The basics of backup generators

Posted on May 01 2015 in Energy

This is a typical air-cooled backup standby home generator powered by natural gas or propane. Photo by Kohler

When severe weather hits, I want to be prepared, and I’m considering purchasing a backup generator for my home. What types would you recommend to take care of the entire house?

Purchasing a backup generator is becoming more common. Residential backup generators are called standby models because they are only used when electricity from the utility grid fails. They are designed to run for a relatively short period of time until power is restored.

First, you must decide how much of your home you’d like to power in order to determine the size of backup generator you will need. There are common necessities, such as refrigeration and lighting, which you will definitely need. Others, such as air-conditioning, washing/drying clothes and vacuuming may not be a top priority during a power outage.

Backup generators are sized by their kW (kilowatt) electricity output. A 12-kW generator can power most electrical needs of a typical family of four. If you can eliminate nonessentials, a smaller, less expensive unit will be adequate, and the fuel costs to operate it will be less.

To get a rough idea of the size of generator you will need, list all electric items you want to power, and total the wattages for all of these. Items and appliances with motors often require more electric current at start-up time, so remember to round up when determining the total wattage. A contractor or installation expert can also advise you on the proper size. It should be noted that installing a whole-house backup generator is not a do-it-yourself project.

For convenience and safety (for both your family and your electric co-op’s emergency line workers), install an ATS (automatic transfer switch). This switch senses when the grid electricity goes off or the voltage drops below a critical point (also known as a brownout). It automatically disconnects your home’s wiring from the utility grid and starts the generator. This occurs quickly, so there is very little down time.

The ATS also runs the generator periodically (called exercising) to ensure everything is working properly. You may hear the generator start the exercise cycle, so don’t be alarmed — the power may not be off.

If you have natural gas available at your house, this is the best fuel to power the backup generator. Natural gas engines run cleanly, require little maintenance and are relatively inexpensive to run. Also, if you have natural gas, you probably have a gas furnace for heat, so the size of the generator required is smaller.

Another clean-running fuel for a backup generator is propane. Many homes with electric heat still have propane available for cooking. In order to power a whole-house generator, a larger propane tank would be required. Even though propane is considered a clean-running fuel, it is more expensive to use than natural gas.

A diesel engine-powered generator also requires a fuel tank. The shelf life of the diesel fuel is only a couple of years — even with a stabilizer — and the overall cost of installing a diesel generator will be higher. However, one advantage of using diesel is that you can always pour more fuel into the tank if you need to run the generator longer than originally expected.

Another option is a smaller, less-expensive portable gasoline-powered generator with several electrical outlets. This will provide enough electricity for the refrigerator and several lamps. These models also provide enough power to operate the blower in a gas, propane or oil furnace for heat.

Never attempt to plug this type of generator into an electrical output with a homemade double-male cord. This can backfeed 120-volt current into the grid, which is dangerous for utility line crews. For more information on backup generators, contact your local electric co-op or speak with a qualified contractor.

James Dulley is a nationally syndicated engineering consultant based in Cincinnati. If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit