Whenever current flows through a resistor heat is produced which represents electrical power in Watts
The unit of electrical power is the Watt (W). One watt of power is equal to the work done in one second by one volt of potential difference in moving one coulomb of charge around a circuit. As an ampere is equal to one coulomb per second, electrical power therefore equals the product of volts times amperes, P = VxI
A resistor can be used at any combination of voltage (within reason) and current so long as its “Dissipating Power Rating” is not exceeded. The power rating of a resistor, also called its “wattage” rating, is an indication how much heat a resistor or resistive element can safely dissipate or convert into heat before becoming damaged. If more heat is generated by the resistor than can be dissipated, the resistor will overheat and become damaged. Resistor power rating is specified in watts.
Power(P) =V×I=I2×R=V2/R Watts
Where: V is the voltage across the resistor, R in ohms, producing the current, I in amperes, for power, P in watts.
When calculating the power in resistors or resistances, the main equation to use whenever there is current flowing in the resistance is I2R.
The physical size of a resistor is no indication of its resistance as a small resistor can have a very low or a very high resistance value. A resistors physical size, however, does give some indication of its power rating. As the dissipated resistor power rating is linked to their physical size, a 1/4 (0.250)W resistor is physically smaller than a 1W resistor and resistors that are of the same ohmic value are also available in different power or wattage ratings. Carbon and metal film resistors, for example, are commonly made in wattage ratings of 1/8 (0.125)W, 1/4 (0.250)W, 1/2 (0.5)W, 1W, and 2 Watts.
Generally speaking the larger their physical size the higher its wattage rating. However, it is always better to select a particular size resistor that is capable of dissipating two or more times the calculated power. When resistors with higher wattage ratings are required, wirewound resistors fitted to metal
heatsinks are generally used to dissipate the excessive heat.
When selecting or replacing a resistor for a circuit, first determine the required resistance value using R = V/I, then calculate the amount of power that will be dissipated by the resistor using any one of the power formulas above.
When selecting the appropriate resistor for a circuit, always try to select a resistor with a higher wattage rating than the actual calculated power dissipation for safety reasons as resistors that conduct lots of current can become very hot.