The offline UPS System is the smallest, generally cheapest and most efficient UPS System of the 3 main UPS technologies.
In normal operation, the AC power is passed straight through to the load – so what comes in goes out. This is why it is designated a VFD UPS System as the output Voltage and Frequency are Dependent on the input voltage. Some of the AC power is taken and passed through to a Rectifier / Charger which converts AC to DC and uses this to keep a float charge on the battery.
Should the mains fail – or go out of tolerance – i.e. becomes either too high or too low, the UPS switches to battery operation. DC power is then delivered to a circuit called an inverter, which converts the DC into an AC power signal, which will provide power to the load. Note the output waveform is no longer sinusiodal, as in practically all offline UPS systems the inverter is a “square wave” type. The exceptions are inverter chargers which are technically offline UPS, designed for use with an external battery for long runtime applications. These products are generally sine wave.