Why should a SIMATIC PC not be switched on in the buffered UPS mode?
A UPS protects a computer from sudden drops in supply voltage and related loss of data. The UPS immediately recognizes the loss of voltage provision and provides the computer with voltage a few milliseconds later from the built-in buffer battery. However, a UPS cannot supply the necessary switch-on current for the computer.
You should therefore only activate your computer when you have the required voltage/power supply.
UPS (Unterruptible power supply)