The electric power P consumed by an electrical device is the product between the voltage V at the terminals (in Volts) and current I passing through it (in amps):

P = V × I

The unit of power is the Watt (W).

- For direct current, P = V × I
- For alternating current used with resistance type components (lamps, heaters), P = V
_{RMS}× I_{RMS}where V_{RMS}and I_{RMS}are the RMS values of voltage and current.

The electrical energie consumed by an electrical appliance is the product of electrical power (P) and the duration of time used (t)

E = P × t

The unit of energy is the Joule (J). Another frequently used unit is the Watt-hour (Wh). Look at your home electricity bill.

1 Wh = 3600 J