Here are some equations that a technician may find useful.
V = IR
In purely resistive circuits (for example conventional lanterns):
P = IV
P = I2R
For AC circuits with reactive or non-linear components the above calculations give the apparent power usually stated in VA (volt-ampere), to find the actual rate of energy transfer (in Watts) you must multiply this figure the Power Factor. That is:
ApparentPower * PowerFactor = RealPower
RealPower = IV * PowerFactor
RealPower = I2R * PowerFactor
In circuits where the current in sinusoidal but out of phase with the voltage:
Where φ is the phase angle between the Voltage and the Current.
Combined Power and Ohms Law Circle
This circle shows the equations which connect Power in Watts, Current in Amps, Resistance in Ohms and Voltage in Volts. The power equations in this circle can only be considered correct for a power factor of unity.