In a UPS uninterruptible power supply, we have a DC type battery which gets charged every now and then. When power goes off, the UPS DC source supplies the necessary power for uninterrupted service.  However, it can last only as long as the charge exists in the battery. Hence, when in UPS, we normally switch off unnecessary lights so that the battery can last longer.

Again when the power comes, it auomatically gets charged and it is ready for the next power backup. Thus, the batteries in UPS gets charged when power is present and discharges when it need to backup for a power failure.