Power Dissipation Calculator - Heat Loss From V, I, and R

A power dissipation calculator turns the voltage across a resistor and the current through it into the watts of heat that the resistor has to shed, using P = V*I, P = V^2/R, or P = I^2*R depending on which two of V, I, and R you measured.

• • Sizing a resistor in a DC circuit: Compute the heat for a 12 V LED dropper, voltage divider, or pull-up to pick the right wattage rating.
• • Verifying a load in a lab experiment: Cross-check the watts on a bench supply against V*I or I^2*R after measuring the load resistance.
• • Designing heater elements and shunts: Estimate heat in cartridge heaters, brake resistors, and current-sense shunts.
• • Calculating battery drain in joules: Multiply dissipated watts by a runtime to get the joule heat a package, PCB, or enclosure absorbs.

A power dissipation calculator applies Ohm's law to give the same watts value from any two of V, I, and R. The result is the steady-state heat the resistor has to shed.

This power dissipation calculator also helps with resistor sizing, lab verification, and joule energy over a measured time. The watts value drives the resistor rating.

For AC where voltage and current peaks do not line up, use an AC wattage tool with a power-factor input. This power dissipation calculator assumes DC or unity power factor.

When you also need to solve for V, I, or R from the other two without the heat-energy step, Ohm's Law Calculator returns the same Ohm's law triangle in a single screen.

The calculator reads the input mode, solves the missing quantity from Ohm's law, then returns the power using P = V*I, P = V^2 / R, or P = I^2 * R, and converts the result into joules over the time interval.

• V: Voltage across the resistor in volts (DC). One of three inputs in all-three mode.
• I: Current through the resistor in amperes (DC). Solved from V/R when only V and R are given.
• R: Resistance in ohms. Solved from V/I when only V and I are given. Zero is rejected.
• t: Time the resistor dissipates the calculated power, in seconds. Drives the heat energy E = P*t.
• P: Power dissipated in watts. Equal to V*I, V^2/R, and I^2*R for the same operating point.
• E: Heat energy in joules. Equal to P * t and drives thermal capacity and runtime estimates.

The three power formulas are equivalent because Ohm's law V = I*R lets you substitute. In practice you reach for whichever version matches the two quantities you actually measured.

When all three of V, I, and R are entered, the calculator checks Ohm's law and flags a consistency warning when the entered resistance disagrees with V/I by more than five percent.

According to HyperPhysics - Georgia State University, the power dissipated by a resistor equals V times I, which equals I squared R, which equals V squared divided by R, so all three forms give the same watts value for any one resistor at one steady operating point.

When the load is AC and the voltage and current peaks do not line up, AC Wattage Calculator carries the same V and I inputs through the power factor to give the real watts.

Four ideas cover most power dissipation problems in physics and electronics labs.

These four ideas cover most power dissipation problems from a 1/8 W pull-up to a 50 W braking resistor.

When the next step is to express the same watts in milliwatts, kilowatts, or horsepower, Watt Converter converts the result across the common power units without retyping the input.

Five steps from a measured V, I, or R to a wattage number plus a safe resistor rating.

1. 1 Pick the input mode: Choose voltage-and-current for meter readings, voltage-and-resistance for a fixed supply and resistor value, current-and-resistance for a clamp-meter reading, or all-three for the consistency check.
2. 2 Enter the measured quantities: Type the DC voltage, current, or resistance you have. Leave blank the field the chosen mode will solve for. Zero resistance is rejected so the calculation never divides by zero.
3. 3 Set the time interval: Enter the seconds the resistor spends at the calculated power. E = P * t gives the joule heat; a zero time still returns the watts because the rate drives resistor sizing.
4. 4 Read the results: The panel reports the watts, the missing quantity from Ohm's law, the joule heat, and the next standard resistor rating above 2x the wattage.
5. 5 Pick the physical resistor: Choose a part whose wattage rating meets or exceeds the recommended value. If the recommendation is above 5 W, split the drop across multiple resistors or add a heat sink.

Practical example: a 24 V supply feeds a 350 ohm heater. Choose voltage-and-resistance, enter 24 V and 350 ohm, and the panel returns about 1.65 W, 68.6 mA, 350 ohm, 98.6 J, and a recommended 5 W resistor.

When the resistor value itself is unknown and you need to back it out from conductor geometry, resistivity, and length, Electrical Resistance Calculator returns ohms from the same V and I inputs.

A wattage-first workflow catches the cases where multiplying volts by amps would lie about the resistor's actual heating.

• • Four input modes in one tool: Switch between voltage-and-current, voltage-and-resistance, current-and-resistance, or all-three consistency check without leaving the page.
• • All three power formulas side by side: P = V*I, P = V^2/R, and P = I^2*R use the same inputs so the user picks whichever two variables they measured.
• • Recommended resistor wattage with 2x derating: The next standard resistor rating above 2x the computed power is reported, so the part number picks itself.
• • Joule heat energy over a time interval: E = P * t converts the dissipation rate into the total heat a package, PCB, or enclosure has to absorb.
• • Ohm's law consistency check in all-three mode: When V, I, and R are all entered, the calculator flags any pair that disagrees with V = I*R by more than five percent.
• • Cross-validation with textbook examples: The default 12 V across 24 ohm case returns 6 W and 360 J over 60 s, matching the standard worked example.

The 2x derating rule is the same idea you see in resistor datasheets as 'rated power at 70 degrees C' and in thermal design notes as 'keep junction temperature below 80 percent of maximum'.

When the joule heat from this calculator becomes the input to a work-energy-power problem over a measured displacement, Work Energy Power Calculator carries the same watts through the rest of the physics.

Five factors shape the power dissipation number, plus two limitations worth knowing.

• • The calculator assumes DC or a resistive AC load at unity power factor. For motor, switching, or rectifier loads use an AC-aware wattage tool with a power-factor input.
• • The joule energy output treats the resistor as the only heat path. In a real PCB some heat leaves through copper traces, so steady-state temperature rise is usually lower.

In teaching, the steady-state, single-resistor assumption matches textbook problems. In the field, treat the wattage as a planning number and verify with a thermocouple or thermal camera on the finished board.

According to Wikipedia - Joule Heating, electrical energy is converted into thermal energy as charge flows through a conductor, and the rate of that conversion equals the product of the resistance and the square of the current, which is Joule's first law of heating.

According to Omni Calculator - Power Dissipation, the power dissipated by a resistor can be computed from any two of voltage, current, and resistance using P = V*I, P = V squared over R, or P = I squared R, and the dissipated energy over a time t is E = P*t in joules.

When the load is not a pure resistor and the real power is lower than V * I because of a non-unity power factor, Power Factor Calculator extracts PF and the reactive var from the same measured quantities.