UPSs and diesel generators are two backup systems which are kept by hosting companies in their data centers in case there are interruptions in the main power supply or the current is shaky and unable to maintain the proper running of the web servers situated inside the facility. UPS is short for Uninterruptible Power Supply or Uninterruptible Power Source and it is an effective battery which works non-stop. It's connected to both the electrical power network and the servers at all times, so anytime the power stops, the UPS is already operational, which prevents the servers from going down even for a second. The diesel generator is an engine that can give you the needed energy to keep the machines operational for a longer period of time. In the event of an outage, the UPS gives the necessary time for the diesel generator to start and then to take over until the main power supply is restored.