What formula does this tool use?

The calculator uses E(J) = P(W) × t(s). Power in watts multiplied by time in seconds gives energy in joules. When you select minutes or hours, the tool converts the duration to seconds before applying the formula.

What is the difference between watts and joules?

Watts measure the rate of energy transfer (power), while joules measure the total amount of energy. One watt equals one joule per second (1 W = 1 J/s). A 100 W bulb running for 10 seconds uses 1,000 J of energy.

How do I convert watts to kilowatt-hours?

Multiply watts by time in hours to get watt-hours, then divide by 1,000 to get kilowatt-hours. Alternatively, enter the values here and the result panel shows kilowatt-hours automatically.

Can I use this for AC circuits?

Yes, but enter the real (active) power in watts, not apparent power in VA. For AC loads, real power P = V × I × power-factor. This tool assumes the wattage you enter already reflects actual power consumption.

Why would I need milliseconds?

Milliseconds are useful for short-duration events such as laser pulses, camera flash discharges, capacitor dumps, and switching transients where power is high but duration is very brief.

How many joules are in a kilowatt-hour?

One kilowatt-hour equals 3,600,000 joules (3.6 MJ). This is because 1 kW = 1,000 W and 1 hour = 3,600 seconds, so 1 kWh = 1,000 × 3,600 = 3,600,000 J.

Watts to Joules Calculator

Enter power in watts and a time duration to instantly calculate energy in joules using E = P × t.

Input Parameters

Power (W)

Time

Quick Examples

Result

Enter values and press Calculate

Formula Reference

Core Formula

E (J) = P (W) × t (s)

With Minutes

E (J) = P (W) × t (min) × 60

With Hours

E (J) = P (W) × t (hr) × 3600

Result copied!

Summary

This calculator converts power in watts (W) to energy in joules (J) by multiplying power by time using the formula E = P × t. The result is shown in joules along with equivalent values in kilojoules, watt-hours, and kilowatt-hours. It is useful for electrical engineering, physics problems, battery sizing, and any scenario where you need to know the total energy delivered by a device over a given period.

How it works

Enter the power value in watts (W).
Select the time unit: seconds, minutes, hours, or milliseconds.
Enter the time duration.
The calculator applies E = P × t (with time converted to seconds).
Results are displayed in joules, kilojoules, watt-hours, and kilowatt-hours.
Use the Reset button to start a new calculation.

Use cases

Calculate energy consumed by an appliance over a known run time.
Determine battery capacity requirements for a given load.
Estimate heat generated by a resistive element in a set interval.
Convert power ratings to energy for physics coursework.
Size a UPS system based on total joule demand.
Evaluate energy output of solar panels over a time window.
Calculate laser pulse energy from power and pulse duration.
Estimate motor work output during an acceleration cycle.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu