What is the difference between an Online and Offline / Standby UPS?
There are two main types of Uninterruptible Power Supplies – Online and Offline / Standby. The terms Online and Offline / Standby refer to the general state of the UPS’s internal inverter under normal mains conditions.
Offline UPS’s are the simplest and least expensive. When mains is present, it is routed straight through the UPS bypassing the inverter to the output. A degree of filtering is providing ensuring limited protection against spikes and other irregularities. The batteries are charged the whole time, but only in the event of a power failure will the load be fed by the inverter. The switching from mains to inverter supply ( generally referred to as Transfer Time ) usually takes a few milliseconds. However, a modern computer’s or office system’s internal power supply has sufficient power stored within it to survive this brief interruption of power – ensuring uninterrupted operation of the load equipment.
Lower Cost Offline UPS generally, when on inverter / battery backup, provide a square / pseudo sine wave output waveform. Given the nature in which switch mode power supplies used in modern computer / office systems draw power this type of output power waveform is usually more than adequate. In situations where a square / pseudo sine output waveform is not acceptable then true sine wave more expensive Offline or Online UPS are recommended.
Online UPS’s take the incoming AC mains supply and convert it to DC which feeds the battery and the load via the inverter. If the mains supply fails, then the batteries feed the load via the inverter with no interruption to the output supply at all. An Online system, by nature of the dual conversion design (AC-DC / DC- AC), ensures a far higher degree of isolation of the load from the irregularities on the mains supply. In general, Plug N Play Online systems are more expensive than similar Offline solutions because the inverter has to be rated for continuous operation.