You want to find a unit big enough to ...
- handle the load (i.e. not trip the internal circuit breaker) when the power goes off
- handle the higher peak loads of motors (fans, hard drives) starting up
- maintain power for at least 15 minutes during an outage
- maintain power for as long as possible after that, within the limits of your budget
I recommend starting with APC's online tool to determine what size UPS you need at http://www.apc.com/tools/ups_selector/
Other manufacturers have similar online tools but APC is the most widely sold UPS.
CAVEATS and NOTES
- A 1 hour runtime is an entirely reasonable expectation in the real world. However, current battery technology makes that very expensive. Typical runtimes for home or office are more like 15 minutes.
- With this, you can wait 5-10mins to see if the power comes back on.
- If it doesn't, you've got another 5mins in which to do a clean, controlled shutdown so all your data will be safely saved.
- As others have said, you cannot accurately determine power usage from the specs alone. Their purpose is to indicate the maximums the electrical components can safely tolerate.
- The APC site lets you size by specs or by device categories.
- I tried "by specs" first. Needs too much info you don't have.
- "By device" gets you reasonable recommendations.
Based on the info supplied, their APC Power Saving Back-UPS 750 (750VA) for $110 should work fine. I've been running an older model like this for several years without complaint. If you can afford it, by all means get a larger unit. There are other brands that are cheaper and probably just as good.
When the battery finally wears out after 5-7yrs or so, it's probably worth just buying a new unit. I've replaced UPS batteries and found a new unit usually gives you better monitoring, switching, and/or surge-suppression technology for just a little more than the cost of a replacement battery.
(edit 2015-01-08)
Battery on said UPS died a few weeks ago and I found that battery prices have dropped a lot so I replaced just the battery (contrary to my advice above). So far so good. With battery prices at about a quarter to a third that of a new unit, if you don't need better electronics, replace just the battery with one from a reliable reseller.