UPSs and diesel generators are 2 backup systems that are kept by web hosting providers in their data centers in the event that there are interruptions in the primary power supply or the current is unstable and unable to support the appropriate operation of the web servers based inside the facility. UPS stands for Uninterruptible Power Supply or Uninterruptible Power Source and it is a powerful battery which works non-stop. It's connected to both the electric power network and the servers at all times, so in case the power stops, the UPS is already working, which helps prevent the web servers from going down even for a second. The diesel generator is certainly an engine that can deliver the required energy to keep the machines working for a longer period of time. In the event of an outage, the UPS gives the needed time for the diesel generator to start and then to take over until the main source is restored.