What are the different types of UPS?

Standby/offline
The Standby UPS is the most basic unit. It provides surge protection and battery backup. It usually will not monitor the battery capacity however. Therefore, it is the least reliable. It is also the least expensive.

Its surge protection capabilities are akin to that of common surge suppressors allowing it to even out spikes. The power supply backup features kick in when the utility power drops below a predetermined level. Then, an inverter will convert the DC of the battery to AC to power the equipment after an internal mechanical switchover. This can take place within a range of less than 4 milliseconds to 25 milliseconds, depending on the detection for lost power capability of the UPS. It generally has a maximum power supply backup capability of 20 minutes.

Line-interactive
The Line-interactive UPS is like the Standby UPS, but it offers voltage regulation capability. It does this with a variable-voltage autotransformer. This transformer has a multi-tap capability that the UPS utilizes automatically to allow more or less magnetic induction as needed in the primary, or input side of the transformer. This subsequently produces more or less output voltage from the secondary. This allows continuous operation during brownout (under-voltage) conditions and over-voltage surges within a certain range. When the transformer is in the process of changing the voltage output it can cause just enough of a power disruption to cause the UPS to make a slight sound from the low warning alarm as it switches power taps.

The common voltage range for autotransformers used for 120v power is between 90v and 140v. If this range is exceeded, utility power is cut out by opening the autotransformer circuits. Power is then maintained via the battery. Power supply backup ranges from 5-30 minutes with expansion possible to several hours.

Online/double-conversion
The Online UPS has a similar technology to the Standby and Line-interactive UPS units, but it has a larger charger/rectifier and a cooling system which allows the rectifier and inverter to run continuously. It also provides electrical isolation and even power maintenance for systems sensitive to power fluctuations. This UPS is more expensive, but it provides necessary features for “noisy” industrial environments, data center load requirements, or extended-run backup generator operations. Because the rectifier drives the inverter even in normal power mode, this unit is also called a Double-Conversion UPS.

DCPowerSupplies.org