How big of a UPS do I need?
First you need to know how much power your devices use. the power listed on the name plate label on the back of a device greatly overestimates the normal power drawn by the device. This is useful mainly for calculating the peak instantaneous power a UPS needs to be able to handle without burning up. It is not useful for estimating how long a UPS can power a load before the batteries are empty.
To calculate typical power you need to measure the device under a load. I use a Kill-A-Watt Meter. These are very inexpensive meters that will show True RMS power used by a device. Measuring AC power can be very complicated. I won't go into that here, except to note that it is not as obvious and intuitive as you might think. You need a special meter. The Kill-A-Watt is the cheapest one that I have seen that actually works. The downside of this is that you must unplug the device you want to measure and plug it into the Kill-A-Watt meter. The value will drift around a bit depending on what the device is doing. You have to watch the device in question for a minute or two to see how it behaves. You might also want to keep the device busy so that it uses more power to get a better idea of the worst case. If you are measuring the power requirements of a server you should give it a good disk and CPU load. Finally, you should add a safety margin to whatever power requirements you come up with. I usually double the value.
OK, so on my server rack I measure the following:
switch: 20W Coyote Point Load Balancer: 160W Dell 1850 Server: 250W Dell 1850 Server: 250W
That gives me a total of 680W. For this type of equipment you can more or less interchange Watts and VA.
Calculating peak instantaneous power
When power supplies power up; hard drives spin up to speed from rest; and when every component in a system is active under maximum load then you will be drawing peak power. This is a large power spike that happens when you first turn a device on. This power pulse usually only lasts a few seconds.
You can calculate your load VA from the ratings printed by the manufacturer on the device. Make a list of items you plan to power. Next to the power plug of each item you should find a nameplate with a rated VA (Volt-Amp rating), Watts, or Amps. We want to convert everything to VA. If the power rating is in Watts then multiply this by 1.4 to get VA. If the power rating is in Amps then multiply this by 120 to get VA.
Pick a UPS with a VA rating greater than the load VA.