Back-up Generators

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Last updated: 
September 15, 2013

Back-up Generators
By Betty Stephens

back up gen

A back-up generator is a back-up electrical system that operates automatically Within seconds of a utility outage an automatic transfer switch senses the power loss, commands the generator to start and then transfers the electrical load to the generator. The back-up generator begins supplying power to the circuits. After utility power returns, the automatic transfer switch transfers the electrical load back to the utility and signals the back-up generator to shut off. It then returns to standby mode where it awaits the next outage. To ensure a proper response to an outage, a back-up generator runs weekly self-tests. Most units run on diesel, natural gas or liquid propane gas.


Automatic back-up generator systems may be required by building codes for critical safety systems such as elevators in high-rise buildings, fire protection systems, standby lighting, or medical and life support equipment. Residential back-up generators are increasingly common, providing backup electrical power to HVAC systems, security systems, and household appliances such as refrigerators, stoves, and hot water heaters.

In 2007, an estimated 12 million generators had been installed across in the United States, with a total capacity of more than 200 gigawatts, according to a Brookings Institute report on distributed power systems. More than nine million of these machines were designed to provide emergency or backup power.


An emergency backup generator is only allowed to operate for 200 hours every year and only in the event of an emergency power failure or for routine testing and maintenance. By contrast, primary generators operate upwards of 8,000 hours every year.

Types of Generators

Home generators can be portable or stationary (standby). They run on a variety of fuels, such as gasoline, diesel, natural gas (NG), and liquefied petroleum gas (LPG). Each type has its pros and cons. Portable gas models are relatively cheap.
You can buy a 4000W set for $300-$400. However, such devices have short run time: you would need to refill their tank several times a day if you run them continuously at rated load. In addition to this, they are not suitable as a long-term power backup since the pumps may not work during a wide spread blackout.

For a long term emergency consider standby sets. They can provide continuous power for the home because they are hooked up to an external fuel source, such as NG line. Some portable devices can also be fueled from an external source and can therefore provide extended run time too. The main differences between them and stationary models are in their connection and activation. A portable device has to be rolled out from the storage, filled with fuel or hooked up to a fuel line, manually started, and connected to your loads.

A fixed standby generator by contrast is already connected to both the house wiring and the fuel source. Therefore it can start immediately either by a push of a button or automatically. Automatic systems have an auto transfer switch. It can sense a power  failure. When grid voltage is restored, such a system will connect you back to the utility lines and turns itself off. You don't even have to be at home to activate it. Note that the typical transfer time of an automatic system is 10-30 seconds, the convenience of an auto starting option and practically infinite run time; permanently connected standby systems offer power levels higher than portables. Their rating ranges anywhere from 5 kilowatt up into hundreds of kW.

Selecting a Generator

When selecting a generator set to supply back-up power for a residence the primary consideration are to choose between a portable or permanently installed unit designed for residential use and how to determine the kW size.

Residential sets are designed to be permanently installed outside the home and are offered in two power ranges, 8 to 17kW to operate selected loads
during the emergency and secondly up to 125kW with sufficient capacity to power the whole home if so desired.

While kW size is a determining cost factor, it is best to specify a quality generator brand with necessary accessories within the budget. A quality set with an UL2200 label assures a trouble-free life, durability and increase the resale value of the home. Specify a set that gives clean, stable and safe electrical power is essential, particularly with today’s sensitive electronics. An auto exerciser should be included to start and run the set each week.

• Determine wattage needs. Determine how much power you need for the items you care about. For help, try our power selection worksheet. If you decide to purchase a portable generator, don't forget you will also need a transfer switch to safely power the circuits in your home.
• Understand electrical terms. You’ll see a lot about watts, volts, amps and more. Review the glossary for definitions.
• Decide: Standby or portable? Considering your budget, convenience, and power needs, chooses which type of generator you want.
• Take care of home power issues. Locate your existing electrical service panel and gas line to target any potential problems before buying a generator. Portable generators should be used with a transfer switch.
• Perform product comparisons. Compare power outputs, run times, and prices, as well as what’s included in those prices, such as accessories, warranties, support and installation.
• Determine financing options. Some retail stores offer financing options for generator purchases. Amazon offers FREE Super Saver Shipping and provides no interest financing on select items.
• Consider included items. Does the generator come with warranty or maintenance package? What about a power cord, oil, wheels, and funnel?
• Consider additional costs. How much more will it cost for installation by a qualified professional? Will you need an accessory like a cover for protection from the elements or a transfer switch?
If you want to use your generator to power part or all of your home, you'll need a sufficiently sized generator and a transfer switch. The transfer switch safely closes off the utility power line to your house's electrical system and opens a direct line to the generator and reverses the process when utility power is restored.


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