Heat sinks are rated by the thermal resistance in °C/W (Celsius per watt) or °K/W (Kelvin per watt). For example, 5°C/W means the heat sink and the component attached to it will be 5°C hotter than the surrounding air (ambient temperature) for every 1 W of heat it is dissipating. Thermal resistance is a heat property and a measure of a temperature difference by which an object or material resists a heat flow (heat per time unit). Thermal resistance is the reciprocal of thermal conductance. The thermal resistance of materials is significant because most electronic components generate heat and need to be cooled. Electronic components malfunction or fail if they overheat, and many components routinely need measures taken in the design stage to prevent this. The standard units of thermal resistance are K/W or the equivalent degrees C/W (the two are the same since as intervals 1 K = 1°C). As a general rule of thumb – the larger the heat sink, the lower its thermal resistance.