What many people do not realize is that the power provided to their home or office is at times irregular and not necessarily continuous 120volts supply. The ideal voltage supply is 120v. However, even in residential areas, this can often be over 160volts or under 100volts. It can get even worse in industrial areas if you live near or work in these zones due to machinery being used and the drop in power that is drawing from these zones.
A basic standby UPS can sense the voltage is not good enough or within tolerance, and will automatically go to battery power until normal power returns.
Other units can adjust the voltage for the connected equipment to keep it at safe levels. High-end UPS models can give a perfectly stable output of 110v at all times regardless of input power.
Other common power problem can be surges and spikes. This is where the incoming voltage jumps rapidly. A UPS can help fix most power problems.