Here are some equations that a technician may find useful.

Ohm's Law

V = IR

Ohm's law is the relationship between current (I) in Amps (A), Resistance (R) in Ohms (Ω) and voltage (V) in Volts (V).

Power

In purely resistive circuits (for example conventional lanterns):

P = IV

This is the relationship between electrical power (P) in Watts (W), current (I) in Amps (A) and voltage (V) in Volts (V). This can be combined with the Ohms Law to give.

P = I^2 R

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:

Apparent Power * Power Factor = Real Power

or

Real Power = IV * Power Factor

and

Real Power = I^2 R * Power Factor

In circuits where the current in sinusoidal but out of phase with the voltage:

Power Factor = |cos φ|

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.