UPS Power design

It is a bit more complex than that, but yeah, your math is basically correct. One thing you have forgotten however is charging current. Assume you have just had an extended outage. The UPS batteries are now depleted with just a few % left in them.  When the power is restored, you might have servers using 3KVA, however another .5KVA or so will be used to recharge the batteries, for the next couple of hours or so.

Larger UPS are generally hardwired.

"Larger UPS are generally hardwired. " do you mean that the UPS power is connected directly to Main supply unit, rather than plugging into the wall socket ?

Any more information about this design ?

Thx

not sure how to understand " What should I construct the power supply if I need to support large power"

if you need to support a load of say 5 KVA, you need to calculate the down time it must support the load : 5KVAx 2 hour =  10 KW support batteries

you can use the calculator here :  http://www.apc.com/template/country_selection.cfm?ref_url=/tools/ups_selector/by_device.cfm?args=US/en/server

I mean for a 5Kva UPS,  I can't plug it directly to a wall power socket, correct ?

Thx

why not?  i don't see any reason

Worst case scenario, a 5000VA UPS will draw close to 30amps. 30AMP wall sockets exist, so you could use one of those.

Below is the USA version.

http://www.stayonline.com/detail.aspx?id=4790

In HK, we are using 220V. Do I need to check maxium current that the wall socket can supply ? Where can I check ?

Thx