An uninterruptible power supply or UPS provides temporary backup power in case the main power source goes out. The market currently has three main types of UPS systems. These are the Standby UPS system, the Line Interactive UPS system and the Double Conversion Online UPS system.
Standby UPS System
The Standby UPS system is commonly used for home personal computers. It is the most basic UPS system and is cheaper compared to the other types of UPS systems. It is also normally referred to as an offline UPS. It is made up of a battery, static long switch, low pass filter, and an AC/DC and DC/AC inverter. This system would automatically shift to battery power once the main power source is interrupted.
The Line Interactive UPS system is mainly used for small businesses, data centers and servers. The battery and inverter of these systems are continuously connected to its output. These systems feature a voltage regulator that automatically adjusts the voltage of the main power source in low voltage conditions. This is important since UPS systems normally switch to battery mode during low voltage conditions. The Line Interactive UPS system is considered as the most efficient UPS system in the market today.
Double Conversion Online UPS System
Double Conversion Online UPS systems are normally used whenever higher voltage is required. While the design of these systems is similar to a Standby UPS system, the primary power source passes through the inverter before it goes into the devices connected to the system. In this design, there would be no power interruption in case the main power source goes out since it only serves as a backup source. But, it should be noted that since the whole power load is converted by both the inverter and the battery charger, efficiency may be reduced and more heat may be generated.
References:
http://www.apc.com/us/en/faqs/FA157448/
http://www.webopedia.com/TERM/U/UPS.html
http://powershield.com.au/what-types-of-ups-uninterruptible-power-supply-are-there/
http://www.howtogeek.com/161479/how-to-select-a-battery-backup-for-your-computer/