Primary formula
P = V x I; P = I^2 x R; P = V^2 / R.
Physics calculator
Electric Power Calculator
Calculate electric power in watts from voltage-current, current-resistance or voltage-resistance relationships.
Last reviewed: July 21, 2026Physics formula engine v3.0.0
Physics calculator
Electric Power Calculator
Enter values, select units and calculate locally. Results use SI internally and show rounded display values.
Decimal.js physics math
Mode
Find power from voltage and current.
Result and visual stay hidden until you choose Calculate.
Formula and assumptions
Input assumptions
Resistance is greater than zero in resistance modes. Voltage and current are compatible branch values. The calculator uses steady ideal formulas.
Precision note
Physics calculators convert compatible inputs into SI units before calculating. Decimal.js is used for arithmetic where practical; trigonometric angle factors are rounded for display.
Physics calculation flow
Parse
Read labelled inputs, selected units and calculation mode from the calculator card.
Convert
Convert compatible values to SI base or derived units without calling external services.
Calculate
Apply the selected deterministic mechanics formula and validate against NaN, Infinity and invalid divisors.
Label
Display the result, supporting unit rows, formula steps, warnings and dynamic visual below the input card.
What Electric Power Calculator does
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time.
The calculator is deterministic and runs from manual inputs only. It is a physics formula tool, so it should not be confused with a unit converter that only changes labels after a value is already known.
Formula and variables
P = V x I; P = I^2 x R; P = V^2 / R.
The calculator converts supported units into SI base or derived units for the actual calculation, then formats the result in the selected display unit.
- P is electric power
- V is voltage
- I is current
- R is resistance
Worked examples
Use the examples below to check expected order of operations and unit handling before applying the tool to your own values.
- 12 V x 5 A = 60 W
- (2 A)^2 x 10 ohm = 40 W
- (24 V)^2 / 12 ohm = 48 W
Unit conversions
Length, time, mass, area, volume, speed, force, pressure, density, frequency, energy and power units reuse the central NexaCalc conversion registry. Acceleration, angle and spring-constant units use the same SI-first convention where applicable.
The displayed conversion rows are supporting context. They do not change the physical assumptions of the selected formula.
Common mistakes
Most errors come from mixing physical quantities that sound similar in everyday language or choosing a unit that belongs to a different dimension.
- Confusing watts with watt-hours.
- Using peak AC values where RMS values are intended.
- Ignoring resistance heating or device ratings.
Assumptions and limitations
This page is intended for educational and planning calculations, not professional engineering certification.
This calculator uses idealized physics formulas and the input values you provide. Real systems may require additional effects such as friction, drag, rotation, deformation, losses or measurement uncertainty.
- Resistance is greater than zero in resistance modes.
- Voltage and current are compatible branch values.
- The calculator uses steady ideal formulas.
- This calculator does not model AC phase angle, power factor, heat dissipation or wiring limits.
- Safety-critical electrical work requires proper measurement and qualified review.
Frequently asked questions
How is electric power calculated?
Electric Power Calculator uses P = V x I; P = I^2 x R; P = V^2 / R. The result is calculated from the values and units entered in the form.
What is the difference between watts and watt-hours?
Electric Power Calculator converts compatible inputs to SI internally, then displays the selected output unit. P is electric power; V is voltage; I is current; R is resistance.
When should I use P = VI?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
When should I use P = I squared R?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
When should I use P = V squared over R?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
Does this calculator handle AC power factor?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
How does Electric Power Calculator relate to Ohm's Law?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
Can current or voltage be negative?
Electric Power Calculator allows signed results only where the physics quantity can carry direction or reference-sign meaning. Divisors such as time, mass and gravity are validated before calculation.
What units are supported?
Electric Power Calculator converts compatible inputs to SI internally, then displays the selected output unit. P is electric power; V is voltage; I is current; R is resistance.
Can this calculator size wires or fuses?
Use this calculator when the question is power transfer in an electrical circuit rather than mechanical work over time. Resistance is greater than zero in resistance modes.
References
- NIST Special Publication 811, Guide for the Use of the International System of Units. Relevance: SI unit symbols and derived-unit naming. Last verified: July 21, 2026. Source.
- BIPM, The International System of Units SI Brochure. Relevance: SI base and derived units used in mechanics, fluids, waves and optics. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 1. Relevance: introductory formulas for motion, force, work, energy, momentum, fluids, waves and optics. Last verified: July 21, 2026. Source.
- NIST CODATA internationally recommended values. Relevance: physical constants for electricity, modern physics, gravitation and radiation formulas. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 2. Relevance: electric circuits, capacitance, magnetic force and electromagnetic waves. Last verified: July 21, 2026. Source.
- OpenStax University Physics Volume 3. Relevance: photons, nuclear decay and blackbody radiation concepts. Last verified: July 21, 2026. Source.
Physics references verified for SI units, mechanics, fluids, waves, sound and optics formulas on July 21, 2026.
Educational disclaimer
This calculator provides mathematical results from the values, conventions and methods you enter. Verify important academic, engineering or professional work independently.