There are three general categories of UPS systems: online, line-interactive, and standby:
An on-line UPS uses a "double conversion" method of accepting AC input, converting it to DC for passing through the rechargeable battery, then inverting back to AC for powering the protected equipment. The batteries are always connected to the DC BUS of the ups, so that no power transfer switches is necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged.
A line-interactive UPS maintains the inverter in line and redirects the battery's DC current path from the normal charging mode to supplying current when power is lost. The line-interactive UPS is similar in operation to a standby UPS, but with the addition of a multi-tap variable-voltage transformer.
In a standby ("off-line") system the load is powered directly by the input power and the backup power circuitry is only invoked when the utility power fails. The offline/standby UPS offers only the most basic features, providing rudimentary surge protection and battery backup.