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Guide

How to Convert Between Energy Units (joules, calories, kWh, BTU)

Learn the main energy units, how they relate, and how to convert between them for food, electricity and engineering.

Open the Energy Converter

An energy converter translates values between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, electron-volts, BTU, and foot-pounds. Different fields use different units for the same physical quantity.

Common energy units

  • Joule (J) — the SI unit of energy.
  • Calorie (cal) and kilocalorie (kcal) — food energy; 1 kcal = 4184 J.
  • Watt-hour (Wh) and kilowatt-hour (kWh) — electrical energy; 1 kWh = 3,600,000 J.
  • BTU — heat energy, common in HVAC; 1 BTU ≈ 1055 J.
  • Electron-volt (eV) — atomic and particle physics.

Key conversion factors

  • 1 kcal = 4.184 kJ = 4184 J.
  • 1 kWh = 3.6 MJ = 3412 BTU.
  • 1 BTU ≈ 1055 J ≈ 0.252 kcal.
  • 1 eV ≈ 1.602 × 10⁻¹⁹ J.

How to use the converter

  1. Enter a value and choose its unit.
  2. Choose the target unit.
  3. Read the converted energy.

Examples

A 250 kcal snack provides about 1046 kJ. A 10 kWh battery stores 36 MJ or about 34,120 BTU. A 9000 BTU air conditioner uses about 2.64 kWh of energy per hour of full-power cooling.

When an energy converter is useful

  • Comparing food energy labels across regions.
  • Converting electricity usage into other units.
  • Sizing heating, cooling, or battery systems.
  • Physics and engineering homework.

Put it into practice

Use the Energy Converter right now — free, in your browser, no sign-up required.

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