By Chris Deziel • Updated Aug 30, 2022
Power, measured in watts, is a fundamental concept in both mechanical and electrical engineering. This guide explains how to calculate watts using simple, reliable equations, while ensuring clarity, precision, and real-world applicability.
The watt was named after James Watt, the Scottish engineer who revolutionized steam engines. He defined power as the product of force and velocity:
P = F × v
In the International System of Units (SI), a watt is one joule per second. Since a joule equals a newton‑meter, the relationship can also be expressed as 1 W = 1 N·m/s. For historical context, Watt’s original unit of horsepower was the power required to lift 33,000 pounds one foot in one minute.
When working with mechanical systems, you can use either of the following equations, provided all measurements are in SI units (meters, kilograms, seconds):
P = F × v
P = W / t
If your data come from the CGS system (centimeters, grams, seconds), convert dynes to newtons (1 dyne = 10-5 N) and ergs to joules (1 erg = 10-7 J) before applying the formulas. Power can also be expressed in kilowatts; simply multiply watts by 1,000.
For electrical applications, the basic power formula is:
P = V × I
Using Ohm’s law (V = I × R) you can rewrite power in terms of voltage and resistance or current and resistance:
P = I² × R
P = V² / R
To ensure accuracy, express voltage in volts, current in amperes, and resistance in ohms. A watt can also be denoted as 1 volt‑ampere, 1 amp²‑ohm, or 1 volt²/ohm. For quick calculations, many professionals use online wattage calculators.
Whether you’re optimizing a mechanical system or designing an electrical circuit, these equations provide a reliable foundation for measuring power in watts.
