Which size of UPS to buy ?

General rule for UPS sizing is: the amount of VA indicated on the UPS will last about 5 minutes under that specific load, eg a 500 VA UPS will last 5 minutes under 500 VA, but may not work under 1000 VA !

The maximum power drawable from the UPS is normally always specified and cannot be exceeded, otherwise the UPS will not be able to deliver the power needed and will indicate an overload.

Several different types of UPS are available for different applications.

Desktop computer:
You need about 500 VA, mainly depending on the CPU used (the higher the frequency, the higher the power consumption), on the amount of hardware (HDD, tape, CD writer, ...) and on the screen size.
Low-end desktops work with a 375VA, while high-end may require up to 650 VA.

Server:
For server, a power dimensionning is needed, especially if you have racks of server or equipment. The easiest way is to physically measure the current that runs trough your mains cable and multiply by your voltage and you get the VA that you are consuming at THAT moment.
If you fully load the servers and measure, you get the max VA that your server will draw, check for a UPS rack that is 10 to 20% bigger than this and you will last 5 to 6 minutes.

A good sizing tool is available at http://www.apcc.com/tool/size/apc/

Laptops:
You may want to connect a laptop to a UPS if you are in an area with long outages. A laptop consumes much less power than a desktop and a 375 VA should cater for more than 15 minutes before you start running on the laptop's batteries.
It is also a cheap way of replacing a faulty battery in a laptop that you mainly use for desktop work.

Shutdown:

Most UPS now come with a serial/USB connection ensuring the shutdown of the computer when they are about to die.
This system works well under windows, is now well supported under linux as well, but is still a bit dodgy under other OS. You should check with the supplier to make sure your OS supports it.

Unix stations generally require more power and it is advisable to target mid-range UPS, between 1000 and 1500 VA.
Few Unix systems are supported by the automatic shutdown and you may want to have a bigger UPS to make sure the Unix machine won't crash if you can't manage to connect an UPS to it.

UPS can also be used to power scanners, inkjet printers and other devices. These have a very low power consumption and the smallest UPS available should do, otherwise one can still connect several devices on one UPS, which is cost-effective.

Special care should be taken when connecting copiers and laser printers: those two draw a lot of power for waring the sheets of paper, to dry the ink, and you need a really big UPS to support these machines. Generally, people don't UPS these, but if you require, get to high-end 2500 to 5000 VA. You UPS reseller can probably advise you on this.