Portable generator. (Anonymous/PR NEWSWIRE)

Every time a storm knocks out power to a significant number of people, you start hearing rumblings about generators. Some people want to invest in a generator but don’t know where to start. What kind? What size?

The answer depends heavily on whether you just want to keep a freezer and maybe a room air conditioner running, or whether you want to your life to go on as usual even when the power is out for days.

Less money, more hassle

Portable generators are sold at home centers and tool companies and cost several hundred dollars to about $1,000. Most run on gasoline, though some run on propane in canisters, like backyard grills, and there are adapter kits that allow the conversion. With a portable generator, you can plug in heavy-duty extension cords (the number of outlets will depend on the generator) and run them directly to the equipment you consider most crucial. Or you can get by with a single extension cord if you install a power transfer switch in your house to direct the power to specific circuits.

The idea of a generator that you can just buy, fire up and use is quite appealing. But the fact that you don’t need professional installation means you’re on your own in making sure it’s running safely and effectively. To guard against carbon monoxide poisoning, you must run a portable generator outside and well away from windows and doors. Running it in the garage is also out because the exhaust can work its way inside even if the big door is open. And don’t even think of modifying an extension cord so you can plug the generator into a house outlet. Generator power needs to bypass the house wiring completely (by running extension cords directly to appliances) or go through a transfer switch, which also acts as a one-way gatekeeper that prevents the current from energizing your home’s transmission line and shocking an electrician working to restore power to your neighborhood.

Besides the safety issues, there are practical drawbacks to portable generators. A portable generator typically runs six to nine hours on a full tank of gas, then needs to be refilled with four or five gallons. So if the power stays out for too long, plan on driving around to find a station with pumps that still operate. Safely storing a few days’ supply is problematic in part because gasoline that’s stored for a long period tends to gum up motors. And there’s the obvious fact that unless someone is home to wheel the generator into a spot where it’s safe to run and then fire it up, a portable unit won’t keep your food from spoiling or your sump pump from preventing basement flooding.

Less work, more money

All the babysitting needs of portables are one reason why built-in standby units are becoming increasingly popular. The units themselves are permanently installed outside, like air conditioners. Instead of running off gasoline, these generators are plumbed to run off natural gas or, where that isn’t available, propane. Assuming the connection isn’t to a tiny propane tank, either option means that standby generators can essentially run as long as they’re needed. Standby units also switch on and off automatically, and even put themselves through a low-power test phase on a regular schedule. And they are always wired to a transfer switch, so you never have to snake out extension cords. You can set up the system to power just a few circuits, or all of them, meaning you can weather a long power outage almost without changing your routine. Running a generator does add noise, though, although standby units are quieter than portables because they are more enclosed. Locating the generator in an insulated enclosure reduces the sound even more.

But the convenience of automatic standby power comes at a price: Installed units often come to about $10,000, though you can shave off a few thousand dollars by going with a small unit that powers just a few circuits.

Homeowners who opt for standby generators often purchase them through electrical contractors, which is fine except for one thing. “Electricians aren’t very friendly with gas, and the gas side of the installation is very important,” says Horacio Muslera, a master electrician and owner of Plus Electric Corp., which installs generators in the Washington area. He warns homeowners need to make sure their electrician checks with their gas company to make sure their gas lines can handle running a generator in addition to the furnace or other gas-run appliances. Otherwise, homeowners could wind up paying thousands of dollars to install a new pipeline from the street.

Regardless of whether you opt for a portable or a built-in, the size generator you need depends on how much you want to run. The most accurate way to calculate this is by using the specific wattages for each appliance you want to back up and then factoring in the surge power that motorized appliances require. Or, instead of scouring through owner’s manuals or searching for appliance data plates, you can use an an online estimating tool, such as the one at www.kohler.com. A sizing chart at www.generatorjoe.net makes it even easier.

In general, a 5,000-watt portable generator is the minimum that’s worth getting to back up vital systems, says Art Aiello, a spokesman for Generac, a generator manufacturer. That’s enough for a fridge, a few lights and even a room air conditioner. Built-in systems start about 7,000 watts, but to keep central AC running, you’ll probably need 17,000 watts to 20,000 watts. Above that, the price jumps because higher-power generators need liquid coolant, not air.

If you do get a built-in system, consider installing a “smart switch” that manages loads by temporarily turning off the most power-hungry appliances (such as AC) when demand on other circuits is high. “You don’t need a monster-size generator to back up your whole home,” Aiello says. You might not get total comfort with this approach. But half comfort looks a lot better than none.

Have a problem in your home? Send questions to localliving@washpost.com . Put “How To” in the subject line, tell us where you live and try to include a photo.

The Checklist Read Jeanne Huber’s roundup of home-improvement tasks you should tackle in July, such as oiling your garage door, at washingtonpost.com/home.