Looking for some low-cost power backup? Even without a once-in-a-century event like Superstorm Sandy, every major storm results in blackouts and, for many of us, fresh memories of spoiled food, a flooded basement, no heat or air conditioning and (if you have well water) no water for bathing or household chores.

Once you've decided you've had enough and start researching generators, you'll find two clear choices. Portable generators for home use start at about $300 and can power selected devices using multiple extension cords or, using a transfer switch, a single plug-in connection you make when the main power goes out. The price goes up if you want your generator to kick in automatically when you lose power: Figure on at least $2,500-plus before installation for a standby, or stationary, generator that can also handle larger needs.

Suppose all you want to do is keep the sump pump in your basement going or keep your food cold during short-term outages: You might get by with a power inverter. These devices start at under $100 and essentially use your car as a generator, which was the premise for our tests during a freak autumn snowstorm that caused broad power outages along the nation's northeast coast.

We tested two models from PowerBright, a brand sold in home centers, warehouse clubs, and other retailers such as Sears and Walmart. The PW900-12, $80, provides 900 watts (1,800 peak) and 7.5 amps; the PW1500-12, $180, is rated for 1,500 watts (3,000 watts peak) and 12.5 amps. Both have two three-prong receptacles and claim to be safe for powering laptops, televisions, gaming consoles, and other electronics.

An inverter resembles a rectangular box about the size of shoe box. At one end are wires for connecting to a power source; at the other are AC receptacles. An inverter takes in direct-current (DC) power—in this case, from your car's 12-volt system—and converts it to the alternating-current (AC) power required by refrigerators, sump pumps, and most other household devices.

That DC to AC conversion isn't as simple as it sounds, however. AC-powered devices are designed to run on current that alternates smoothly. Because many inverters don't supply a pure sine wave—and those that do typically cost as much as a more-capable generator—anything you power with an inverter is likely to run less efficiently.

Once you've powered a couple of devices, you might be tempted to plug a half-dozen more into a power strip and then plug the strip into the inverter to increase your options. But unless you're just charging iPods and cellphones, it probably won't work and wouldn't be a good idea. The reason: The inverter can't draw more from your car's battery than the alternator can replenish without draining the battery.

Our special report, "Lessons Learned From Superstorm Sandy," will help you be prepared for when a natural disaster strikes.

We tested the inverters both in our labs and in the home of a staffer who had lost power—one among millions of northeast homeowners following a Halloween snowstorm. We also consulted the manufacturer and other industry experts.

We found that inverters can keep at least some of your home's essentials energized. But you'll need to prioritize: Even the smaller, 900-watt PW900-12 was able to run a freezer, two refrigerators, two sump pumps, lights, and chargers for various electronics for our staffer at home—but not all at once. Fortunately for him, only one of two installed sump pumps needed to come on, and it could do so even when a refrigerator or freezer was also running.

Either inverter supplies enough power to run most circulating hot-water home heating systems. But for other heating systems, you'll need to weigh your options. A forced-air system might require a larger inverter, and an inverter might not work at all with other heating options. These systems are hard-wired and need to be connected to a transfer switch, installed by an electrician, to power on.

What's more, knowing both the starting (peak) watts and continuous-load wattage for what you're powering is especially important with inverters. The rule of thumb for continuous-load wattage is to multiply the amps—as stamped or printed on the device—by 110 volts. But when a device first starts up, its starting load can be two to 10 times that of its continuous load, particularly for sump pumps and refrigerators. That means you may have to lower your expectations with an inverter. or consider stepping up to a full-fledged generator.

Still other inverter caveats are worth considering. For instance, we used a fully charged, spare car battery to run a refrigerator for roughly six hours before the battery ran down. But unless you have an extra battery lying around, you'll need to keep your car running so the alternator can keep its battery charged. Idling the car with the inverter attached used about a third of a gallon of gasoline per hour; we idled ours for 14 hours one day with no apparent ill effects. But that also means you'll need to keep the car outdoors with the exhaust facing away from the house and at least 15 feet away to reduce the risk of carbon-monoxide entering the home. And you'll need a direct connection to your car's battery terminals; the 12V cigarette-lighter adapter isn't up to the task.

Other inverter requirements: You'll need one extension cord per device you power, though if you have more devices than inverter receptacles, you might be able to alternate between, say, a refrigerator and a sump pump. Each cord will need a route from the device to the car. That typically means you'll have to keep at least one window ajar—an issue when you're also trying to keep what little heat that remains in the house. (A dryer vent or basement window might be an alternative.)

Today's power inverters are safer than ever, with a number of features that help keep you out of trouble—such as a cooling fan, overload indicator, and automatic shutdown for those who ignore the indicator. Still, using an inverter without first reading the manufacturer's instructions can result in property loss, injury, and even death.

As with a generator, you'll need extra gasoline to power your car in the event of a blackout. If you want to keep some extra fuel on hand, remember to store gasoline in a cool, well-ventilated area away from sources of heat or sparks. Don't count on your local gas station to supply more when you need it. A power failure is likely to disable the station's pumps. Here are some other tips for safe use:

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