When they detect a power failure or an important fluctuation in the input voltage they use the batteries to generate the AC supply. This means that the devices connected to the UPS have no power supply for a very brief period of time (4-10 ms). Nevertheless, most electronic devices, such as computers, are not affected by this as they have a small amount of accumulated energy which allows them to withstand this period without switching off.

Square waves are not is not the signal which is normally generated by the power companies. For this reason some devices will not function correctly with this type of wave (motors, measuring equipment, etc.).