All the electrical units of measurement are interrelated: a pressure of 1 volt causes a current of 1 amp to flow through a resistance of 1 ohm. Volts divided by ohms equal amps. For example, a light bulb with a resistance of 3 ohms, in a 12 volt system, consumes 4 amps.
The side and tail lights, however, which you may need to leave on when the car is parked, are always wired independently of the ignition switch. When fitting extra accessories, such as a rear window heater which consumes a heavy current, always wire it through the ignition switch.