A variety of systems—offline, online and line-interactive—are available, based on the degree of protection and power conditioning you need.
Offline. This is the simplest type of UPS and is typically used for a single computer or other application where a minimum level of protection is required. The UPS device charges under normal operating conditions and then switches over to provide power during an outage.
Line-interactive. High-quality line-interactive units have built-in surge and noise suppression. However, they will only supply AC output with power from the battery upon a complete outage or when the AC line voltage falls outside a wide range. So, line-interactive systems can maintain power briefly during an outage or power quality disturbance, but online systems provide a higher degree of protection. In some cases, a constant voltage transformer is inserted between the power supply and the equipment being protected to correct minor voltage sag.
Online. These systems supply power continuously by combining a double power conversion and an inverter. The double conversion involves rectifying AC to DC to charge the battery and inverting DC to AC to supply the load. Online units have no switchover time, and their power conditioning offers the best protection against power quality problems. They are the best choice for applications with mission-critical equipment, such as servers and routers. Disadvantages include higher costs, more heat generation and increased energy use.
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