UPSs and diesel generators are two backup systems which are kept by hosting providers inside their data centers in case there are interruptions in the main power supply or the current is unstable and unable to sustain the appropriate functioning of the machines located in the facility. UPS refers to Uninterruptible Power Supply or Uninterruptible Power Source and it's a highly effective battery that works non-stop. It is connected to both the power network and the servers all of the time, so if the power stops, the UPS is already functioning, which prevents the servers from going down even for a second. The diesel generator is certainly an engine that can supply the necessary energy to keep the web servers working for a longer period of time. In the case of an outage, the UPS gives the necessary time for the diesel generator to start and then to take over until the primary power supply is restored.