How much fuel do you need to run a generator?

Neglect your leaf blower, and you’ll be raking up everything. Shrug off maintaining the snow blower, and you’ll be shoveling. But ignoring your portable generator means your home goes dark when the power goes out. The heating or cooling system shuts down. Food spoils. For homeowners with a well, you have no water for the shower or toilets.

Key to keeping a portable generator running is minding the fuel, engine oil, filters, and spark plug, along with starting up the machine every month or so. You can see the basic steps in our new generator prep video, above. In addition to maintenance, however, being at the ready also means keeping sufficient fuel on hand. Here's a guide:

Gasoline. During a major storm, you might not be able to get on the road to fill up gas cans, and sometimes stations themselves lose power and can’t pump. So keep enough gas on hand to buy you some time, even if you can’t store the 12 to 20 gallons you’d need for 24 hours of straight running. And remember to add stabilizer to the gas before fueling the generator, preferably one developed to counter the effects of ethanol. Whatever gas you don’t use can go into your car’s gas tank.

Propane. For a portable generator, you’d need four to eight 20-lb. propane tanks on hand to run the generator for 24 hours—a lot of grill tanks. Larger cylinders are available. But if you have a stationary generator, tanks won’t suffice; you’ll need at least a 250-gallon propane tank.

Natural gas. Mostly a fuel for stationary generators, natural gas is the easiest to use since homes equipped for gas have a ready supply. If you’re installing a new stationary generator that’s larger than about 8 kilowatts (kW), however, you’ll probably need to upgrade your gas meter and probably your gas lines—at a cost of $1,000 or more—to ensure that the generator has all it needs after what the furnace, oven, and other appliances require. Word to the wise: If you have a choice between natural gas and propane for a stationary generator, propane supplies more energy than gas, with differences of 1 to 1.5 kW common for the models tested by Consumer Reports.

Diesel fuel. Very few generators use diesel fuel. But for those that do, diesel fuel tends to be slightly more efficient than traditional gasoline. The now-discontinued Gentron GG2303 portable we tested, for example, needed 8 to 12 gallons for a 24-hour period.

Considering a new generator?
Take the time to determine your needs, keeping in mind that the greater the load on a generator, the more fuel it consumes. See our buying guide before checking our Ratings of 45 portable and stationary (standby) models. Our top recommended models include the $900 Generac RS7000E, a portable, and the $3,200 Kohler 8.5 RES-QS7, a small stationary generator. Prices noted don’t include installation.

—Ed Perratore (@EdPerratore on Twitter)