An online UPS system might be considered the top-end option for most usage scenarios: it’s typically the most costly type of UPS power supply and offers the greatest security against unexpected current fluctuation or sudden power loss.
The term ‘online UPS’ has nothing to do with internet connectivity as you might expect; rather, a fully online UPS power supply (often called a ‘true UPS’) refers to one that is constantly filtering, storing and delivering electrical flow to your PC, even when all conditions are normal.
In short, an online UPS is always performing the task it was designed to perform, rather than only jumping to action when needed - which of course means that, in the event of sudden power loss, there is genuinely zero interruption as far as the computer is concerned, and the supply of clean, stable power continues as if nothing happened.
While this offers by far the most seamless and safest (from a hardware/data perspective) option in terms of protection for computers and their components, it also means that power draw and running costs are considerably higher for this type of UPS unit - not to mention the significantly increased cost of the technology itself.
Naturally, this can be of considerable value in certain critical roles and environments, where any performance or data loss would be catastrophic, but for most home users it might be considered overkill. That said, while online UPS systems were once extremely costly investments for high-end business networks only, in recent years they have become a lot more affordable for enthusiast home users keen to buy into the most comprehensive protection for their own systems and files.
In contrast to the version outlined above, an offline or standby UPS does exactly what you’d imagine from the name: rather than constantly cycling, storing and converting power through to a computer or server, it ‘idles’ until absolutely needed.
In practice, whenever the standby UPS detects that electricity flow from the wall socket is registering as normal, it effectively allows itself to be bypassed. The instant it detects a problem - which is usually within 5 milliseconds for most quality models and brands - the offline UPS power supply switches over to its internal battery backup instead.
Although that 5 milliseconds usually puts it well within tolerance range for most home-use, consumer-grade electronics, it does nevertheless count as a very slight ‘interruption’ - which is why offline models aren’t technically considered ‘true’ UPS devices by professional users. However, they tend to be a perfectly functional (and much more cost-effective) stand-in for most domestic desktop PC owners, and are only really seen as unsuitable in very critical applications where any risk of data or hardware loss is unacceptable.
Based on each of the above definitions, it won’t surprise you to learn that hybrid UPS systems - which often include so-called ‘line-interactive’ UPS units - offer something of a middle-ground solution and an effective balance between cost and performance.
Whereas an online UPS constantly cycles power through its battery, and a standby UPS system switches over to battery power only if and when it detects a problem, a hybrid or line-interactive UPS power supply offers various operating modes. These generally include double conversion mode, economy mode, and active filter mode - and the hybrid UPS is able to detect which is most appropriate at any given moment. This approach is achieved through a variable power transformer system and means that total power draw over any typical period of extended use is much lower than for a fully online equivalent.
Moreover, during ‘brownout’ scenarios - which are much more common than full blackouts, and involve potentially damaging fluctuations in current rather than a total loss of power - the line-interactive UPS is far more effective at monitoring, managing and adjusting to flow variations than an offline version.
The hybrid model will not continually have to switch between supplying wholly mains or battery power, meaning the long-term health of the battery is greatly increased and further strengthening the overall value proposition of a line-interactive UPS.