UPS is Uninterruptible Power Supply, this is also known as power backup with battery. UPS usually will automatically backup power source for computer when the home or office power suddenly drop or turn off. With UPS even the main power source down, this unit will always give power to computer. UPS have a limit time to give backup power, this is depend on how big the UPS capacity.
How is UPS work?
With simple explanation inside UPS have battery with recharge module, it also have module to determine whether main power source on or off, so when main power source on, the recharge module will charge the battery til full. When main power source suddenly off, then the module will switch on the battery to changed into backup power source.
Offline vs Online UPS
What is Offline UPS? This UPS type have module to switch between using battery or using bypass main power source. When main power source is on, then module will bypass main power source become the output off UPS, but when main power source is off, then the module will switch from bypass become through the battery which is change into AC power source output. Because this type of UPS have to change between bypass or battery, then it have little time of delay, about less then 4ms delay time.
What is Online UPS? This UPS type is better than Offline, because no matter main power source is on or off, this Online UPS output already come from battery which is already change into AC power source output. That's why this type off UPS does not have delay time when main power source is off or on. I recommended this Online UPS to backup server power source.
At the market, Online UPS usually already have stabilizer module for make output power source become pure sinus wave which is really good for every gadgets. As far as I know, the price for Online UPS is more expensive then Offline UPS. So, like what I said before, I use Online UPS to backup server computer power source, and Offline UPS to backup workstations computer power source.