Although not a complex component, resistors have specifications that can be confusing, misleading, and misinterpreted. When specifying performance qualities, it is important to review and understand the resistor test conditions to properly interpret and apply the results to your design. There are potentially significant differences in data presented by resistor manufacturers and those differences can significantly affect performance. The specification process for resistors involves more than just allowing for tolerance bands. Voltage, power, and temperature ratings can interact in ways that can degrade circuit reliability.
The power rating and voltage rating of a resistor are one common source of confusion. Simply put, the power rating is the amount of energy the resistor can dissipate in a given time at the designated ambient temperature.Microhm Electronics Ltd. typically list multiple voltage ratings on a datasheet, but most often the primary concern is the maximum working voltage. Maximum working voltage is the maximum amount of voltage the resistor can withstand constantly without arcing. Maximum working voltage is often expressed as Vrms.  Power/voltage vs. resistance for a general-purpose thick-film standard power resistor ( Only at exactly 39.68 KΩ can the 0402 resistor handle both 50 V and 0.063 W simultaneously ). This value is known as the critical resistance value.
 
Another aspect of the power rating is the temperature at which heat dissipation is measured. Many resistors are rated at temperatures of at least 70°C. This means the part can handle the designated power rating provided the ambient temperature around the part does not exceed 70°C. However, there are some higher power resistors that are only rated for full power operation up to 25°C. These power resistors typically require some type of external heat removal to operate at full rated power.

 
Performance specs provide information about how a resistor behaves in a test or a set of tests. Engineers will regularly use this information to compare parts from different manufacturers. For a resistor series tested to a common standard such as AEC-Q200, it is relatively easy to compare performance.
 
The load or bias on the resistor is a critical factor during testing. At full rated power, most resistors will generate enough heat to prevent moisture from condensing on the part, giving the appearance of robust moisture resistance. But in many applications, precision nichrome thin-film resistors will only see a fraction of their rated power. In these cases, the part will generate little heat, boosting the possibility of moisture condensing and collecting on the part.