The best way to determine the power consumption of your server room is to ask an electrician to measure the total current (in amps.) and multiply this value x 120 volts. The resulting value will be the power expressed in VA (Volt-Amps).
Why an electrician? Well, is easier and more accurate (to avoid forgetting some devices) to perform the measure at the electrical panels. Just make sure the instrument, usually a clamp meter, is True-RMS.
If you want this measure to size an UPS, make sure to get some extra "space" for future increments and to avoid use the UPS at maximum load.
Regards,
Jose.-
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Pablo Mir
pm@pablomir.com