Power Transfer: How to Combat Blackouts with a Transfer Switch and Portable Generator

portable generator

By Joseph Truini for The Home Depot

Power outages strike without warning. They’re unavoidable, indiscriminate and more than a little inconvenient. When the electricity is out, it only takes a few hours for food to spoil, freezers to defrost, and your home to turn bitterly cold in winter or insufferably hot during the summer. Evenings are spookily dark and quiet, and particularly unsettling. And if your home draws water from a deep well, then a blackout means no water for drinking, washing or flushing of toilets. Welcome to the 17th century.

You can use a portable generator to provide temporary power to a few plug-in appliances, but its usefulness is limited and it requires stringing extension cords throughout the house. However, you can greatly enhance the versatility and performance of a portable generator with the addition of an ingeniously simple electrical device, a transfer switch.

What is a Transfer Switch?
A transfer switch is essentially a mini-electrical panel that gets installed next to your home’s main electrical panel. Each wire in the transfer switch is connected to an existing circuit in the main panel. You can choose which circuits to connect, but it’s best to choose essential appliances, such the refrigerator, furnace, well pump, water heater and range.

A junction box is installed near the transfer switch and an electrical cable is run from the box through a wall to the outdoors. There, the cable is connected to a power inlet box that’s mounted to the exterior of the house. At the junction box, a power cord is connected to the electrical cable; the power cord has a plug that fits into the socket on the transfer switch.

A separate power cord, which has a plug on each end, connects the generator to the power inlet box mounted outside.

It’s important to note that a transfer switch isn’t meant to power every electrical circuit in the entire house. (To achieve that feat, you’d need a whole-house standby generator). However, a transfer switch can provide enough electricity to keep most of your house up and running until power is restored.
A transfer switch is wired into the main electrical panel. The model shown here is a six-circuit transfer switch.

Operating a Transfer Switch
Once the power goes out, you must plug the power cord connected to the junction box into the transfer switch. Check to be sure that each switch is in the down or “off” position.

Next, place the portable generator outdoors near the power inlet box.

Important: Generators produce deadly carbon monoxide gas, which is colorless and odorless. Never run a generator indoors, in a garage (even if the door is open) or near an open window or door where the fumes could drift inside.

Now, plug one end of the double-plug power cord into the generator; plug the other end into the power inlet box. Attach a long, bare copper wire to the grounding lug on the generator. Take the opposite end of the wire, wrap it tightly around a steel bar and then drive the bar into the ground. This step, which is often overlooked, grounds the generator and helps prevent electric shocks.

Start up the generator. Then, go inside and flip on one of the circuits on the transfer switch. Confirm that the appliance is operating correctly, then flip on the other switches. However, be aware that running all circuits simultaneously can overload the generator. (See the box below for more details).
The generator gets plugged into the inlet box, which then carries electricity to the transfer switch.

Cost Considerations
Transfer switches come in various sizes based on the number of circuits. The most popular size is the six-circuit transfer switch, which is designed for use with a 5,000-watt generator. If your home’s electrical system requires a bit more power, consider a 10-circuit transfer switch and team it with a 7,500-watt generator.

A six-circuit transfer switch typically costs between $200 and $250; a 10-circuit model costs about $300 to $400. A power inlet box runs about $55. And figure another $400 to $600 for installation by a licensed electrician.

Larger homes require large-capacity transfer switches, such as this 10-circuit model.

Warning: Don’t attempt to install a transfer switch yourself. Because you’re dealing with high voltage electricity, it’s important to hire a qualified professional to ensure your safety during the process and after the switch is installed.

Giving accurate prices for portable generators is trickier, since there are several manufacturers and each offers models in a wide range of prices. However, you can expect to pay about $600 to $800 on average for a 5,000-watt generator, and $800 and up for a 7,500-watt generator.
This 7,500-watt generator produces enough power to supply a 10-circuit transfer switch.Watts Up?
This chart shows how many watts of electricity it takes to run various household appliances. It lists both running watts and starting watts. Running watts represent the amount of electricity used when the appliance is operating normally. Starting watts is the extra surge of electricity needed when an electric motor cycles on. Note that not all appliances require starting watts.

To avoid overloading the generator and possibly damaging an appliance, confirm that the total number of running watts and starting watts doesn’t exceed the wattage rating of the generator. And if the total number of watts at the transfer switch does exceed the generator rating, that’s fine—you just won’t be able to power on all the circuits at the same time.

  • Appliance             Running Watts          Starting Watts
  • Box Fan                 200                              0
  • Coffemaker            1,000 to 1,500             0
  • Computer               800 to 1,000                0
  • Electric Range       2,100                           0
  • Freezer                  500 to 750                   750 to 1,000
  • Furnace                  600 to 900                   1,400 to 2,400
  • Lights                      Check bulb wattage    0
  • Microwave Oven    600 to 1,500                0
  • Refrigerator             800 to 1,000               2,000 to 2,500
  • Air Conditioner        10,000 BTU                1,200 3,600
  • Sump Pump            800 to 1,000                1,300 to 2,500
  • Toaster Oven         1,200                            0
  • TV, 27-inch 400 to   600                              0
  • Water Heater (elec) 3,000 to 4,000            0
  • Water Heater (gas), 500                             1,500
  • Well Pump               1,000                           2,100

Editor’s Note: Joseph Truini is a home improvement expert and author who writes about DIY projects for The Home Depot. He provides advice on topics from plumbing to generators. Visit The Home Depot to find the transfer switch options that Joe talked about in this article.   

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