Online Double Conversion UPS Systems are considered to provide the best power protection available. Most modern systems provide a sine-wave inverter which is always on – hence ‘online’. As a result the output is independent of the input giving the classification VFI: Voltage and Frequency are Independent from the input voltage. However, actually technically this may not be true as in many cases the output frequency will track the input frequency (in case a revert to bypass is required).
In normal operation, the AC power feeds the rectifier / charger which maintains a float charge on the batteries and converts the AC power to a DC power “bus” which feeds the inverter circuit, which converts DC to a stable AC power waveform. Hence the term “double conversion”.
Should the mains fail – or go out of tolerance – too high that exceeds design parameters or too low that current input is unsustainable then the unit will revert to battery power. At which point the input mains is disconnected and the DC bus will be maintained from the batteries. This is seamless and the inverter is operating all the time, meaining this is a true “no break” technology.
The drawback of online technology is the high fan noise (some systems have automatic fan speed reduction at low loads) and lower efficiency than other technologies (usually in the order of ~90%). As a consequence many manufacuturers have incorporated “ECO Mode”, however if you want to operate in ECO mode a sine wave line interactive UPS System would be a better option.