MATHS

The units that are used to measure electricity are:
Volts:  Electrical force of pressure behind the electrons in a circuit. Like water pressure or PSI for air. (Usually 12v on a narrowboat).
Amps:  The number of electrons flowing past in a second. Like litres per second in a pipe, it defines the electrical current in a wire.
Watts:  Total amount of electrical energy, per second.

The formulae to work them out are:
Watts = Volts x Amps
Volts = Watts divided by Amps
Amps = Watts divided by Volts
For example: a 20 Watt light bulb in a 12 Volt system:
20 Watts divided by 12 Volts = 1.66 Amps

For those who like facts and figures we also have
Lumens: (lm) measure of the perceived power of the light.