UPS planning

How big of a UPS do you need.

First you need to know how much power your devices use. Unfortunately, the power listed on the label on the back has almost nothing to do with the amount of power actually drawn by the device. 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 should also try to keep the device busy. If you are measuring a server you should give it a good disk and CPU load. You might want to then double this value for safety.

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.