Key takeaways
- In a power outage, the automatic transfer switch senses the loss of utility power and signals the generator to start.
- Once the generator is running at the right voltage and frequency, the switch moves the house to it. The lights are usually back within seconds.
- When utility power returns and holds steady, the switch moves the house back and the generator runs on its own for a cool down before it stops.
- The whole sequence depends on the transfer switch, the battery and the fuel supply being ready, which is what maintenance is for.
A power outage with a standby generator in place goes very differently from one without. The lights go out, and then, before most people have found a flashlight, they come back on. Nobody goes outside, nobody pulls a cord, nobody flips a switch. Behind that moment is a short sequence the system runs on its own, and knowing what happens in each step makes it easier to understand what the equipment needs from you between outages.
We sell and install Generac home standby and commercial generators, and the sequence below is how an automatic standby system works in general. For how we put one in, see our Generac generator installation page.
Step 1: the utility fails
The outage starts on the utility side: a storm takes down a line, a transformer fails, or the grid sheds load during a heat wave or a freeze. In most of the San Antonio area that utility is CPS Energy. Whatever the cause, the voltage at your service drops to nothing or falls well below normal.
Inside the house, the lights go out and anything without a battery stops. This is the only part of the outage you will notice for more than a moment.
Step 2: the transfer switch senses it
The automatic transfer switch sits between the utility service and your panel, and it watches the utility voltage all the time. When the voltage drops below the level it is set for, the switch waits a short moment to make sure the loss is real and not a flicker that clears on its own. Utility power often blinks during a storm and comes right back, and starting a generator for every blink would wear it out for nothing.
Once the switch is satisfied the outage is real, it signals the generator to start.
Step 3: the engine starts
The generator's engine starts on its own battery, the same way a car starts. Fuel comes from the natural gas line or the propane tank. The engine runs up to speed, and the generator's controller waits until the output voltage and frequency are stable before it reports that it is ready to take the load.
This step is where a neglected generator fails. A weak battery, old oil, a clogged air filter or a fuel valve that was closed and never reopened can all keep the engine from starting or from coming up to speed. That is why the battery and the fuel supply matter as much as the engine.
Step 4: the switch transfers the house
When the generator is ready, the transfer switch disconnects the house from the utility and connects it to the generator. That disconnection from the utility is the most important thing the switch does. It keeps the generator's power from flowing back out onto the utility lines, where it could injure a line worker who believes the line is dead, and it keeps the utility from feeding into the generator when power returns.
In the house, the lights come back on. From the moment the power went out, the whole sequence usually takes a matter of seconds.
The switch is the heart of this sequence, and there are several kinds. We compare them in our guide to generator transfer switch types, and our transfer switch installation page covers how one is put in.
Step 5: the loads run on the generator
Now the house runs on the generator. What runs depends on how the system was set up. With whole house coverage, every circuit has power, and load management may hold off a large load such as a second air conditioner or a water heater until there is room for it. With essential circuit coverage, only the circuits on the generator panel have power and the rest stays dark.
- Air conditioners may start a little after the lights, because many systems wait before restarting a compressor.
- Load management modules may cycle large appliances so they take turns.
- Clocks and some electronics may need to be reset, because they lost power for a few seconds.
- The generator will be audible outside while it runs.
How much of the house runs is decided at sizing time, which is covered in our guide to Generac generator sizing for Texas homes.
Step 6: the utility comes back
While the house runs on the generator, the transfer switch keeps watching the utility side. When the utility voltage returns, the switch does not move the house back right away. It waits until the utility power has been steady for a while, because restored power can drop again as crews work on the lines.
Step 7: retransfer and cool down
Once the utility is steady, the switch moves the house back to utility power. Inside, you may see a brief blink as the transfer happens. The generator then keeps running for a few minutes with no load on it, to cool down, before it shuts itself off. After that it returns to standby, ready for the next outage, and it will run its regular exercise cycle on the schedule it was set for.
What you will notice in the house
From inside, a power outage with a standby generator feels like a short interruption rather than an event. The lights go out, the house is quiet for a moment, and then the lights return and the refrigerator starts humming again. If you are watching closely, you may notice the air conditioner take a little longer to come back than the lights, or a second system wait its turn. When the utility comes back, there may be another brief blink as the switch moves the house back.
A few things do need attention. Clocks on ovens and microwaves may flash. Computers without a battery backup will restart, so anything unsaved is lost in the first blink. Garage door openers, gate operators and some smart home devices may need a moment to reconnect. None of that is a fault in the generator; it is the few seconds between the outage and the transfer.
When the sequence does not finish
Sometimes the lights stay out. The most common reasons are simple ones: the generator was left in the off position after service, the battery was too weak to start the engine, the fuel valve was closed, or the generator's own breaker was off. Many units show a status light or a code on the controller that says why they did not start, and some report it to an app.
- Check that the generator is set to automatic, not off or manual.
- Look at the controller for a status light or fault message.
- Confirm the fuel valve is open and, on propane, that the tank has fuel.
- Do not open the unit or the transfer switch. Both carry live power, and the switch can be energized from the generator side.
If the cause is not one of those, the system needs service, and the time to find that out is before the next storm rather than during it.
What keeps the sequence working
Every step above depends on equipment that sits unused most of the year. The battery has to hold a charge, the oil and filters have to be in good condition, the fuel valve has to be open, and the transfer switch has to be clean and working. The regular exercise run tells you the engine starts, but it is not a full test under load.
Our generator service and repair page covers what that service involves, and the Generac generator maintenance schedule post lists what to check and when. If you do not have a standby generator yet, start with our generator installation service or book the Generac generator sizing visit.