Watt Calculator

About the Watt Calculator

The watt (W) is the SI unit of power, defined as one joule per second. In electrical engineering, power is the rate at which energy is transferred in a circuit. For DC circuits the relationship is straightforward: P = V × I (voltage times current). For AC circuits, the power factor reduces the real power delivered: P = V × I × cos(φ). Three-phase systems multiply by √3.

Power can also be computed from resistance: P = I²R (useful when current and resistance are known) or P = V²/R (when voltage and resistance are known). Mechanical power is calculated as P = τω (torque times angular velocity). All these formulas give the same quantity — the rate of energy conversion — but from different measurable inputs.

This calculator supports five power formulas, AC power factor and three-phase options, dBm conversion for RF work, and energy cost estimation. Compare your result against everyday power references (LED bulb to EV charger) and convert between watts, kilowatts, horsepower, BTU/hour, and more.

When This Page Helps

Power calculations appear in every branch of electrical, mechanical, and thermal engineering. This calculator handles five different power formulas and automatically converts to every common unit. The energy cost estimation helps with real-world planning — just enter hours and electricity rate to see what a device costs to run. It is useful anywhere you need to compare circuit load, motor output, heat transfer, or operating cost.

How to Use the Inputs

1. Select the calculation mode matching your known quantities.
2. Enter voltage and current for the standard P = VI mode.
3. Adjust the power factor and phase for AC circuits (or leave at 1 for DC).
4. Use I²R or V²/R modes when you know resistance instead of both V and I.
5. Enter hours of operation and electricity rate for cost estimation.
6. Read the output in watts plus all common unit conversions.
7. Compare your result on the power scale visual.

Example Calculation

Tips & Best Practices

• For AC circuits, the power factor matters: a motor drawing 10 A at 120 V with PF 0.8 only uses 960 W of real power, not 1,200 W.
• Three-phase power is √3 times single-phase for the same voltage and current — a major efficiency advantage in industrial systems.
• dBm is commonly used in RF: 0 dBm = 1 mW, 30 dBm = 1 W, 40 dBm = 10 W.
• To estimate monthly cost: power in kW × hours/day × 30 × rate.
• Horsepower varies by definition: mechanical HP = 745.7 W, metric HP = 735.5 W, boiler HP = 9,810 W.
• For battery-powered devices, divide watt-hours by voltage to get ampere-hours (Ah) for battery sizing.

Power in AC Versus DC Circuits

DC circuit power is simply P = V × I. AC circuits complicate this because voltage and current may be out of phase. Real power (watts) = V × I × cos(φ), reactive power (VAR) = V × I × sin(φ), and apparent power (VA) = V × I. The power triangle relates these: S² = P² + Q². Only real power does useful work; reactive power oscillates between source and load.

Electrical Safety and Power Ratings

Every component has a maximum power rating. Resistors are rated in watts (¼ W, ½ W, 1 W, etc.); exceeding the rating causes overheating and failure. Wire has a current-carrying capacity (ampacity) determined by allowable temperature rise — which is fundamentally a power dissipation (I²R) issue. Circuit breakers trip at current levels chosen to keep wiring below its temperature rating.

Power Measurement Instruments

A wattmeter measures real power by multiplying instantaneous voltage and current. Modern digital power analyzers sample both at high rates and compute true RMS power, power factor, harmonics, and efficiency. For RF, power meters use thermistors or diodes to measure average power in dBm. Kill-A-Watt devices plug inline to measure household device power consumption and accumulated energy.

Frequently Asked Questions

• What is the difference between watts and volt-amps?

  Watts measure real power (energy actually consumed). Volt-amps (VA) measure apparent power (V × I regardless of phase). The ratio is the power factor: W = VA × PF. For a purely resistive load, W = VA.

• How do I convert watts to amps?

  Divide power by voltage: I = P/V for DC or single-phase AC with PF = 1. For AC with a power factor: I = P/(V × PF). For three-phase: I = P/(√3 × V × PF).

• What is a kilowatt-hour?

  A kWh is an energy unit: 1 kW consumed for 1 hour = 1 kWh = 3.6 MJ. It is the billing unit for electricity. A 100 W bulb running 10 hours uses 1 kWh.

• Why does my power factor matter?

  Low power factor means you draw more current than needed for the real power consumed. This increases I²R losses in wiring, requires larger conductors, and may incur utility surcharges for industrial customers.

• How is mechanical power related to electrical power?

  Both are measured in watts. An electric motor converts electrical power P = VI into mechanical power P = τω minus losses. Motor efficiency is typically 80-95%, so mechanical output is less than electrical input.

• What is dBm and when is it used?

  dBm is a logarithmic power unit referenced to 1 milliwatt: P(dBm) = 10 × log₁₀(P(mW)). It is used in RF engineering, telecommunications, and fiber optics because it simplifies gain/loss calculations to addition/subtraction.