Just from looking at the specs of the 650va UPS from APC, it’s about 390 Watts of Output. Also need to take into account the mouse, keyboard and Monitor.

I pulled this off Google..

“1 W = 1 V x 1A x PF where PF= Power factor, a measure of how efficient the conversion is. So a 1kVA UPS = 1000VA * PF, assuming PF = 0.7 (which is about standard) you get 700W, which is sufficient to power a 700W system. However, a 700w PSU will never consume 700W so it is safe to assume that you won’t need a UPS of that capacity.”

And here is a UPS selector. http://www.apc.com/tools/ups_selector/index.cfm
I typed in 700watts and 1500VA pops up. 700 watts might be overkill.

APC Smart-UPS Battery Backup Power Supply (SMC1500)
http://www.amazon.com/APC-Smart-UPS-Battery-Backup-SMC1500/dp/B007ZT2KKM

although his system won’t draw 750W, that 650VA won’t cut it
he also need to factor in the amount of time the system needs to stay powered – so yeah, that 1kVA UPS doesn’t sound as far fetched; it will give him roughly 10 minutes to save his work

10 minutes is more than enough time to shut a computer down anyway, especially with an SSD, under normal conditions.

The other thing this person needs to take into account is whether or not Windows and driver updates would severely lengthen the time of the shut down, that leaves no time left to do much else beforehand, other than save a document file or save anything else. I would advise a 1300 or 1350VA UPS just to be safe if he can afford it, this would leave roughly 20 minutes of runtime at full load, or up to 1 hour if the load is less.

i have a similar setup on my HTPC/Server Rig (3570k @ 4.7ghz, 16gb ram, R9 290x, 4 HDDS and 1 SSD paired with a 750W PSU) and a 1000va is about 5-7 min at max at full load or about 10-15 min idle maximum (it’s enough to survive a power loss but not a really bad one…