Many standby generator installations have transfer switches, which regulate the directional flow of power. These set-ups are designed to switch the power source from the main grid to back-up generator in case of an emergency. Then, when power is restored to the grid, the power source can be transferred back to the main grid.
Some transfer switches are manual, meaning someone has to physically go and switch the power source from the main grid to the emergency diesel generators in order for the back-up power to start. As you can imagine, this can be very frustrating if it is located in a dark place and dangerous if this means a person needs to go outside during a storm or other weather situation.
A better option is to make sure that your generator has an automatic transfer switch. This will immediately switch the power from the main grid to emergency power in case of a power outage, and then back to the main grid once power is restored. Not only is this safe and easy for the homeowner, but it also eliminates potentially dangerous situations for utility workers.