Line-Interactive & Offline UPS

It suits desktops, networking, CCTV, billing and similar office loads where a few milliseconds of switching is acceptable and mains voltage varies rather than being badly distorted. It is not the best choice for servers, medical devices or sensitive controllers that need zero transfer time and isolation from mains disturbances. For those, choose an online UPS.

The automatic voltage regulator corrects low or high mains voltage by switching transformer taps, so the output stays near nominal without using the battery. That reduces battery wear and keeps equipment running through sags. AVR has a limited correction window, beyond which the UPS switches to battery.

It is the time the UPS takes to switch to battery when mains fails, typically a few milliseconds. Most computer power supplies hold up long enough to ride through that gap. The exact figure is on the UPS datasheet, and equipment that cannot tolerate any gap requires an online UPS.

Add the actual wattage of the computer, monitor, router and other connected devices, divide by 0.6 to 0.8 depending on the UPS power factor rating, and select the next standard VA above that. Leave margin for future additions. The UPS should be loaded at 80 percent or less of its watt rating.

Generally no. Laser printers draw large short peaks when the fuser heats, which can overload a small UPS. If a printer must be backed up, size the UPS for its peak consumption or connect it to a surge-protected outlet without battery backup, as the manufacturer recommends.