What is a portable generator?
Updated August 2026
A portable generator is a wheeled engine and alternator you roll out during an outage and connect either to individual appliances by extension cord or to the house through a transfer switch. Residential sizes run 3,000–9,500 running watts, and 5,000–7,500 is the sweet spot that covers a refrigerator, a furnace blower, a sump pump, a well pump, and lights at the same time. Dual-fuel models add propane, which stores indefinitely and never gums a carburetor.
Where it is
Storage and operation are two different places. The unit stores in a garage or shed, drained or stabilized, but it operates outdoors only: the CPSC advises at least 20 feet from the house with the exhaust aimed away from doors, windows, and vents. A garage with the door open does not count, and neither does a covered porch or a breezeway. Fuel belongs outside living space too, in approved cans, and 5–10 gallons is a realistic reserve for a two-day outage at half load.
What it does
There are two legitimate ways to use one. The first is direct cords: 12-gauge outdoor extension cords running to the refrigerator, a lamp, and a space heater, which is cheap and awkward but safe. The second is a transfer switch or an interlock kit that lets the generator feed selected house circuits, which is the only correct way to power a furnace, a well pump, or anything hardwired. The illegitimate third way kills people. A double-ended male cord, known in the trade as a suicide cord, plugged into a dryer or wall outlet pushes voltage backward through your service drop and up the neighborhood transformer, where it becomes thousands of volts on a line a repair crew may be handling. Carbon monoxide is the other killer: the CPSC warns a single portable generator can emit as much CO as hundreds of idling cars, and units certified to the newer CO shutoff standard cut themselves off as the gas builds. Both hazards, and the slower damage of stale fuel, show up in the real cost of a neglected generator.
Maintenance
A generator that sits is a generator that will not start. Run it under load for 30 minutes every 1–3 months, change the oil after the first 20–30 hours and then every 50–100 hours of use, and either treat the fuel with stabilizer or run the carburetor dry before storage. Air filter and spark plug get replaced annually or at 200 hours. The rest of the routine, including the battery on electric-start models, is on the portable generator maintenance checklist. A varnished carburetor from stale gas costs $100–200 at a small-engine shop, always during the week everyone else needs one too.
What it costs to replace
A replacement portable generator runs $500–2,000 for the generator, plus $500–1,500 for a transfer switch or $150–400 for an interlock kit.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
What size portable generator do I need?
Add the running watts of everything you want on at once, then add the largest single starting surge on top. A refrigerator runs at 150–200 watts but surges to 1,200. A gas furnace blower runs 600–800 and surges near 2,000. A 1/2 horsepower well pump runs 1,000 and surges past 3,000. Most households land at 5,000–7,500 running watts. Buying a size up costs $200–400 more and spares you the load-shedding arithmetic at 2 a.m.
How long can a portable generator run continuously?
Tank size decides it. A 4–10 gallon tank at half load gives 8–12 hours, which means one refuel per day and one overnight gap unless you get up. Shut the engine down and let it cool several minutes before adding gas, since fuel spilled on a hot muffler ignites. Running 24 hours a day for several days is hard on the engine and on you, which is the practical argument people eventually make for a standby unit.
Is it ever safe to plug a generator into a wall outlet?
No. Backfeeding through a receptacle bypasses every overcurrent protection in the house, puts exposed male prongs at line voltage in your hand, and energizes the utility drop toward line workers who believe the circuit is dead. It also lets utility power return into a running generator and wreck it. An interlock kit costs $150–400 installed and removes the hazard permanently. Treat that as the minimum entry fee for feeding house circuits from a generator.
How many years should one last?
A homeowner-grade unit typically gives 1,000–2,000 run hours, which for most households means 10–15 calendar years because the meter barely moves. What actually kills them is storage, not running: ethanol fuel left sitting varnishes the carburetor, and moisture rusts the cylinder on units parked wet. Owners who stabilize fuel and exercise the engine quarterly routinely pass 15 years on a $700 machine.