I am planing to buy a  UPS for our servers. we have two servers and external HP backup drive. One server is compaq and another one is assembled P4/2.8. how can I calculate the power consumption of both server and backup drive?
pleas help.

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You can calculate the power needed for every system, or look at the figures printed on the power supply (needs opening the box), then add them

calculate the needed power :      

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Hi raja0123,
For these two servers planning for about 400-500VA. However you will find that practically whatever UPS you buy the rating will be sufficient but the battery runtimes will vary significantly. Don't forget about the monitor either.
As a minimum I would suggest a 1000VA UPS which should give you a runtime in the order of at least 30 minutes.
And do not forget there are 2 different kinds of UPS'es :
1- runs off the AC, and switches to battery if the power falls out
2- runs off the battery always (best, more expensive)
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I have a better Idea depending on how critical these servers are.  Run them off of the UPS as said, I agree with the 1000VA, then have a standby generator, the UPS will provide the power until the generator comes on.  Not cheap, but if the server is critical, it is worth it.
You might also wish to consider doing what I do.

I have a rack of compaq servers which all have redundant power supplies. I have a couple of UPS's with compaq power supply going to a different UPS. This means I have added fault tolerance and when the batteries need changing every 3 years or so I can shut down one UPS at a time without having to switch off the servers.
APC - - has a UPS configurator (obviously it recommends APC solutions), which has knowledge about the actual power consumption of a lot of modern computer equipment.

I think everyone who answered made a good contribution but as you can only split it 2 ways maximum I will leave it to you.
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