How do i find out the exact wattage of my prebult HP computer?

[dnst3qface]

i need to know this because ill be adding hardware and knowing the wattage would be good before purchasing or even having it and knowing if i cant even use it? so how do i do this? the product support page has nothing about the power supply. nor do the user guides that came with the hp products have this info. so what am i to do?

[cnfrisch]

What is the specific model number of your HP computer?

[DaveinCharlotte]

OK, I have an HP Pavilion p6531p desktop PC and an HP 2010i monitor.

How much wattage does this pair use in normal operation and when asleep?

Knowing this will let me determine how long my UPS will keep them running. 

[Big_Dave]

This $20 Kill-A-Watt meter will do the job.   I am using the APC ES 750 for my UPS.