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Page history last edited by Brian Matthews 14 years ago

Electricity Units used in Energy Modeling


One kilo-Watt-hour is equivalent to one 1000 watt light bulb running for one hour. 


The utility company may regard a 100 kw pulse happening over the duration of 5 minuites to be a 100 kw BILLABLE demand for that full hour.   The facility didn't use 100 kw for 60 minuites, but the utility company may regard THE PEAK OF ANY HOUR to be that value designated to be billed for that hour.   This matter is largely disregarded unless you are computing the cost of use.        


Brief summary of power equations:


1=  3.412 kBtu

         1 kWh


1= 3.412 kBtu/hr

           1 kw


1=          kWh                       

     1000 watts * 1 hour



1 =          kW            This is approximate, see equations below for more detailed calculations.

     1.3 Horsepower



1 Phase


1 phase (single pole) Typical USA wall outlet, Volts = 110Vac or 120Vac depending on local voltage



kW = Volt *Amp 




2 Phase 

2 phase: This is commonly refered to a 220 plug or service, it is common for household clothes dryers.   



kVA =  Volts * Amps *1.41


kW =  Volts * Amps *1.41 *PF     



HP for 2 phase =  Volts * Amps * %EFF * PF * 1.41



For the PF (Power Factor) value see this link for the PF of low horsepower motors common to 220 circuits:  www.amptechsupply.com/electrical_data/motordata.htm



 3 Phase


3 phase: Volts= often are 460 or 440 a nameplate voltage on a piece of equipment sometimes will say 410 volts to let people know that if the power company cant keep up with 440 410 will still work.  Amps is measured from one of three legs, ballanced load between all legs is assumed. Phase angle is assumed to be unitary.



kVA =  Volts * Amps *1.73



KVA is simply KW without a PF taken into account.  Sometimes motors have unitary power factor =1 so in this case KVA=KW

But commonly KW is calculated: 


kW =  Volts * Amps *1.73 *PF    



PF often is .86 or .82 for motors above 15-20 hp (any large motor) see here for pf for low horsepower motors:  www.amptechsupply.com/electrical_data/motordata.htm


There are two different power arrangements for 3 phase circuits.  For more information on 3 phase circuits go here.


HP for 3 phase =  Volts * Amps * %EFF * PF * 1.73



% EFF is the efficiency of the motor  See here for a list of common efficiencies for motors