakes that shorten runtime
- Counting on the benchmark hours while running near maximum load.
- Filling the tank with old, untreated fuel that causes rough running.
- Skipping oil checks, which can trigger a low-oil shutdown on many models.
- Forgetting that starting surges from motors need extra headroom.
- Leaving the unit on when nothing important is plugged in.
Related guides
- Portable Power Stations For Cpap During Power Outages
- How To Prepare For A Power Outage Checklist And Gear
- Dual Fuel Generators For Hurricane Home Backup
Frequently asked questions
How many hours will a 5-gallon tank last on a portable generator?
There is no single answer, because runtime depends on how much power you draw. As a rough illustration, a unit that burns about half a gallon per hour at moderate load would run close to ten hours on five gallons, while a heavier load could cut that sharply. Look for the consumption or runtime figure in your manual, then adjust for your actual appliances.
Does a generator use less fuel at a lower load?
Yes. Fuel burn generally rises with electrical load, so running only a refrigerator and a few lights typically uses far less fuel than powering a whole list of appliances at once. Inverter models that adjust engine speed to match demand are often more economical at light loads than conventional fixed-speed engines, though the exact savings vary by design.
Is it safe to refuel a generator while it is running?
No. Manufacturers consistently instruct owners to shut the engine off and let it cool before adding fuel, because gasoline vapor can ignite on hot engine parts or exhaust. Use an approved fuel container, refill outdoors with ventilation, wipe up any spills, and keep a plan for brief shutdowns so appliances such as a freezer are not left off for long.
How long can gasoline be stored for a generator?
Untreated gasoline is commonly said to degrade within a few months, and ethanol blends can absorb moisture over time. Many owners add a fuel stabilizer and rotate stored fuel regularly. Follow the generator manual for storage guidance, keep fuel in approved containers away from living spaces and ignition sources, and avoid putting old fuel into the machine when you need it most.
Do propane generators run longer than gasoline ones?
Not necessarily. Propane contains somewhat less energy per gallon than gasoline, and many generators produce slightly lower rated output on propane, so runtime per gallon is often shorter. The advantage is that propane stores well for long periods. A 20-pound cylinder holds roughly 4.7 gallons by common figures, so check the manufacturer’s propane runtime rating before you plan around it.
