There are three different topologies for Uninterruptible Power Systems; Online, line-interactive, and standby.
Online UPS is also known as a double conversion that accepts AC input and rectifies to DC and then inverts back to 120 V/208 VAC to feed the electricity for load equipment. Since Online-UPS converts the power twice on the circuit, it is called “Double Conversion” and outputs a pure-sine wave which is favorable for the load. On the main circuit board, there is a dual-directional circuit for the rechargeable battery. This rechargeable battery will back up the load equipment during a blackout. There is no power down when the power source transfers from a double-conversion circuit to the rechargeable battery. Online UPS is the most reliable UPS, and it is the reason why many manufacturers have employed SANUPS for highly critical loads in various industries.
Line-interactive UPS is similar to offline UPS in its circuits but enhanced by the adding a transformer (voltage regulator) to condition the utility power. It slightly improves efficiency and reduces battery usage. Though Line-interactive UPS is not as reliable as Online-UPS, it is still prevalent among the consumer and IT professional markets.
A Standby UPS powers the load directly by commercial power. There is no modification added to the output by the UPS. A Standby UPS only invokes when the utility power fails. In other words, Standby UPS would not protect the load from unfavorable power conditions. This type of UPS does not require as many parts as an Online-UPS, and it is often seen in consumer markets because the capacity demand is small (less than one kVA) and the price is inexpensive.