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 source or the current is unstable and unable to support the appropriate running of the web servers situated in the facility. UPS stands for Uninterruptible Power Supply or Uninterruptible Power Source and it's an effective battery that works non-stop. It's connected to both the power network and the web servers constantly, so when the power stops, the UPS is already up and running, which prevents the web servers from going down even for a second. The diesel generator is certainly an engine which can supply the required electrical power to keep the web servers working for a longer period of time. In case of an outage, the UPS gives the required time for the diesel generator to start and then to take over until the primary power supply is restored.