UPSs and diesel generators are 2 backup systems which are kept by hosting service providers inside their data centers in case there are interruptions in the primary power supply or the current is unstable and unable to maintain the appropriate running of the machines located in the facility. UPS is short for Uninterruptible Power Supply or Uninterruptible Power Source and it's a powerful battery which works non-stop. It is connected to both the power network and the web servers all of the time, so if the power stops, the UPS is already functioning, which helps prevent the servers from going down even for a second. The diesel generator is certainly an engine which can provide you with the required electric power to keep the machines operational for a longer period of time. In case of an outage, the UPS gives the necessary time for the diesel generator to start and then to take over until the primary source is restored.