The answer to this question very much depends on the following main factors:
1. How critical the load backed by the online UPS is
2. How sensitive your equipment is to power cuts
3. How important lower running (and in particular energy) costs are
When looking for online UPS, you will see headline figures of up to 99% efficient in what is often called ECO mode, which generally involves running the load directly from the mains (via a static switch internal bypass) with the inverter switched off. As soon as a power cut is detected by the online UPS, the unit then switches to true Online operation (through the inverter) in a matter of milliseconds. Going back to the three factors above, if your equipment has enough lag in its power drag to not be interrupted by a 0.2 second power cut, then ECO mode would be fine. If however this is not quick enough, then it would be recommended that you remain within the less efficient Online mode.
The only other thing to mention is that modern online UPS also often have a smart ECO mode where the online UPS will automatically decide which mode to operate in based on the stability of your mains. It will try and operate in the most efficient mode wherever possible but if it starts to see that your mains is slightly unstable, it will switch over to Online mode. Different online UPS manufacturers even have self learning algorithms that constantly improve their decision making based on the site.
Post time: Sep-29-2017