Electricity Explained

The­ three most basic units in electricity are:

  • Voltage (V); measured in volts
  • Current (I, uppercase "i"); measured in amps
  • Resistance (r); measured in ohms


Measuring electricity

We can measure electricity in a number of different ways, but a few measurements are particularly important as stated below:


The voltage is a kind of electrical force that makes electricity move through a wire and we measure it in volts.  The bigger the voltage, the more current will tend to flow.  For example a 12-volt car battery will generally produce more current than a 1.5-volt flashlight battery.  The volt is the basic unit for electric potential.  The higher the electric potential, or volt, the greater the amount of electrical energy that can be transferred through a circuit.


Voltage does not, itself, go anywhere: it's quite wrong to talk about voltage "flowing through" things. What moves through the wire in a circuit is electrical current: a steady flow of electrons, measured in amperes (or amps).  Current is a flow of electrical charge carriers, usually electrons or electron-deficient atoms.  The common symbol for current is the uppercase letter I.  The standard unit is the ampere, symbolized by A.

Types of Current

  • A direct current (DC) is produced when free electrons move in only one direction in a conductor.  Pulsating direct current is a current in one direction which regularly varies in intensity.
  • Alternating Current (AC) is produced when the current regularly changes its direction and intensity.



Voltage and current together give you electrical power.  The bigger the voltage and the bigger the current, the more electrical power you have.  We measure electric power in units called watts.  Something that uses 1 watt uses 1 joule of energy each second.  The electric power in a circuit is equal to the voltage × the current (in other words: watts = volts × amps).  So if you have a 100-watt (100 W) light and you know your electricity supply is rated as 120 volts (typical household voltage in the United States), the current flowing must be 100/110 = 0.90 amps. If you're in Ireland you're household voltage is 220 volts.  So if you use the same 100-watt light, the current flowing is 100/220 = 0.45 amps.

A watt is an electrical unit of power.  This term is commonly used to rate appliances using relatively small amounts of electricity.  Wattage is stamped on light bulbs and all appliances.  Wattage = Amps x Voltage. Electrical power is measured in watts.  In an electrical system power (P) is equal to the voltage multiplied by the current.


Power is a measurement of how much energy you are using each second.  To find out the total amount of energy an electric appliance uses you have to multiply the power it uses per second by the total number of seconds you use it for.  The result you get is measured in units of power × time, often converted into a standard unit called the kilowatt hour (kWh).  For example: If you used an electric toaster rated at 1000 watts (1 kilowatt) for a whole hour, you would use 1 kilowatt hour of energy; you would use the same amount of energy burning a 2000 watt toaster for 0.5 hours or a 100-watt lamp for 10 hours.