In. exactly. the same way as you do so for a singlephase load. It's the product of the power of the load, expressed in kilowatts, and the during of operation, expressed in hours.
http://wiki.answers.com/Q/How_do_you_calculate_kwh...

I was A2A this question but, I think that I might want to start out with what kW and kWh are.... Units of Measurement kW is a unit of Power and stands for kilowatt or 1 thousand watts
http://www.quora.com/SolarEnergy/Howisthecost...

You need to know voltage too to calculate power (P) P = V*I. To calculate energy (E) in kWh you need time (T) of consumption in hours (h) E = P*T/1000 kWh.
http://answers.yahoo.com/question/index?qid=201001...
