A UPS for my new iMac 21.5"

As I wrote earlier you need no additional software for APC UPSs. Plug it in and it works. This probably holds true for other UPS manufacturers as well.

OS X already includes software to monitor the UPS's charge state, will inform you that it is running on backup power, and will shut down your Mac according to options you specify:

APC's Customer Care Team's answer is correct in a manner not unlike the following story:

A man flying in a hot air balloon realizes he is lost. He reduces his altitude and spots a man in a field down below. He lowers the balloon further and shouts, "Excuse me, can you tell me where I am?"

The man below says, "Yes, you are in a hot air balloon, about 30 feet above this field."

"You must be an engineer," says the balloonist.

"Yes, but how did you know?"

"Everything you told me is technically correct, but it's of no use to anyone."

The man below says, "You must be in management."

"Yes, but how did you know?"

"You don't know where you are or where you're going, but you got there due to poor planning and a lot of hot air. You are in the same position you were before we met, but somehow that's my fault, and now you expect me to be able to help."

Yes. Any one of these except for the cheapest: http://www.apc.com/products/family/index.cfm?id=29

If it includes software on a CD throw it away; you don't need it.

If I'll connect a UPS , The Energy Saver in System Preferences will change ?

Yes. Upon connecting the USB cable, two tabs and an additional "Show UPS status in the menu bar" option will appear as in the following:

Without a USB cable the Mac has no way of determining a UPS is connected.

John Thank You Very Much for the mid day Laugh. And for the information.

Does Mac power requirements a pure sinvewave UPS ? Or step approximated sine wave ?

A pure sine wave UPS is unnecessary and a waste of money for any equipment that uses a switching power supply such as recent iMacs. The "step approximated" variety are OK.

If you need to power audio equipment or motors, definitely. Anything with a linear power supply, probably. Otherwise, no.

🙂

Just as I said each port is USB 3 but if you plug a USB 2 device in first without plugging a USB 3 in before that the port will be set to USB 2 speeds, only that port though no the others. It makes no difference with a UPS though and since you will leave it plugged in don't worry.

I would say yes, I have one connected and if you search on here you will see many tales of woe from people that didn't have one and their Mac crashed or they lost valuable pictures and documents when the Mac would not start again after a power outage. Protecting your Mac from power surges, brownouts and power failures is important, I would not be running mine without one.