Sizing and endurance are two different problems
Choosing a generator is treated as one question and it is two. The first is whether the machine can carry the load at all, which is a watts problem dominated by starting surges: anything with a motor draws several times its running current for a second or two, so the peak that sizes the machine is the running total plus the single largest surge rather than the sum of every surge. That question is worked in full by the generator sizing calculator, and it is the right place to start if you have not chosen a machine.
The second question is how long you can keep it running, which is a fuel problem and is what this page is for. It is the question that decides whether the purchase turns out to have been the right one, because the failure mode of a correctly sized generator in a long outage is not electrical. It is an empty can at ten at night.
Fuel burn is not proportional to load
An engine burns fuel to keep itself turning before it delivers anything. That is why a generator at a quarter load does not use a quarter of the fuel it uses at full load, and why an oversized machine running a small list is expensive to keep alive. A reasonable model for a small engine is a fixed component plus a component proportional to load, and this page uses that shape: it takes your published half-load figure, assumes roughly 30 percent of that burn is the fixed part, and scales the rest with the load fraction.
That 30 percent split is an approximation and it is the least certain number on this page. If the specification sheet gives a second data point, put it in the full-load field and the line is fitted exactly through your two figures instead of estimated. Where only run-time claims are published rather than gallons per hour, divide the tank capacity by the claimed run time and note the load fraction that claim was measured at, because a run time quoted at 25 percent load is a very different number from the same tank at 75 percent.
| Load fraction | Burn as a share of the half-load figure | What it means |
|---|---|---|
| 25% | about 65% | Lightly loaded, poor fuel per kilowatt-hour |
| 50% | 100% by definition | Where most published figures are measured |
| 75% | about 135% | Working, and no surge headroom left for much |
| 100% | about 170% | Continuous overload territory in practice |
An inverter generator changes this materially, because the engine speed is no longer tied to output frequency and it can throttle right down under light load. If your list is small and the outages are long, that difference in fuel burn at a quarter load is often larger than the difference in purchase price over a few seasons.
The hours a day figure is the biggest lever here
Most people plan on running a generator continuously and most people do not need to. A full freezer holds temperature for something like two days with the lid closed. A refrigerator holds about four hours. A house with any insulation at all loses heat over hours, not minutes. Running the machine for four hours in the morning and four in the evening covers the same appliances on a third of the fuel of continuous running, and it gives you a quiet night.
What that schedule does not cover is anything that has to run continuously: a sump pump in a wet basement, medical equipment, a furnace in genuinely cold weather where the pipes are the concern. Those are the loads that decide whether cycling is available to you, and they should be identified before the outage rather than during it. Enter their real hours per day in the list on this page and the daily kilowatt-hour figure tells you which side of that line you are on.
Refuelling is the other reason to think about hours. Every tank is a stop with the engine shut down and cooled — fuel onto a hot engine is a fire, and it is the most common generator accident that is not carbon monoxide. Two refuelling stops a day is manageable. Five is a job.
Storing the fuel you just calculated
The total on this page is frequently the number that surprises people. Three days at twelve hours a day for a mid-sized portable is commonly fifteen to twenty-five gallons, which is four or five cans, which is more fuel than most households keep and more than many are comfortable storing. That is a real constraint and it is worth confronting before the storm rather than in it, because fuel stations without power do not pump.
Fuel does not keep indefinitely. Gasoline degrades over months, more quickly with ethanol content and heat, and the symptoms show up as a machine that cranks and will not fire. Stabiliser extends it, rotating the cans through your car keeps it fresh, and running the generator under load for half an hour every few months both proves the machine and clears the fuel system. Propane sidesteps the shelf-life problem entirely, at the cost of a few percent of output and a much bulkier storage arrangement.
If the outage plan is longer than a couple of days, the arithmetic starts pointing away from fuel altogether. A battery bank charged by a generator running at a sensible load for a few hours a day uses far less fuel than the same generator idling through the night, and the battery runtime calculator and the inverter and charge controller sizing calculator cover that side of it. The home maintenance schedule is where the twice-a-year test run belongs so it actually happens.
The two things that are not negotiable
A generator produces carbon monoxide in quantities that kill quickly and it produces it silently and without odour. It runs outdoors, a good distance from the house, away from windows, doors and air intakes, and never in a garage or under a deck or on a porch, open door or not. Every year people die from generators that were placed somewhere that seemed ventilated. Put working carbon monoxide alarms in the house before you need the generator, not after.
Connecting a generator to the wiring of a building is licensed electrical work. It requires a transfer switch or a listed interlock that makes it physically impossible for the utility supply and the generator to be live at the same time. Backfeeding a house through a receptacle sends power back out through the service transformer as a lethal voltage on lines that utility crews are working on, and it bypasses the overcurrent protection on that circuit at the same time. There is no procedure for it here because there is no acceptable amateur version of it. Running cords from the generator directly to individual appliances is the alternative that needs no permit and no risk, and for the list most people back up, it is sufficient.
Questions people ask
How long will a generator run on one gallon of gasoline?
For a mid-sized portable carrying a typical essential-load list, somewhere around one to two hours per gallon is the usual answer, which surprises people who expected more. The figure is entirely governed by the load fraction and the specific engine, so the useful version of the question is the one this page asks: at your load, on your machine, what is the burn. A lightly loaded engine delivers fewer useful kilowatt-hours per gallon than a well loaded one, which is why oversizing has an ongoing cost rather than being free insurance.
Why is my run time shorter than the specification says?
Almost always because the published figure was measured at 25 or 50 percent load and you are running considerably more than that. Manufacturers state the load fraction in the fine print and the headline number always uses the flattering one. Altitude, ambient temperature, a dirty air filter and stale fuel all take a further bite. Divide your tank size by the run time you actually observe, and you have a real gallons-per-hour figure for your machine at your load, which beats any specification sheet.
Is it cheaper to run the generator all the time or in blocks?
Blocks, by a wide margin, provided nothing on the list genuinely needs continuous power. The reason is the fixed component of the burn: an engine consumes fuel to keep itself turning whether or not you are drawing anything, so the hours when the fridge is coasting and everyone is asleep are hours of pure loss. Four hours morning and evening keeps a refrigerator and a freezer comfortably in range and roughly halves the fuel against continuous running. The exceptions are a sump pump with water coming in, medical equipment, and freezing weather where the concern is the pipes rather than the food.
Should I use propane instead of gasoline?
Propane solves the shelf-life problem, which is the most common reason a stored generator fails when it is finally needed, and it stores safely for years in a tank. The trade-offs are that the same engine typically produces a few percent less on propane, tank storage is bulkier per unit of energy than a fuel can, and a large tank being refilled during a regional outage faces the same supply queue as everything else. Tri-fuel machines exist and their headline rating is the gasoline one. If your generator sits unused for years at a time, propane is usually the better answer for reliability alone.
Can I refuel without shutting the engine down?
No. Gasoline spilled onto a hot exhaust or engine block ignites, and this is one of the most common generator injuries after carbon monoxide. Shut it off, let it cool, then fill it, which means planning refuelling stops around your run schedule rather than treating them as interruptions. It also means the appliances go dark for that period, which is another argument for a scheduled block of running rather than a continuous one people are reluctant to break.