A standby generator is an engine that runs a few minutes a week and occasionally for days. That duty cycle is unusual and it produces a specific set of failure modes.
The exercise cycle
The controller runs the engine automatically — weekly or biweekly, typically for five to twenty minutes.
It circulates oil, charges the battery, keeps the fuel system wet and dries condensation out of the engine.
It is also the diagnostic. A generator that fails its exercise cycle reports it, and most current controllers send that to a phone. Without exercise, the first indication of a problem is an outage.
Do not disable it to keep the neighbours happy. Reschedule it to a reasonable hour instead.
The scheduled items
Oil and filter at the manufacturer's interval — typically annually or after a stated number of running hours, whichever comes first. After a long outage, the hours may come first.
Air filter, annually or as conditions require.
Spark plugs, at the stated interval.
Battery, which is the single most common cause of a failure to start. Lead-acid batteries last a few years and they fail in cold weather. Replace on schedule rather than on failure.
Valve clearance, at longer intervals on air-cooled engines.
Coolant, on liquid-cooled units.
Between services
Keep it clear. Leaves, snow, grass clippings and anything stored against it block the cooling air.
Check under the enclosure for rodent nests, which are a genuine and common problem in a warm, sheltered machine that is quiet most of the time.
Look at the pad for settlement and standing water.
Confirm the exercise ran, if there is no remote notification.
After an extended run
An outage of several days puts more hours on the engine than a year of exercise cycles.
Oil and filters after a long run, regardless of the calendar, and check the air filter — a generator running continuously during a storm ingests a great deal.
Fuel considerations
Natural gas does not degrade and needs nothing.
Propane does not degrade either, and the tank level does. A tank that has been supplying a generator through an outage needs refilling promptly, because the next event may follow.
Diesel, on the larger units, degrades in storage and needs treatment and periodic replacement or polishing.
The service contract
Most dealers offer an annual maintenance agreement covering the scheduled items plus a functional test.
Whether it is worth the price depends on what the same visit would cost separately, and on whether the household will otherwise do it. A generator that is not maintained is a very expensive object that does not work, so the honest question is not the price but whether the alternative is actually going to happen.
The test that matters
Once a year, at a convenient time, simulate an outage: turn off the main, let the generator start and transfer, and run the house on it for half an hour with the normal loads on.
That exercises the transfer switch, the shedding modules and the household's understanding of what does and does not work — none of which the automatic exercise cycle tests, because it never transfers the load.
Records
Keep the service dates, the hours reading and the battery replacement date with the manual.
Running hours are what most of the intervals are actually based on, and after a few multi-day outages a generator can be due for service well before the calendar says so. The controller displays the hours; nobody looks at it unless they are in the habit.
Warranty
Standby generator warranties generally require documented maintenance at the stated intervals, and the exercise cycle to be enabled.
That is worth knowing before disabling the exercise or skipping a service year, because the components most likely to fail — the controller, the alternator — are exactly the expensive ones the warranty covers.