Backup Generators · Problems

Backup generator problems

Most backup generator failures trace to four things: a dead starting battery, stale or contaminated fuel, skipped maintenance, and a unit that was undersized for the load. Bob Vila's maintenance guidance calls for oil changes roughly every 100 hours of operation, air filter and spark plug service at about a year or 100 hours, and professional service every two years or 200 hours — the intervals people skip are the ones that strand them.

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It won't start when the power goes out. On standby units, the usual culprit is the starting battery. That's why the weekly exercise cycle exists — the unit runs itself for a few minutes on a set schedule to keep the battery charged and the oil moving. If you never hear yours run, something is wrong and you should find out now rather than during a storm.

It starts, then quits under load. That's often an undersized generator or too much on the transfer switch. Sizing math is running watts plus the single largest starting surge; central air alone can need roughly 3,000 running watts and about three times that to start. A common rule of thumb is to keep the generator at no more than 90 percent of its rated capacity.

Portable unit won't start after storage. Gasoline left sitting varnishes the carburetor. Bob Vila's guidance is to run a portable every two weeks for five to ten minutes if there's fuel in the tank, and to use a stabilizer for shorter storage periods rather than leaving untreated fuel to sit.

It runs but nothing in the house has power. That's a transfer switch or breaker problem, not an engine problem. It's also the point at which people start improvising dangerous connections. Don't. Call the installer.

How to choose

Listen for the exercise cycle: Standby units typically self-test weekly for several minutes. Note the day and time. Silence for a few weeks is your early warning.
Treat the battery as a consumable: A starting battery is the most common reason a standby generator does nothing during an outage. Ask your service tech to load-test it at every visit.
Recheck sizing when you add big loads: A new heat pump, EV charger or well pump can push you past what the generator can carry. Central AC needs roughly 3,000 running watts and about 9,000 to start.
Don't store untreated gasoline: Stale fuel is the number one reason a portable generator that worked last year won't start this year.
Distinguish engine faults from switch faults: If the engine runs and the house is dark, the problem is downstream. That's a warranty and installer conversation, not a fuel one.

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All backup generators problems

Power Outages

No Backup Power

Undersized Generator

Transfer Switch Missing

Fuel Storage Concerns

Quiet Hours Restrictions

FAQ

Backup Generators problems — common questions

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Why won't my standby generator start during an outage?

Most often a dead starting battery, followed by stale fuel or a fault flagged during a skipped service. Standby units run a weekly exercise cycle largely to keep the battery healthy — if yours hasn't been running, that's the first thing to check.

How often does a generator need an oil change?

Bob Vila's guidance is an initial change after about 30 hours on a new portable, then roughly every 100 hours of operation, with air filters and spark plugs at about a year or 100 hours, whichever comes first.

My generator trips out when the AC kicks on. What's wrong?

Almost always undersizing. Motor loads draw a large surge on startup — central air can need about three times its running wattage to start. Either the unit is too small or too many circuits are on the transfer switch.

Is a generator that runs but powers nothing dangerous to troubleshoot myself?

Yes. The fault is on the transfer switch or panel side, which is live electrical work. Call the installer rather than experimenting with cords.

Sources

Where these figures come from

Bob Vila — Generator Maintenance · retrieved 2026-07-26
Bob Vila — How Big of a Generator Do I Need? · retrieved 2026-07-26
Bob Vila — How to Choose a Generator · retrieved 2026-07-26