Wattage for 27-inch IMAC ?

1. The 170 W is actually just the CPU chip¹!

2. The actual formula for determining AC wattage using rated volts and amps is PF x V x A (where PF is power factor). Not usually published I ignore PF and just multiply volts and amps as you have. However, because Apple doesn't know the details of each individual iMac configuration going out the door, they have to state the maximum rating which assumes that every port is in use, every accessory has been added, the graphics board is going full tilt, the RAM is fully populated, etc., etc., etc.. Ergo, 480 W. But the power supply used in Mike's specific configuration was based on the known loading in that configuration so they only installed a 310 W power supply. Eh. Their call. They've made billions at this game so who am I to argue with their ratings.

Now, back to sizing the UPS. Go ahead and use the 480 W -- a margin of safety won't hurt you at all -- and add in the wattage of all the other devices you want on battery backup. Take that total wattage and multiply by 1.25. That's my rule-of-thumb for UPS selection. Then select a UPS with equal or higher watts rating (ignore the VA rating). I also prefer UPS units with replaceable batteries but that is your choice (keeping in mind every couple years the battery will need to be replaced).

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¹ For example: http://www.bit-tech.net/reviews/tech/intel-core-i5-4670k-haswell-cpu-review/6/

Total wattage will depend on screen size, processor (65 or 95 watt), hard drive type (SSD or platter), bluetooth / wireless enabled, etc...

I would choose my UPS based on whether I need software support or not...

This $50 APC 450 watt is a good basic one, but lacks the software connection, and also lacks the USB charging port.

I use it for all my extras, external drives, printers, etc...

http://www.officedepot.com/a/products/383084/APC-Back-UPS-BN450M-Battery-Backup/

Look on the side of this APC 650 and you will see a software support socket. Plug it into a Mac running Snow Leopard (I believe) or higher, and it is auto recognized, and under "System Preferences", "Energy Saver", and you will see an new tab "UPS", which you can configure to auto shutdown your computer.

http://www.officedepot.com/a/products/321880/APC-Back-UPS-BN650M1-Battery-Backup /

Also has a USB port to charge your iPad, iPhone, etc...

Lower than Snow? Download the PowerChute software from APC....

Hint - Change the sensitivity to "High", plug the UPS in for an hour, turn it on, let it charge. Turn it off, the push the button to turn it on, but do not let go, hold for ten seconds, Button will turn red, and flash, push the button to cycle through, Red, Green, Amber. Stay on amber, . Wait 15 seconds, it will turn off.

Now turn on as normal.

A big part of the question for a UPS back up is how long do you want the back to be able to power the Mac. If we assume the power consumption is 310W and you want to have 30 minutes to shut down your Mac before the UPS runs out of juice, you will need a 1000W or greater UPS according to the APC "UPS Selector".

EveryMac indicates a 310w power supply http://www.everymac.com/systems/apple/imac/specs/imac-core-i5-2.7-27-inch-alumin um-mid-2011-thunderbolt-specs.html

Step 12 of this iFixIt tear down also indicates 310w http://www.ifixit.com/Teardown/iMac+Intel+27-Inch+EMC+2309+and+2374+Teardown/123 6

Even though the tear down is a late 2009 27" model their parts list says it's the same power supply.

Mike's right, and it's likely that Apple are too on the actual consumption, since the supply may not be rated at full power in that enclosure, and would have some overhead for unpowered accessories.