Generator buying guide

Generators are typically sold by wattage. How much they put out determines not only how many lights and appliances you can run at once but how well they run. For example, a refrigerator often requires about 600 watts, a portable heater 1,500 watts, a window air conditioner 1,000 watts, and lights 60 to 200 watts. Our wattage calculator provides an average wattage rating for most appliances and devices to help you to tally your needs.

Manufacturers claim that small portable units produce up to 3,600 watts, midsized portables up to 6,600 watts, and large stationary models up to 12,000 watts. While even a small generator can power several plug-in appliances, you'll need a larger one and a transfer switch for furnace fans and other items hardwired to your home. What's more, we've found that many generators fell short of their claims and began shutting down when we loaded them to the max. An overloaded generator can overheat and damage a computer or the electric motor in some sensitive devices.

Check a generator's oil level, fuel filter, and spark plug according to the owner's manual, and test-run the generator at least once each season. Be sure the gasoline remains fresh so the generator is ready when you need it. Untreated gasoline can go bad in as little as a month; adding a stabilizer can extend its life for up to a year. Then be sure to store it in a container that meets safety standards set by the American National Standards Institute.

Today's generators are better and safer than models we'd tested in the past. But a carelessly used generator can be deadly. Prevent carbon-monoxide poisoning by running a portable generator outdoors at least 15 feet from the house--never in the basement, garage, or other enclosed space.

Store gasoline in a cool, well-ventilated area away from sources of heat or sparks, and turn off the generator before you refuel it. A typical portable generator uses about 10 gallons of gasoline a day, so you'll need to store at least 20 gallons. 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. Some generators can also run on propane or natural gas, easing fuel-storage requirements.

And always use a transfer switch (about $600 installed) to run hardwired items like central air or a furnace fan and to route power to wall outlets. Transfer switches also eliminate the need to run extension cords into and out of the house. Besides saving work and hassle, they help eliminate shock hazard since even heavy-gauge cords can be hazardous when the ground is wet. Transfer switches are usually included with stationary units.

These small and midsized models typically put out anywhere from 2,500 to more than 6,000 watts. They cost as little as $500 or so and are adequate for many homes, which is why they're the biggest sellers. Most portables run only on gasoline, but some can also use liquid propane or connect to a natural-gas line.

These large models mount permanently outside the house and are growing in popularity. Their roughly 6,000 to 12,000 watts let most power an entire house, including central heat and air conditioning.

Stationary generators often run on either propane or natural gas. Most portable models run only on gasoline, though come equipped to run on a propane tank or natural-gas line or can be converted with kits.

A single person can move a portable generator with wheels, but you'll still need two people to lift it. Models with pneumatic tires are easier to roll.

This feature protects the engine from damage by shutting down the generator if the oil level falls below the minimum. It's typically on stationary generators and on some portables.

The gauge lets you check a portable's fuel tank at a glance. It's especially useful during a prolonged power failure.

Portable generators with a battery-powered, pushbutton starter save you the hassle of pull-starting the engine. But we've found pull-start models relatively easy to start. All stationary models have this feature.

A feature on higher-end portables, it makes wattage output smoother and more consistent via a microprocessor-controlled circuit.

Four or more outlets let you best use a generator's wattage by spreading the load through more outlets--important if you're using extension cords instead of a transfer switch.