What is the purpose of a backup generator?

A backup or standby generator provides electrical power in the event of a power outage. Within seconds of the power loss the generator starts and the electrical load is transferred to the generator. When combined with an Uninterruptible Power Supply (UPS), it can provide complete continuity of power with zero downtime. Once power returns to the mains supply the electrical load transfers back and the generator reverts back to standby until the next outage. Most generators run on diesel, natural gas or liquid propane gas.