An uninterruptible power supply (UPS) usually performs two prime roles in the electrical grid. From one end, it is constantly feeding the electric power without any interruption while on the other side, it prevents the electrical devices from numerous variation in AC power. 

Differences between the online and offline UPS system

Online UPS:

• The online UPS generates power with the aid of an inverter and rectifier combination. At the same time, it includes both charging and backup control such that the backup can be charged for power supply in case of power loss. 
• In comparison to offline UPS, heat dissipation is much more because it is online continuously. So, it will require large heat sink as the online UPS comprises of rectifier that gives direct power to both inverter and the battery. 
• Online UPS is protected from external distortions such as from surges, spikes and power failure. 
• Online ups are less effective as they are always in an online state.
• They are complex in design and costly compared to offline UPS. 
• Their applications are mostly in intensive care units, medical and telecommunication equipment. 

Offline UPS: 

• The Offline UPS supplies AC power directly to the load by switching the transfer switch on. However, in the event of a power outage, the offline UPS provides the battery backup power.
• In contrast to online UPS, the heat dissipation is very low as offline ups are turned on just in case of power loss. 
• The offline UPS is not generally immune to the external distortion. 
• The offline UPS is more efficient as it is only in the online state during power failure and thus has a high output.
• They are simpler in design and are less costly than online UPS.
• They are mostly found in computers, printers, scanners.