Power factor measures the phase angle between the instantaneous voltage and the instantaneous current in a circuit. Voltage (E) leads current (I) by 90° in an inductive (L) circuit and voltage (E) lags current (I) by 90° in a capacitive (C) circuit.
A popular mnemonic is ELI the ICE man since E is leading (coming before) I in ELI, and E is lagging (coming after) I in ICE.
power factor ( P_{f} ) = cos ( q ) 

Apparent Power ( P ) = I * V  
True Power = P  
I = Current  
q = Phase angle  
V = Voltage 
Power Formulas  
Apparent Power P = I * V P = I^{2} * R P = V^{2} / R 
True Power P = I * V * cos (q) P = I^{2} * Z * cos (q) P = V^{2} * cos (q) / Z P = P_{a} * P_{f} 
Impedance Z = V^{2} * cos (q) / P Z = P / [I^{2} * cos (q)] 

Current I = P / [V * cos (q)] I = sqrt {P / [Z * cos (q)]} 
Voltage V = P / [I * cos (q)] V = sqrt [P * Z / cos (q)] 

