Resistor power, wattage, and derating margin
The calculated resistor power is only the load. The selected resistor rating must also survive ambient temperature, PCB copper, package size, airflow, and reliability margin.
Is calculated resistor power enough to choose a resistor package?
No. Calculated power tells you the heat generated electrically. Package choice needs margin above that value, plus derating for temperature and layout conditions.
Model summary
- Power from voltage and resistance: P = V^2 / R.
- Power from current and resistance: P = I^2 * R.
- Power from voltage and current: P = V * I.
- Practical rating target: selected wattage >= calculated power * margin factor.
Worked example
A 1 kOhm resistor with 10 V across it dissipates P = 10^2 / 1000 = 0.1 W.
With a 2x design margin, the minimum practical target is 0.2 W.
A 0.25 W resistor may be acceptable in benign conditions, but temperature derating and PCB layout still need review.
Design decision
Use the calculated power to shortlist a wattage rating. Then check package derating, maximum element voltage, ambient temperature, and whether the load is continuous or pulsed.
Common mistakes
- Choosing a resistor rated exactly equal to calculated dissipation.
- Ignoring pulse energy, transient loads, or startup conditions.
- Assuming all package sizes with the same nominal wattage behave the same on the PCB.
When the approximation breaks down
- The calculator-style power equations do not replace pulse-rating curves, thermal simulation, or manufacturer derating curves.
- High-voltage resistors also need voltage rating, clearance, and sometimes series sharing checks.