The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. When city power sags, outages and other anomalies are frequent, online UPS provides advantages to sensitive IT equipment, backup generator, etc.
The basic technology of the online UPS is the same as offline or line-interactive UPS. But it costs more due to the integration of rectifier and inverter into the UPS. Basically in an online UPS, the batteries are always connected to the inverter, so no power transfer switches are necessary. The rectifier also drives the inverter. When there is a blackout, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power restores, the rectifier resumes carrying most of the load and begins to charge the batteries.
The main advantage of the online UPS is its ability to provide an electrical firewall between the incoming city power and sensitive electronic equipment. While the standby and line-interactive UPS merely filter the city power. The online UPS also provides a layer of insulation from power quality problems. It allows control of output voltage and frequency of input voltage and frequency.