Think of a transfer switch as a miniature circuit breaker panel that draws electricity from your generator instead of from the power company. When electric service is out in your area, you plug your portable generator into an outdoor outlet that’s connected, through the house, to a transfer switch inside.
The switch itself is installed by an electrician, usually alongside your main circuit breaker panel. The electrician can help you figure out which circuits you’ll want to power in an outage. Heating and cooling equipment are essential, as are water heaters and well pumps.
Without this switch installed, you’ll need to run outdoor-rated extension cords from your generator into the house. And yes, multiple cords: Because they can be overloaded, you’ll need a dedicated cord for anything that draws a lot of power, such as a space heater or window air conditioner. Keep in mind, too, that without a transfer switch, you can power only electronics that have a standard plug. You won’t be able to connect anything that’s hardwired to your circuit panel, like a furnace or an air conditioner compressor, and you’re also out of luck if you have an electric range or dryer, because both use large, 220-volt, four-prong plugs. A transfer switch allows you to power any of those—and skip the extension cords.