OctaCalc / Power Calculator

Power Calculator

Calculate electrical power in watts by entering voltage and current — the power calculator applies P = V × I instantly.

Power
—W

For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.

How to Use This Power Calculator

Enter the voltage across your component or circuit in volts, and the current flowing through it in amps. Click Calculate Power to get the power dissipated or consumed in watts. If you know the current in milliamps, divide by 1000 first (e.g. 150 mA = 0.15 A).

Power Formula

Electrical power is the rate of energy transfer. For a DC circuit:

P = V × I

Power can also be expressed as P = I²R or P = V²/R using Ohm's Law substitutions.

Example

Scenario: A 12V motor drawing 2.5 A
P = V × I = 12 × 2.5
P = 30 W

Small circuit: 3.3V microcontroller drawing 80 mA (0.08 A)
P = 3.3 × 0.08
P = 0.264 W (264 mW)

Frequently Asked Questions

What is the power formula used in this calculator?
This calculator uses P = V × I, where P is power in watts, V is voltage in volts, and I is current in amps. This is the fundamental power equation for DC circuits.

Can I use this calculator for AC circuits?
For simple AC circuits with purely resistive loads, P = V × I still applies using RMS values. For circuits with reactive components (inductors or capacitors), you would also need to account for power factor, which this calculator does not cover.

My current is in milliamps — what do I enter?
Divide your milliamp value by 1000 to convert to amps before entering it. For example, 250 mA becomes 0.25 A.

What is the difference between watts and milliwatts?
A milliwatt (mW) is one-thousandth of a watt. Small components like LEDs and microcontrollers often consume power in the milliwatt range. To convert, multiply watts by 1000 — for example, 0.05 W = 50 mW.

How does this relate to Ohm's Law?
Power, voltage, current, and resistance are all linked. If you know resistance instead of current, you can derive power using P = V²/R or P = I²R — both are rearrangements of P = V × I combined with Ohm's Law (V = I × R).