UPS stands for Uninterrupted Power Supply. The name itself justifies the operation and its usefulness. For any converter system connected to main power line/grid there are two possible mode of operation. One is “online” and other is “offline”. Online term refers to the condition when converter is in operation when power in the line is present. Offline mode refers to a condition when converter operation start after the main line is off or not available.
Therefore, online UPS is a stage which is introduced between the load and mains. As this system operate when main power source is present, it keeps the load voltage free from voltage distortion. Additionally, protects sensitive loads from voltage fluctuation. If in capacity this UPS system provides suirable reactive power support to maintain required reactive power at point of common coupling.
Unlike online UPS, offline system works when main line is not present. For example we can take an example of our household inverter system. When power is not present from main line, UPS enables conversion of battery energy into electrical energy for household items to run appropriately.
Hope this helps you in understanding difference between online and offline UPS.