A standby generator that sits fine during its weekly exercise cycle but won’t actually start when the power goes out for real is one of the most frustrating standby failures — because the unit “worked last week.” Here’s where that gap usually comes from:
- Nothing happens at all — no crank, no lights on controller → start with Dead or Weak Starting Battery
- Engine cranks but won’t fire during a real outage → jump to Fuel Supply During an Actual Outage
- Exercise cycle runs fine but real outages fail → see Utility Sensing Delay Issues
The number one cause, by a wide margin, is a starting battery that’s weak enough to run a quick exercise cycle but not strong enough for a real cold start under load — especially in summer heat, which shortens standby battery life faster than people expect. The second most common cause on propane or natural gas units is a fuel supply issue that only shows up during long outages: a low propane tank level sensor stopping short of empty, or (rarer) a gas valve solenoid that’s failing intermittently.
- Test the starting battery under load, not just resting voltage — a battery can read 12.4V resting and still fail to crank under real load.
- Standby generator batteries typically last 2–3 years in hot climates like Arizona, shorter than the 3–5 years quoted for milder climates.
- The weekly exercise cycle is a light-load, short-duration test — it doesn’t prove the unit will handle a real outage with your house’s full load.
- Check your propane tank level before assuming a mechanical fault — many “won’t start” calls during long outages are simply an empty tank.
- Some controllers have a programmable utility-sensing delay (to avoid nuisance transfers on momentary blips) that’s set too long for your comfort — this is adjustable, not a fault.
Dead or Weak Starting Battery
Standby generators use a conventional lead-acid or AGM starting battery, typically a Group 26 or similar size, and it’s maintained by a small trickle charger built into the controller. That trickle charger can mask a failing battery for months — the battery reads full resting voltage because it’s constantly topped off, but doesn’t have the cranking amps left to actually turn the engine over under compression, especially after the generator’s been sitting.
Test it properly: with the generator off, check resting voltage (should read 12.6V+ for a healthy 12V battery), then have someone attempt a manual start from the controller while you watch voltage — a healthy battery holds above 9.6V during cranking; a weak one sags to 7V or lower and the engine cranks slowly or not at all. If it sags badly, replace the battery — Group 26 standby batteries run $60–120, and this is a five-minute swap with two terminal bolts on most units.
Fuel Supply During an Actual Outage
The weekly exercise cycle typically runs 5–20 minutes, which barely touches fuel supply. A real outage might run the generator for hours or days, and that’s when problems that don’t show up in testing appear: a propane tank that reads “enough” on the gauge but trips the tank’s internal low-level safety shutoff (most propane tanks stop supplying gas below roughly 20% to protect the tank), or a natural gas line pressure drop if multiple neighbors’ generators or other gas appliances are drawing on the same main during a widespread outage.
Check your propane tank level directly, not just the generator’s fuel gauge if it has one — many owners assume a “half full” reading means plenty of fuel, not realizing the tank’s own safety valve limits usable draw. For natural gas units, if the generator starts fine but loses power or surges specifically during high-demand periods, that’s gas pressure at the meter, not the generator.
Utility Sensing Delay Issues
Standby controllers are programmed with a delay — commonly 5–10 seconds by default — before they declare utility power actually lost and start the transfer sequence. This exists to prevent the generator from starting for every brief blink or lightning-related flicker. In a real sustained outage this delay is invisible; you won’t notice a 10-second gap when the whole house has been dark. But if you’re testing by flipping your main breaker briefly to simulate an outage, a short test can end before the delay elapses, making it look like the generator “failed” to respond when it was actually just following its programmed delay correctly.
If you want to confirm real-outage performance, the valid test is leaving the main breaker off for a full 2–3 minutes, not a quick flip, and checking your specific controller’s programmed delay setting (found in the controller’s setup menu, adjustable on most Generac and Kohler standby units) if you want a faster response for your situation.