Runtime is meaningless until you know the load. Start with the watts your house actually pulls, then come back to the fuel math.
Size your generator first →How long a generator runs on a tank is the second question every homeowner asks, right after how big it needs to be. The answer on the box is real, but it was measured under conditions you will never reproduce during an outage. Runtime is not a property of the generator. It is a property of the generator plus your load, and the load is the part you control.
The runtime printed on the box is a best case
Manufacturers publish runtime at 25 percent load, and sometimes at 50 percent. A 7,500-watt portable advertised at “up to 11 hours” is usually being asked for about 1,875 watts while it does that. That is a refrigerator, a few lights, a modem and a phone charger. It is not a well pump, a furnace, a chest freezer and a coffee maker at the same time.
Inverter generators muddy this further, in a good way. They throttle the engine down to match demand, so at low load their advertised runtime is closer to honest. A conventional generator holds engine speed at 3,600 rpm regardless of load, which means it burns a meaningful amount of fuel even when almost nothing is plugged in.
The arithmetic: gallons per hour at your real load
A gallon of gasoline holds roughly 33 kWh of chemical energy. A small air-cooled engine driving an alternator converts something like 15 to 25 percent of that into electricity, depending on how hard it is working. So the practical figure is that a portable generator delivers somewhere between about 4 and 8 kWh of usable electricity per gallon, with the high end only reached when the engine is loaded to 50 to 75 percent of its rating.
That single fact explains almost every runtime surprise. Run a 7,500-watt machine to power 600 watts of lights and a modem and you will get maybe 2 kWh per gallon, because most of the fuel is fighting engine friction and windage. The generator is not broken. It is just badly matched.
What that looks like on a mid-size portable
Approximate figures for a common 7,500-watt conventional portable with a 6.6-gallon tank. Treat these as the shape of the curve, not as your machine’s spec sheet.
| Load | Output | Rough burn | Runtime |
|---|---|---|---|
| 25% | ~1,900 W | ~0.45 gal/h | ~14 h |
| 50% | ~3,750 W | ~0.75 gal/h | ~9 h |
| 75% | ~5,600 W | ~1.05 gal/h | ~6 h |
| 100% | 7,500 W | ~1.4 gal/h | ~4.5 h |
Notice that doubling the load from 25 to 50 percent does not halve the runtime. It cuts it by about a third, because a chunk of the burn is fixed overhead. The efficient place to sit is the middle of that table.
Propane runs cleaner and shorter
Propane holds about 91,500 BTU per gallon against roughly 114,000 for gasoline, so a dual fuel machine loses somewhere near 10 percent of its output and burns through volume faster. A standard 20-pound cylinder holds about 4.6 gallons, which typically means around 5 hours at half load on a mid-size portable. Two cylinders is one overnight, not one weekend.
What you buy in exchange is storage. Propane does not gum a carburetor, does not go stale, and can sit in the shed for a decade. If the machine will spend most of its life not running, that trade is usually worth it. We go through it properly in dual fuel generators and in propane vs gasoline for generator fuel.
Refueling is the part that bites
A 9-hour runtime sounds like it covers a night. It does not, because you cannot refuel a hot engine. You have to shut down, wait for the cylinder and muffler to cool, then fill. Realistically that is a 15 to 20 minute interruption, and it lands at 3 a.m. if you started the machine after dinner.
Safety: Never pour gasoline into a running or hot generator. Spilled fuel on a hot muffler ignites. Shut it down, let it cool, fill it outside and away from the exhaust, and keep the cans well clear of the machine.
The other constant hazard is where the thing sits. Carbon monoxide from a generator has killed people through open windows and attached garages with the door up. Read where to put a generator so it doesn’t kill you before the first outage, not during it.
How much fuel a three-day outage really takes
Most people do not run a generator continuously. The sane pattern for a house on a well is cycling: run four to six hours in the morning, four to six in the evening, off overnight. That keeps the fridge and freezer cold, refills the pressure tank, charges phones, and runs the furnace blower long enough to hold the house temperature.
Ten hours a day at roughly 0.75 gallons per hour is 7.5 gallons a day, so a three-day outage is about 22 gallons. Five 5-gallon cans covers it with a margin. That is a meaningful amount of gasoline to store safely and to rotate, which is exactly why storing gasoline for a generator deserves its own plan rather than four cans forgotten behind the mower.
Five ways to stretch the tank
- Cycle rather than run continuously. A full freezer holds roughly 48 hours closed, so it does not need the generator all night.
- Load it properly. Running one large machine at half load beats running it at 10 percent for the same job.
- Stage the big draws. Pump water, run the washer and cook while the engine is already loaded, not spread across the day.
- Move the small overnight loads to a battery. A modest power station covers the modem, a CPAP and a couple of lamps without an engine running.
- Keep the air filter clean and the oil fresh. A choked filter costs fuel every hour it runs.
What we’d actually do
Work backwards. Decide how long you want to be independent, assume ten run hours a day, and multiply by the burn rate at the load you measured rather than the one on the box. If the fuel number that falls out is more gasoline than you are willing to keep on the property, the fix is not a bigger tank. It is a smaller load, an inverter machine that throttles, or propane so the fuel can sit for years without going stale. Two honest weak points worth naming: cycling means the house gets cold and the fridge warms between runs, and cans of gasoline are a genuine fire load in an attached garage. Both are manageable, neither is free.
Frequently Asked Questions
How long will a 5,000-watt generator run on 5 gallons of gas?
At about half load, roughly 8 to 10 hours. At full load, closer to 5. Light loads do not extend it as much as you would expect on a conventional, non-inverter machine.
Can you run a generator 24 hours straight?
Not on one tank, and most manufacturers want an oil check every 24 to 50 hours anyway. Plan on a cool-down and refuel stop at least once a day, and keep spare oil on hand.
Does running a generator at low load damage it?
Extended very light loading on a conventional generator can cause wet stacking and carbon buildup. It is not instantly harmful, but the engine is happiest between 40 and 75 percent of its rating.
How many gallons per hour does a 22 kW standby generator use?
On propane, roughly 2 to 3.5 gallons per hour depending on load, which is why standby units are normally fed from a 250 to 500 gallon tank rather than cylinders.