The best tip for buying a portable home generator: Don’t wait until the lights go out. Whether you live in a place at the mercy of snowstorms, hurricanes, heat waves, or an outdated electric grid, a power outage can range from annoying to life-threatening.

Gasoline-powered portable generators, which cost from about $400 to $1,500, based on their output and features, are not only readily available at home-improvement stores, they’re also easy to use.

“If you can operate a lawn mower, you can operate a generator,” says Art Aiello, spokesman for Generac, one of the largest manufacturers of portable generators. “In winter, it doesn’t take a lot of snow or ice to bring down a power line in certain neighborhoods. If you have the ability to pull it out, add fuel, and attach extension cords, a portable is a sound solution.”

The trick, as any good Scout would say, is to be prepared. Here’s what you need to know about buying a portable generator.

Assess your situation. How frequently do you experience power outages? Are they a few hours or overnight? (Most outages usually last only three hours, says Paul Hope, senior home editor for Consumer Reports.) Is your home habitable without power? Could you make it for more than a few hours without heat or air conditioning? How much food do you typically have in the refrigerator and freezer?

Calculate how much power you need. Generators are sized based on the power they provide an average home. Go through your home and identify the lights, electronics and appliances you want to run and the watts they consume. That’s simple enough to figure out with lamp or light fixture that takes a 60-watt bulb, and tougher to determine with your refrigerator. Aiello says to look for the data plate found on most appliances. It should tell you the volts and amps. Then do the math: Volts multiplied by amps equals watts. Or go online and look up the specs. Generac also has an online portable-generator sizing calculator you can use as a guide.

Keep in mind that extension cords running from a generator can power anything that plugs into a wall socket, but not hard-wired appliances such as a furnace, sump pump or central air conditioner. If you want to run any of those during an outage, you will need to have a licensed electrician install a manual transfer switch (it looks like a mini-circuit breaker) next to your main electrical panel. A heavy-duty cord from the generator plugs into the switch and allows you to power select circuits in your home. Prices vary depending on your market. One customer paid about $1,200 for the switch and installation, more than for the generator, which cost about $550.

(No portable generator can power an entire house; for that you'll need a different category of generator, called a home standby or whole-house generator, which will cost $2,500 to $4,500, plus installation, something that can run in the thousands, depending on where you live.)

Pick your features. Models come with either a recoil handle (similar to a lawn mower) or electric push-button start, and one or more outlets to plug in extension cords. Some include covered outlets, illuminated control panels or automatic shutdown in case oil runs low. Portables can weigh from 100 to 250 pounds, so you’ll want to buy a wheel kit. With the wheels installed, a generator can be rolled like a wheelbarrow into place.

Find some space. Although you can store a portable generator in a garage or shed, it must be at least 20 feet away from your home when running because it produces carbon monoxide. You may be glad it’s 20 feet away, because portable generators can be obnoxiously noisy. (This may not please your neighbors, however.)

Most hold about 6 to 10 gallons of fuel, enough to run 8 to 16 hours before needing to be refilled. That means you also need an appropriate place to safely store 25 to 30 gallons of gasoline. You could keep just a few gallons on hand and plan to refuel, but remember that gas stations require electricity to operate their pumps. If your entire community is offline or roads are impassible, you won't be able to get gas.

A generator is more of a comfort than a cost-efficient purchase, Hope says. He and Aiello agree that a smart consumer will buy with forethought, in the offseason. Find a local dealer or home-improvement store and compare models. Try the starter yourself (if that pull-start cord is too difficult, look for an electronic ignition). Some retailers will even let you fire it up and check the noise level. Once you get your generator, practice putting it in place and starting it. Then, hope you won’t have to.