Back-up (stand-by) Generators can be set-up to go on as soon as an electric outage occurs, giving you uninterrupted electrical service. There are several ways which your system can make the transfer ranging from automatically making the transfer providing you with uninterrupted service to manually making the transfer where there would be an interruption of service. How you choose to make the transfer depends on you and your needs. 

Using only the Generator as a Grid Stand-by, to come on if your Utility Power fails. You would want to use a Transfer Switch or an Automatic Generator Start Relay (AGSR) which will allow you to have electric when there is a problem with your grid source.  When the switch detects an outage it turns on the generator to provide you with electric and than shuts off the generator once your utility electric is restored.  This method gives you basically uninterrupted service, meaning that once your electric source stops (no electric) the switch has to turn on the generator, the generator in turn has to stabilize before it can send electric to you.  Meaning that you could be without electric for up to a minute or two.  If you have equipment which need to have no interruption than you may want to consider using an inverter in place of the transfer switch.

Using the Generator as a System Back-up, to come on when your system falls below a certain voltage and or when there is no electric from the Utility. In this instance, your system would include a battery back-up and your system transfer would be done by either an Inverter or an Automatic Generator Start Relay.  Here, your main source of electric is the utility company.  When you loose electric from the grid, your system switches to the Inverter which uses the batteries to make your 120 VAC electric.  Once your battery voltage drops below a certain voltage, the Inverter or an AGSR starts the generator.  Once the batteries reach a certain voltage level or the grid is back on line, the generator is shut off until it may be required again. With this set-up, you could incorporate Solar, Wind and or HydroElectric as your main source of electric, keeping your batteries charged and using the inverter to supply electric to the house.

Using the Generator as a Back-up for your Solar, Wind and or HydroElectric System.  This example can be used with or without a Utility connection.  Your main source of electric comes from your own power plant, whether it be solar, wind or hydro.  When your system has depleted your batteries to a certain level, your Inverter or AGSR turns on the generator and once the batteries are filled to a certain level, the generator is shut down.  Some inverters will see if the Utility is available before it starts the generator.

When it comes to back-up / stand-by generators we prefer the stationary generators as oppose to the portable generators for a number of reasons.  Mainly because they interface with both solar systems and with utility service and they can be started automatically as mentioned above.  However, sometimes there are other considerations one must take into account and for this reason, I have attached an article by Bill Hoffman so that you can make up your own mind.  Please click here to read his article.