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Power Calculations


Power (and Watts) are computed as:
P = VA

So if you had a 208 volt device that was pulling 25 amps, you would end up with: 
(208v) * (25a) = 5200 watts = 5.2 kW

To look at this over time, we have the term "Watt hour" which is this number (assuming it's stable) multiplied by the number of hours.  Thus for one hour we would get: 
(5.2kW) * (1hr) = 5.2k Watt Hours = 5.2 kWhr


British Thermal Units are (usually) a measure of heat produced by computer equipment.   A BTU is defined as: 
1 Whr = 3.413 BTU
1 kWhr = 3,413 BTU

Thus for the equipment mentioned above, it would produce the following BTU's: 
(5.2 kWhr) * (3,413) = 17,747 BTU/hr = 17.7k BTU/hr

Power Factor: 

Power Factor is a concept for AC power and things that induces some resistance like stuff on the power line. (it's called Impedance, and it is computed looking at inductance and capacitance.)  

If you look at a UPS, is will sometimes list something called "Apparent Power".  This is the REAL Watts being used, as it includes the power factor as well.  This is because the real calculation for Watts.  Thus the actual calculation for watts is: 
W = P * S = (real power) * (apparent power) = (VA) * (power factor)