Power ratings of DC Circuit:

Electrical equipment can only stand a certain amount of heat production without damage and the safe power which a piece of equipment can consume without damage is its ‘power rating' or ‘wattage rating'. Each component is given a wattage rating and if this is exceeded the component will overheat.

The more power consumed by a device the more heat or light it produces in a given time; a 100w lamp gives more light than a 60w lamp. The rating 6V 12W on a lamp means that if is connected to a 6V supply, its resistance is such that it develops 12W of power and that it is intended to work at this rating.
Note that:

• The above bulb consumes 12W only at the correct voltage. If the voltage is increased more power is developed and the component may be damaged.

• A fluorescent tube of 12W rating produces more light than a 12W filament bulb because the tube produces much less heat and is therefore more efficient.

This power rating has a different meaning from that of a bulb.  In this case we must always keep below the stated value.

To keep below the stated power value, there are maximum permissible values of voltage and current, which may be calculated as follows:

Maximum Current    P  =  I²R

Therefore and this is the maximum current to avoid damage to the resistor.  

Maximum Voltage    P  =  V^2/R

Therefore V  =  √P  *  √R and this is the maximum voltage to avoid damage to the resistor.