What is a Standby or Off-line UPS?

A standby UPS is a Voltage and Frequency Dependent (VFD) uninterruptible power supply as defined by BS EN 62040. The output from the UPS is dependent upon the input voltage and frequency. There is usually no AVR/AVS to step the mains power supply voltage up or down and little spike/electrical noise filtering. The inverter if off-line (standby mode) and switches on to protect the load with battery power when the mains power supply fails.