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Every real component wastes some energy as heat, which is what actually determines its temperature rise and cooling needs. There are three common starting points for finding this loss. If you know the voltage and current at the device, dissipation is simply P = V×I (or the resistive forms I²R and V²/R). If you only know the system's efficiency, the loss is the fraction (1−η) of whichever power figure you have. And if the device is switched on and off repeatedly, its average heating depends on how much of the time it is actually on — the duty cycle.
| Method | Formula |
|---|---|
| From V & I | P = V × I |
| From R & I | P = I² × R |
| From R & V | P = V² / R |
| From input power & efficiency | Ploss = Pin×(1−η) |
| From output power & efficiency | Ploss = Pout×(1−η)/η |
| Duty-cycled average | Pavg = D × Pon |
This dissipated power P is exactly the input needed by the Junction Temperature and Heat Sink calculators — find P here, then feed it into those tools to size cooling.
It is the electrical power converted to heat in a component rather than doing useful work — the rate at which a device warms up. It is measured in watts and is the key input to thermal (cooling) design.
Multiply them: P = V×I. This works for any two-terminal device where you know the voltage across it and the current through it, such as a diode, LED, or a switch's on-state drop.
Use P = I²R if you know the current, or P = V²/R if you know the voltage across it. Both give the same answer as P=V×I for a purely resistive component.
If you know the input power, loss is Pin×(1−η). If you only know the output power, use Pout×(1−η)/η instead, since the input is Pout/η.
Manufacturers often specify a converter's or regulator's efficiency rather than its internal loss breakdown, so this lets you quickly estimate the heat it generates without knowing every internal loss mechanism.
When a device is switched on and off repeatedly (e.g. PWM control), its average heating is less than its full on-state power, scaled by the fraction of time it is actually on: Pavg=D×Pon, where D is the duty cycle (0–1).
Average power sets the long-term temperature rise, but peak (on-state) power still matters for instantaneous voltage/current ratings and short-term thermal transients, especially for slow-thermal-mass parts.
The dissipated power P found here is exactly what you plug into a thermal-resistance formula, Tj = Ta + P×θ, to find how hot the device actually gets — see the Junction Temperature calculator.
The V×I and resistive formulas here capture steady-state (conduction) loss. Devices that switch on and off at high frequency also have switching loss, which is calculated separately — see the Switching Loss calculator.
It depends entirely on the package and cooling: a small SOT-23 transistor might handle under 0.5 W without a heatsink, while a TO-220 device with a heatsink can handle many watts. Always check the datasheet's maximum power rating.
Yes. Total dissipation is simply the sum of every loss mechanism present — conduction, switching, quiescent/bias current, etc. Compute each separately (using the appropriate tool) and add them for the total thermal design power.
Not the dissipation itself (which depends on electrical conditions), but it strongly affects the resulting temperature rise and therefore how much dissipation is safe — see the Junction Temperature and Heat Sink calculators.
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