UPSs and diesel generators are 2 backup systems which are kept by website hosting companies inside their data centers in the event that there are interruptions in the primary power supply or the current is shaky and unable to sustain the proper running of the servers located within the facility. UPS stands for Uninterruptible Power Supply or Uninterruptible Power Source and it's an efficient battery which works non-stop. It's connected to both the electric power network and the web servers constantly, so in case the power stops, the UPS is already operational, which prevents the machines from going down even for a second. The diesel generator is an engine that can supply the necessary electrical power to keep the machines working for a longer period of time. If there is an outage, the UPS provides the required time for the diesel generator to start and then to take over until the main power supply is restored.