Power resistors are usually larger than their lower power counterparts. The increased size helps to dissipate heat and often is used to provide mounting options for heatsinks. High power resistors are also often available in flame retardant packages to reduce the risk of a hazardous failure condition.

 
The power rating of a resistor defines how much power a resistor can safely handle before the resistor begins to suffer permanent damage. The power dissipated by a resistor can be found easily using Joule's first law, Power = Voltage x Current. The power dissipated by the resistor is converted to heat and increases the temperature of the resistor.  To avoid these failure modes, such as a shift in resistance value, reduction in an operating lifetime, open circuit, or temperatures so high, power resistors are often derated based on expected operating conditions.

 
Most electronics applications use low power resistors, typically 1/8th watt or less. However, applications such as power supplies, dynamic brakes, power conversion, amplifiers, and heaters often demand high power resistors. Generally, high power resistors are resistors that are rated for 1 watt or greater loads and are available in the kilowatt range.