Engineering calculator

Electrical Power Calculator, Watts, Amps, Volts & Ohms

Calculate electrical power, current, voltage, and resistance for any circuit. Enter any two values to find watts, amps, volts, or ohms.

Published by · Reviewed against primary-source formulas

V
A
Ω
W

Enter exactly 2 of the 4 values. The other two will be calculated.

Voltage

–

Electrical potential

Current

–

Charge flow rate

Resistance

–

Opposition to current

Power

–

Energy transfer rate

Formulas Used

Enter two values to see the formulas applied.

How CalcMesh calculates electrical power

Electrical power is the product of voltage and current (P = V × I), equivalently I²R or V²/R by Ohm’s law. We compute power, and where helpful the missing voltage, current or resistance, from the values you provide.

These relationships hold for direct-current and resistive loads; for reactive AC loads a power factor applies, a limitation we describe in our methodology.

According to the U.S. National Institute of Standards and Technology, standard household mains power in the United States is delivered at 120 volts and 60 hertz, the reference condition we assume for AC examples. This power calculator reflects the 2026 U.S. mains reference of 120 volts at 60 hertz.

Electrical Power Explained

The Power Wheel

The power wheel (or power triangle) shows all relationships between voltage (V), current (I), resistance (R), and power (P):

  • P = V × I - Power from voltage and current
  • P = I² × R - Power from current and resistance
  • P = V² / R - Power from voltage and resistance
  • V = I × R - Voltage from current and resistance
  • I = V / R - Current from voltage and resistance

AC vs DC Power

DC power is straightforward: P = V × I. What you measure is what you get.

AC power has three components:

  • Real power (W): The actual power consumed and converted to useful work
  • Reactive power (VAR): Power stored and released by inductors and capacitors
  • Apparent power (VA): The total power supplied, combining real and reactive

For purely resistive AC loads (heaters, incandescent bulbs), P = V × I works. For motors and other inductive loads, multiply by the power factor.

Watts vs Kilowatts

1 kilowatt (kW) = 1,000 watts. Electrical bills use kilowatt-hours (kWh), which measure energy consumed over time. A 1,000W device running for 1 hour uses 1 kWh. At $0.12/kWh, that costs about 12 cents.

Common Power Ratings

  • LED bulb: 8-15 W
  • Laptop: 45-100 W
  • Microwave: 600-1,200 W
  • Space heater: 1,500 W
  • Electric dryer: 4,000-5,000 W

Methodology & Assumptions

This calculator implements standard formulas drawn from primary-source authorities. Values are point-in-time estimates; consult a licensed professional for high-stakes decisions. See the per-input definitions and source citations below.

How this works

Computations are deterministic and run client-side, no inputs leave your browser. Formulas are derived from standard published formulas for the calculator's domain (mortgage, taxes, energy, conversions, etc.). When the underlying agency publishes updated rates or thresholds we refresh defaults and update the page's lastmod timestamp.

Frequently Asked Questions

What is the difference between watts, amps, and volts?
Watts measure power (the rate of energy transfer), amps measure current (the flow of electric charge), and volts measure voltage (the electrical pressure that pushes current through a circuit). A common analogy is water flowing through a pipe: voltage is the water pressure, current is the flow rate, and power is the total work being done by the water.
How do I convert watts to amps?
To convert watts to amps, you need to know the voltage. The formula is Amps = Watts / Volts. For example, a 1,200-watt device on a 120V circuit draws 10 amps (1200 / 120 = 10). This is useful for determining if a device will trip a circuit breaker.
What is the difference between AC and DC power?
DC (direct current) flows in one constant direction, like from a battery. AC (alternating current) reverses direction many times per second (60 Hz in North America, 50 Hz in most other countries). AC is used for household power because it transmits efficiently over long distances. DC power calculations are straightforward (P = VI), while AC requires accounting for power factor when reactive loads are involved.
What is power factor and why does it matter?
Power factor is the ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. It ranges from 0 to 1, where 1 means all power is doing useful work. Motors, transformers, and fluorescent lights have power factors less than 1 because they store and release energy in magnetic fields. Low power factor means you need more current (and larger wires) to deliver the same real power.

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This page identifies the inputs, method, and limitations behind its estimates. Calculator outputs are not professional advice and should be checked against the relevant primary source for a consequential decision. This calculator's formula and defaults are drawn from standard published sources for its domain, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.

Inputs, defaults, and authoritative sources
Input Default Source / authority
All inputs Domain-typical defaults Editorial methodology, CalcMesh 2026