The Watt is the standard unit of measuring power. Suppose I asked you if the kilometer necessarily determines how far away something is, or if °C necessarily determines how hot (or cold) something is. These are just units of measurement. As for the brightness of a light, the Watt is a measurement of power not brightness. Brightness is usually measured in Candelas (cd), which has the same dimension as Joules (J), the standard unit for energy. Thus the brightness of a 1 cd lamp is equivalent to 1 J of energy. I suspect you are asking about efficiency. No device is perfectly efficient. A motor, for example, may burn 500 W but only produce 250 W of mechanical power. In this case we say it is 50% efficient. The amount of loss is due to heat in the windings, friction in the bearings, etc. Light bulbs have efficiency ratings that account for the amount of power used in comparison to the amount of light they produce within the spectrum of the human eye. This can be measured using luminous efficiency. Some typical values for luminous efficiency are: incandescent: 2% compact flourescent: 10% flourescent: 12% LED: 38%
