Editing Uninterruptible power supply (section)

===Online/double-conversion===

In an online UPS, the batteries are always connected to the inverter, so that no power transfer switches are necessary. When power loss occurs, the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. When power is restored, the rectifier resumes carrying most of the load and begins charging the batteries, though the charging current may be limited to prevent the high-power rectifier from damaging the batteries. The main advantage of an online UPS is its ability to provide an "electrical firewall" between the incoming utility power and sensitive electronic equipment.

The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations.<ref>{{Cite web|url=https://powercontrol.co.uk/blog/choosing-ups-system-for-business/|title=Choosing the Right UPS System for Your Business - Power Control Ltd - Uninterruptible Power Supply Solutions, UPS Maintenance & UPS Installation|access-date=2021-01-12|archive-date=2021-01-14|archive-url=https://web.archive.org/web/20210114004421/https://powercontrol.co.uk/blog/choosing-ups-system-for-business/|url-status=dead}}</ref> Although it was at one time reserved for very large installations of 10&nbsp;kW or more, advances in technology have now permitted it to be available as a common consumer device, supplying 500&nbsp;W or less. The online UPS may be necessary when the power environment is "noisy", when utility power sags, outages and other anomalies are frequent, when protection of sensitive IT equipment loads is required, or when operation from an extended-run backup generator is necessary.

The basic technology of the online UPS is the same as in a standby or line-interactive UPS. However, it typically costs much more, due to it having a much greater current AC-to-DC battery-charger/rectifier, and with the [[rectifier]] and inverter designed to run continuously with improved cooling systems. It is called a ''double-conversion'' UPS due to the rectifier directly driving the inverter, even when powered from normal AC current.

Online UPS typically has a static transfer switch (STS) for increasing reliability.