Power Dissipation Calculator

Power dissipation is the rate at which an electronic component converts electrical energy into heat, measured in watts. It equals P = V × I, the voltage across the component times the current through it. Every component, from resistors to transistors to LEDs, turns part of its electrical power into heat, and exceeding its rating causes drift, permanent damage, or fire.

Use any of three equivalent forms: P = V²/R, P = I²R, or P = V × I. Example: 12 V across a 100 Ω resistor gives I = 0.12 A and P = 144/100 = 1.44 W. The resistor must be rated above this with margin, so this calculator also applies the standard 2× safety rule and temperature derating.

Choose a power rating at least twice the expected dissipation: P_rating ≥ 2 × P. The headroom covers tolerance stack-up, ambient temperature derating, and supply transients. For 1.44 W of dissipation, 2× gives 2.88 W, so the smallest standard rating that satisfies the rule is 3 W (standard series runs 1/8, 1/4, 1/2, 1, 2, 3, 5, 7, 10 W and up).

A resistor's rated power only applies up to a threshold ambient temperature, typically 70 °C for film types. Above that it derates linearly to zero at its maximum operating temperature, often 155 °C. For example, a 1 W resistor in 100 °C ambient is only good for about 0.65 W. Always derate first, then apply the 2× margin rule to the derated value.

For a BJT in saturation use P = Vce(sat) × Ic, for example 0.3 V × 2 A = 0.6 W. For a MOSFET add conduction loss P = Id² × Rds(on) and switching loss P = f × E_sw, where f is switching frequency and E_sw is the energy lost per switching event. Example: 10 A through 10 mΩ gives 1 W conduction; at 100 kHz with 10 µJ per switch the switching loss is another 1 W, for 2 W total.

Use P = Vf × If, forward voltage times forward current. A diode dropping 0.7 V while carrying 1 A dissipates 0.7 W. An LED at 3.2 V and 20 mA dissipates 0.064 W. Add every diode and LED to the total thermal budget of the board, since their heat adds up with the rest of the circuit.

Use Tj = Ta + P × Rθja, where Ta is ambient temperature, P is dissipated power, and Rθja is the junction-to-ambient thermal resistance from the datasheet. Example: 2 W with Rθja = 62.5 °C/W (typical TO-220 without heatsink) at 25 °C ambient gives Tj = 25 + 125 = 150 °C, which is at the typical silicon limit, so a heatsink is needed.

Compute the maximum allowed heatsink resistance: Rθsa = (Tj_target − Ta)/P − Rθjc − Rθcs, where Rθjc is junction-to-case and Rθcs is case-to-sink (thermal interface). Example: 10 W, 40 °C ambient, 120 °C target, Rθjc = 1.5 and Rθcs = 1.0 gives Rθsa ≤ 5.5 °C/W. Choose a heatsink rated at or below that value under your actual airflow conditions.

Rθja, junction-to-ambient thermal resistance in °C/W, tells how much hotter the silicon gets per watt dissipated without a heatsink. It is listed in the thermal section of the datasheet. Typical free-air values: TO-220 about 62.5, SOT-23 about 200, SOIC-8 about 130 °C/W. Real layouts differ, so always confirm against your own datasheet before trusting the estimate.

This calculator covers general power dissipation for any component, plus thermal estimation and heatsink sizing. For resistor-specific work such as color code decoding, E-series value selection, and LED series resistors, use the Resistor Calculator at https://nrtechstudio.com/tools/resistor-calculator/. For heat produced by current in conductors, see also the Joule Heating calculator.