A UPS performs three primary functions:
First, a line-interactive or double-conversion UPS conditions the incoming dirty power from the utility company. It filters out the irregularities and gives you clean, uninterruptible power.
Second, it provides ride-through power to cover for sags or short-term outages (3 minutes to several hours, depending on what you need).
Finally, it enables seamless system shutdown during a complete power outage.