What is the primary difference between double conversion and delta conversion UPS systems?

Double conversion UPSs require a 1.3X generator oversizing. Delta conversion UPSs require a 3X generator oversizing.

Double conversion UPSs are practical in size ranges from 0 - 20 kW. Delta conversion UPSs are practical in size ranges of 20 kW to 1 MW.

In a double conversion UPS, 100% of the power is converted from AC to DC and then again from DC to AC. In a delta conversion UPS, only the difference between input and output power is converted.

In a double conversion UPS, the transformer has a special ferro-resonant capability, which provides limited voltage regulation and output waveform shaping. In a delta conversion UPS, a standard transformer is used.