An uninterruptible power supply (UPS) stores energy in batteries or super-capacitors and powers the load. Their main application is as a backup or emergency power supply, when the mains power fails. Unlike a standby generator, the UPS provides near-instantaneous protection, without interruption of power to the load. UPSs come in many types and capacities. Offline or standby UPSs, powered by batteries, protect computers and other hardware, and typically offer energy for less than an hour. Line interactive UPSs, with much higher capacity, are mainly for high-end and industrial use and offer comprehensive protection, including protection from power failures, regulation of voltage, harmonics, and power factor correction. When purchasing a UPS, pay close attention to the specs, such as capacity, input and output voltage, buffer time, and more. The best UPS depends on the application. For instance, a basic single phase, entry-level UPS will suffice for home or office PC, but providing backup to a data centre would require a three-phase UPS with 10-40 kVA capacity, or more. UPSs for industrial use may require anywhere between 10 kVA to 3 MVA capacity and the option to regulate input voltage range. Also consider the safety features available in the UPS, such as intelligent battery protection to prevent overcharging, the range of exhaust discharge protection, and more.