Uninterruptible Power Supply Systems: There are three distinct types of uninterrupted power supplies, namely, (i) on-line UPS (ii) off-line UPS, and (iii) electronic generators. A UPS differs from an auxiliary or emergency power system or standby generator in that it will provide near-instantaneous protection from input power interruptions, by supplying energy stored in batteries, supercapacitors, or flywheels. Understanding these distinctions is crucial for selecting the optimal uninterruptible power supply that matches your equipment's. A Uninterruptible Power Supply (UPS) generally consists of a rectifier, battery charger, a battery bank and inverter circuit which converts the commercial ac input into dc suitable for input to the battery bank and the inverter. Energy Storage: UPS systems use batteries, flywheels, or supercapacitors to store energy for use during power interruptions. For ordinary household appliances such as incandencent lamps, tubes, fans, TV and fridge, the mains ac supply does not make much difference, but when used for computers, medical equipments and telecommunica¬tion systems, a clean, stable interruption free power. Single phase and three phase UPS systems are essential for ensuring uninterrupted power supply in businesses and organizations.