Power Factor (PF) = cos (θ)
Apparent Power (P) = I * V
True Power = P
I = Current
θ = Phase angle
V = Voltage
A calculator is included in the Free
Espresso Engineering Workbook.

Power factor measures the phase angle (θ) 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
Formulas 
Apparent Power P_{A} = I
* V [VA]
P_{A} = I^{2} * R [VA]
P_{A} = V^{2} / R [VA]

True Power (Real) P_{t} =
I * V * cos (θ) [W]
P_{T} = I^{2} * Z * cos (θ) [W]
P_{T} = V^{2} * cos (θ) / Z [W]

Reactive Power (Q) P_{R} = I * V *
sin (θ) [VAR]
P_{R} = I^{2} * Z * sin (θ) [VAR]
P_{R} = V^{2} * sin (θ) / Z [VAR]

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

Voltage V = P / [I * cos (θ)]
V = sqrt [P * Z / cos (θ)]

Impedance Z = V^{2} * cos (θ) / P
Z = P / [I^{2} * cos (θ)]

where

P =
P_{T} ± jP_{R} P = power in Watts V = voltage in Volts
I = current in Amps
R = resistance in Ohms (Ω)
Z = impedance in Ohms (Ω)
θ = phase angle between I and V in degrees
PF = power factor
P_{A} = apparent power in Volt·Amps (VA)
P_{R} = apparent power in Volt·Amps (VA)


