The All-Important Transfer Switch
When the power goes out at the Wymans', they have to flip a manual transfer switch (which isolates the house from the incoming power lines), then push the button that fires up their unit. For another $1,000, an automatic transfer switch could do all that work for them. This brainy device constantly monitors incoming power, and when it detects an interruption or a serious voltage reduction (a brownout) automatically isolates the house and starts the generator. All told, the process takes 10 to 20 seconds. "You're going to be in the dark for a few seconds, but the power will come back up as soon as the generator comes on," says Bill Perry Sr., owner of Central Maine Diesel and installer of the Wymans' system. The reason for the delay is that the transfer switch waits to see if the power will come back on, so that the generator won't cycle on and off every time the lights dim. When power from the utility returns, the transfer switch waits for a sustained flow of current before it shuts the generator down.

The first step in choosing a generator is to determine how much electricity your household needs in an emergency. That will tell you how big the generator's output needs to be. Small 4- to 8-kw units, for instance, can power lights and a few critical appliances, while the 25-kw behemoths allow a household to function as though the power lines never went down. A 10- to 15-kw generator, the size most commonly used for residences, is usually sufficient to power most home appliances, including such energy-suckers as water heaters and window air conditioners. To keep power needs (and generator size) in bounds, an essential-loads distribution panel is a necessity. Only circuits that require emergency power are connected to the panel. Normally, it receives and distributes power from the utility, but during an outage, after the main transfer switch is activated, this panel gets all its power from the generator. (When the outage is over, the panel reverts to utility power.)

A properly sized generator should deliver 10 to 20 percent more power than the total expected demand. Running a unit too close to its rated output reduces fuel efficiency and makes the engine run harder. If completely overtaxed, the generator's breakers will trip (to protect it from overheating). "A generator that's too small is worse than not having any power at all," says Ron Ford, sales manager at Kohler, which manufactures both residential and industrial generators.

You pay a substantial price for the convenience of a permanently installed generator, and that price increases substantially as output goes up. It starts at about $3,000 for the 6- to 8-kw models and reaches from $5,000 to $10,000 for the 11- to 20-kw units. Add to that anywhere from $800 to $2,500 for installation, including pouring a concrete pad, wiring, and installing a fuel line and tank, all of which are coordinated by the dealer. (Find them in the yellow pages or through manufacturer Web sites.) Plus, there's the cost of maintenance and fuel.

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