The power plants produce electric energy from 10,000 to 20,000 volts. To avoid energy lose, electricity is sent at 220,000 volts.
Power Plants transform primary energy into mechanical energy with the turbines, then the alternator uses the mechanical energy to produce electric energy.
When the primary energy is mechanical, like the hydroelectric or wind power plants, it moves directly the turbine. When it's termic, energy is applied to a fluid, like water, which becomes steam and moves the turbine.
Hydroelectric stations like Niagara falls or Hoover dam can produce electric energy almost without any kind of pollution, althought energy has to travel large distance to be useful in house and industries. This electric energy transfer loses energy by releasing termic energy. This unwilling energy called joule heating is I2R (Current and Resistance).
All cables have some kind of resistance, for example, one kilometer of normal cable used to send electric energy to houses has a resistance of 0.2W ohms.
The joule heating can be decreased by reducing the resistance using high conductive and large diameter cables, but those are heavy and expensive.
The loses are proportional to the square of the current in the conductors, so it is very important to mantain it low.
The electric energy per second (power) that is send to large distances is obtained by the relation P=VI. To reduce the current and mantain the power, the voltage is increased.
Some of those conductor lines use voltages of 500,000V. The low resultant current decreases the loses of I2R in the conductors.
When we pay the light service to an electric energy company, we just buy the electric energy, not the power. The energy used by any kind of dispositive is the relation of consumed energy in joules per second (Watt), multiplied by the time that the dispositive was working.
J/S * S = Energy Joules
Like the Joule, the watt-second is a relatively small scale of energy, so the electric energy companies use a higher scale called kilowatt-hour (kwh)
1 kwh = 1000J/S * 3600s = 3.6 * 106 J