Power Dissipation Calculator — P = I²R, V²/R, VI
Calculate how much power a resistor or resistive component dissipates as heat. Enter any two of voltage, current or resistance — the calculator derives the third and shows power in watts plus energy per hour, day and month.
V
A
Ω
P = V × I = I²R = V²/R
- 1
Formula: P = V × I
Both voltage and current are known — apply P = V × I. - 2
Power = V × I
12 × 2 = 24
How does this calculator work?
P = V × I = I²R = V²/R. Enter any two of voltage (V), current (A) or resistance (Ω) to get dissipated power in watts. The calculator also shows energy consumption per hour, day and month — useful for cost estimation of heaters, motors and electronic components.
Formula
How this is calculated
When electric current flows through a resistance, electrical energy is converted to heat — a process known as Joule heating (or ohmic dissipation). The rate of energy conversion is given by three equivalent formulas: P = V × I, P = I² × R, and P = V² / R. All three are consistent with Ohm's law (V = I × R) and give the same power when any two of V, I and R are known.
This calculator accepts any two of the three electrical quantities, applies Ohm's law to derive the missing one, and then computes dissipated power using the most convenient form of the formula. It also shows the energy dissipated per hour (in joules, since 1 W = 1 J/s), per day (in watt-hours) and per 30-day month (in kilowatt-hours) — useful for estimating operating costs of resistive heaters, incandescent bulbs, or electronic components.
Assumptions and limitations: the calculator assumes a linear (ohmic) resistor at constant temperature and DC conditions. In AC circuits, the formula P = V × I applies to RMS values of voltage and current for purely resistive loads. For loads that include inductors or capacitors (motors, power supplies), true power depends on the power factor cos(φ): P = V × I × cos(φ). Use the Power Factor Calculator for that case. Resistance also varies with temperature in practice — at high power, a resistor's self-heating can change its resistance (positive temperature coefficient for most metals, negative for NTC thermistors).
Frequently asked questions
Joule heating (P = I²R) is the conversion of electrical energy into heat in a resistive conductor. It is the intended effect in electric heaters, toasters and incandescent bulbs, but an unwanted loss in power cables, wiring and transistors. Engineers minimise I²R losses by using thicker cables (lower R) or higher voltage (lower I for the same power).
For purely resistive AC loads (heaters, incandescent bulbs), yes — enter the RMS voltage and RMS current. For inductive or capacitive loads (motors, transformers, capacitor banks), true power is P = V_rms × I_rms × cos(φ), where cos(φ) is the power factor. The Power Factor Calculator handles that case.
Calculate the expected dissipated power with this tool, then select a resistor rated at least twice the calculated value — a 2× safety derating accounts for tolerance, ambient temperature, and component lifetime. Common resistor ratings are ¼ W, ½ W, 1 W and 2 W.
Also known as
TG we-Calculate Editorial Team. (2026). Power Dissipation Calculator — P = I²R, V²/R, VI [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/power-dissipation-calculator
TG we-Calculate Editorial Team. "Power Dissipation Calculator — P = I²R, V²/R, VI." TG we-Calculate. 2026. https://we-calculate.com/calculator/power-dissipation-calculator.
TG we-Calculate Editorial Team, "Power Dissipation Calculator — P = I²R, V²/R, VI," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/power-dissipation-calculator
@misc{wecalculate_power_dissipation_calculator, title = {Power Dissipation Calculator — P = I²R, V²/R, VI}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/power-dissipation-calculator}}, year = {2026}, note = {TG we-Calculate} }
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