How a home standby generator works

A home standby generator is a permanent machine on its own base outside the house. It's wired into the house and connected to its natural gas or propane supply. When the utility power fails, it starts on its own and takes over the circuits it's wired to. When the power comes back, it hands them back and shuts down. It's meant for outages, not as a main source of power.

The transfer switch

The automatic transfer switch does the switching. It watches the utility power, starts the generator when the power drops, and moves the house circuits over to it. A generator that powers circuits the utility normally feeds goes through a transfer switch, which isolates the house from the utility lines while the generator runs.

Without that isolation, generator power flows back out into the utility lines, which is called backfeed. Backfeed is an electrocution risk to utility workers and to neighbors on the same transformer, and it bypasses the house's own circuit protection. It's the same danger as plugging a portable generator into a wall outlet.

Which circuits it runs

A standby generator powers the circuits wired through its transfer switch, not necessarily the whole house. The generator's rating sets how many circuits it can carry at once, and its real output drops in some conditions:

• About 3.5% less for each 1,000 feet above sea level
• About 1% less for each 10°F above 60°F

Around the unit

A standby generator runs outdoors only, because its exhaust carries carbon monoxide. Keep it the distance from the house, windows and doors that its manual gives, and keep the space around it clear:

• Air openings free of leaves, grass and snow
• No downspout, roof runoff, sprinkler or sump pump discharge spraying or flooding it
• Room on all sides for servicing
• Nothing built over it, such as a wooden deck

Keeping it ready

Set to automatic, the generator starts once a week and runs about 12 minutes to keep itself ready. It doesn't switch the house over unless the power is actually out. A typical schedule for an air-cooled unit:

• Fuel lines and connections, and the oil level, checked monthly
• Oil and oil filter changed every 100 hours of running, and at least yearly
• Air cleaner and spark plugs inspected yearly
• Battery terminals cleaned and tightened monthly
• Enclosure louvers kept clear, weekly

Service work is best left to an authorized dealer.

Carbon monoxide

Generator exhaust can kill within minutes in an enclosed space. Put carbon monoxide alarms with battery backup on each level of the house, whatever kind of generator you have. Get to fresh air right away if you feel dizzy or weak. A portable generator needs to run outdoors and at least 20 feet from windows, doors and an attached garage. Running a portable generator safely covers that kind.

An annotated example

What to do next

Find the generator's owner's manual and note its exercise day. Put the weekly, monthly and yearly checks on your calendar. Whatever kind of generator you have, put carbon monoxide alarms on each level of the house.