How Standby Generators Restore Power Automatically During a Power Outage

A single home has power in dark neighborhood without electricity after a storm.
Power Outages Caused by Storms and Other Factors Leave Your Home and Family Without Essential Electric Power

The first generators required a team of engineers to keep them working. They kept the steam engines running that provided the mechanical power for the generator, and made constant adjustments to the generator to regulate the power production. As the technology matured, the need for constant supervision gradually declined until generators could operate on their own. As cost declined and reliability increased, systems designed to provide backup power found a place in the market.

Called Standby Generators, they always stand by, ready to supply power whenever needed, without operator intervention. Modern home standby generators use natural gas or propane for fuel. Where neither fuel is available or accessible, a diesel generator is another alternative.

Buyer Guide: What Size Standby Generator Do I Need?


Guardian 20kW Home Backup Generator
Guardian 16-kilowatt Standby Generator Installed in a Home’s Backyard Near the House

A standby generator system is permanently installed outside the residence or business and connects to a long-term supply of fuel to eliminate frequent refueling. Residential and commercial systems usually run on natural gas or LP gas. An automatic transfer switch connects the generator to the building’s electrical system and selects either generator power or utility power.

When the generator detects a power outage, the engine starts automatically and turns a power generating unit called an alternator. The alternator converts the mechanical energy of the internal combustion engine into electrical energy. After a few seconds to stabilize the engine speed and electrical output, the transfer switch isolates the circuits it controls from the utility grid and supplies power from the generator to the home or business.

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Automatic Transfer Switch

20-Kilowatt Kohler Standby Generator with 200-Amp Automatic Transfer Switch

The automatic transfer switch makes the generator a permanently installed solution to power outages. Depending on the model, it may be a Load Center ATS with its own circuit breakers or it controls the main circuit breaker panel or a sub panel. Some Automatic Transfer Switches include power management capabilities for 240-volt, high-current loads.

A transfer switch isolates the standby generator from the utility power lines. When an outage occurs, it automatically disconnects the utility service and connects the generator as soon as it is ready to supply power. A typical switchover takes less than a minute from startup to transfer. Isolation prevents the generator from feeding the utility lines and endangering utility workers. When the utility restores power, isolation protects the generator from damage.

When the utility restores power, the transfer switch disconnects the generator and reconnects the utility supply—automatically.

Buyer Guide for Automatic Transfer Switches

Power Management

Cummins Quiet Connect Standby Generator In a Home's Backyard with Deck and Family
Cummins Quiet Connect 20-Kilowatt Home Standby Generator with Power Management

When high-voltage devices (air conditioners, electric water heaters, electric dryers, etc.) all try to operate at the same time, the standby generator may not have the capacity to handle the full load. With a power management option, high-voltage loads only run when the generator has sufficient power. This may force one air conditioner to wait while the other runs, and other high-power loads may also have to wait.

Power Management Systems usually choose loads to operate based on a preset priority because homes and businesses often have multiple loads that need management while running on standby power. With the right power management option, these loads each get their share of power as necessary to keep the appliances operating during an outage.

Priority based load management allows a smaller, more fuel-efficient, and less costly standby generator to do the job of a larger generator which costs more and uses more fuel.

Standby Generator Systems with Power Management

House Lights On with a Champion Home Standby During an Outage
“A Best Buy” The Champion Standby Generator

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Generator Controller

Generac Evolution Controller for Guardian Standby Generators
The Generac Evolution Controller Includes a Two-Line Multilingual Display

The controller lies at the heart of the generator. It handles all standby generator functions from start up to shut down and monitors the generator for problems. Some models handle the power outage detection, others rely on the transfer switch to detect an outage. In either case, a short delay of a few seconds ensures the outage is not momentary. The engine starts and stabilizes at full speed after about five seconds. At the same time, the alternator output reaches full voltage and the system is ready to handle the full load electrical load.

After the utility restores power, the controller runs the engine in a cool-down cycle, usually for about a minute, and then shuts the generator down. To keep the standby generator lubricated and ready to run, the controller will also exercise the generator on a set schedule. It starts the unit, lets it run for a short period, then shuts it down again.

How it Works: Standby Generator Components

Standby Generator System

Briggs and Stratton Home Backup Generator
Briggs and Stratton 20-Kilowatt Home Standby Generator System with ATS and Symphony II Power Management

Manufacturers design transfer switches and generator controllers to work together as a unit and provide more features than using separately designed and manufactured switches and generators. This eliminates compatibility issues, simplifies installation, and reduces cost while improving reliability. A standby generator system is often called a ‘Genset’ for generator set. The set includes the generator and the transfer switch, and any optional electrical equipment necessary, such as power management modules.

The modern standby generator system is always ready to supply power during an outage to keep homes safe and businesses operating.

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