UPSs and diesel generators are 2 backup systems which are kept by hosting companies inside their data centers in case there are interruptions in the primary power supply or the current is unstable and unable to maintain the proper functioning of the servers situated inside the facility. UPS is short for Uninterruptible Power Supply or Uninterruptible Power Source and it is a highly effective battery which works non-stop. It's connected to both the electrical power network and the web servers constantly, so in case the power stops, the UPS is already operational, which helps prevent the machines from going down even for a second. The diesel generator is literally an engine that can provide you with the needed power to keep the machines working for a longer period of time. In case there is an outage, the UPS gives the needed time for the diesel generator to start and then to take over until the primary power supply is restored.