Continuous Power

Continuous power — also called “DC power” or “steady-state power” — is the value most engineers focus on when picking a resistor. Unlike pulse and peak ratings (which assume the heat is briefly stored and then dissipated), continuous power is the equilibrium: heat in = heat out, body temperature stable, indefinite operation. It is the most demanding rating because the resistor must hold up to material degradation, oxidation, mechanical creep and termination fatigue over thousands of hours.

The continuous rating is conservative — manufacturers test parts at the rated power for 1000 h or more and require resistance drift below a few percent at end of life. A 50 W resistor rated continuous at 70 °C ambient will typically reach 200 – 250 °C body temperature in still air. To run continuously above the rating you must add forced air, heatsinks or active cooling, or accept that drift and lifetime will degrade.

In practice designers derate the continuous rating by 50 % for high-reliability applications (military, automotive, BESS), 30 – 40 % for industrial, and 10 – 20 % for consumer. Always check the derating curve for the actual operating ambient. For pulse-heavy applications the continuous rating is just the lower bound — the average over the duty cycle must also stay below it, even when peak pulses are far higher.