Sales of portable generators tend to spike around major storms, from just before to weeks afterward. But getting a portable generator up and running in a hurry often means bypassing the recommended installation procedure and making do with extension cords.

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“It’s not only less safe to run a generator without a transfer switch, but you’ll limit the devices that your generator can power,” explains Dave Trezza, who oversees generator testing for Consumer Reports. “A transfer switch will let you power things on your circuit breaker panel, including hardwired appliances like a water heater or well pump.”

Plan ahead, and do it right: Portable generators rated for 5,000 watts and above need a transfer switch, a small board that looks a bit like a circuit-breaker panel, installed within a few feet of your circuit breaker panel. (The advantage of stationary, or standby generators? They have a built-in transfer switch that automatically switches on when the power goes off.)

How a Transfer Switch Works

Skipping the transfer switch and connecting your portable generator directly to your electrical service panel could fry appliances, endanger utility workers who might be working on power lines, or damage your generator. Plus, you don’t always know when power is restored, unless you see your neighbors’ lights go on.

A transfer switch lets you easily and safely power what you need most—your refrigerator, your furnace or boiler, a sump pump, and more. Think about it: Power to these devices can come from your utility or your generator but not both. The transfer switch works by having a dedicated set of switches for each of the circuits you want to power.

With a transfer switch (about $500 to $900 to install, including labor), you’ll have one connection to make—plugging one end of a cable into the portable generator and the other into a dedicated box connected to the transfer switch.

Looking for a portable or stationary generator? Check CR’s generator buying guide before plugging into our generator ratings.

An Alternative: The Interlock Device

Most manufacturers of service panels and many third-party manufacturers make a small connector called an interlock device. Each is intended for a specific panel model, but all work the same way: They allow you to attach a portable generator to the service panel—without a transfer switch—and they eliminate the hazards of a direct connection.

An interlock kit, $50 to $150, should also be installed by an electrician, who can tell you whether it meets local building codes and whether it will work with your electrical system.

When in place, the interlock covers your service panel’s main cutoff switch, so you can’t switch it on while the generator is running. Once utility power is back, you slide the interlock back to its usual position. Thus, power flows in only one direction.

This option is more manual than using a transfer switch, but that’s the price you pay to save hundreds on buying and installing a transfer switch.

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