How does a UPS work?

[packman 0]

Because of recent electricity supply outages, I'm thinking of getting an uninterruptible power supply on which to run my PC and monitor.

 

I understand in principle the characteristics of UPSs but one thing that I'm unsure of is the following. When a UPS is in use and the system is idling, ie. normal conditions, is the UPS supplying the power to the load (in my case, the PC and the monitor), or is some sort of bypass supplying the power?

 

I ask because it seems to me you could design a UPS to work in two different ways:

 

1. The UPS permanently takes over the supply of power to the load, being permanently charged from the supply side. When an outage occurs, the UPS merely continues to supply power to the load, for as long as it's capable.

 

2. The UPS is constantly on charge but a bypass around the UPS allows the normal electricity supply to directly power the load. When an outage occurs, the UPS cuts in and supplies the necessary power, again for as long as it can.

 

Which of these two scenarios do UPSs normally implement?

[GTwannabe 0]

The wall feeds the UPS power, and the UPS feeds your devices. A UPS draws ~15-20% more power from the wall than it supplies to the attached devices.

 

If the UPS has non-battery backup ports (for high-draw devices like laser printers), those ports only get surge protection and/or voltage regulation.