Energy Unit Converter

Engineering, physics & everyday use

Convert energy between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU (IT), and advanced physics units such as electronvolts (eV) and Hartree energy.

Standards: thermochemical calorie (1 cal = 4.184 J), international-table BTU (1 BTU = 1055.056 J), exact 1 eV = 1.602176634×10⁻¹⁹ J.

Convert between joules, calories, kWh, BTU & more

Single input → all units updated

Enter the energy quantity you want to convert (any positive or negative real number).

Nutrition “Calories” on labels.

Atomic scale energy unit.

Presets:

Quick reference: common energy equivalences

The table below lists approximate relationships between some of the most common energy units. All values are rounded for clarity.

Quantity Approximate equivalents
1 kWh 3.6 MJ ≈ 860 kcal ≈ 3412 BTU
1 BTU (IT) ≈ 1055 J ≈ 0.252 kcal ≈ 0.000293 kWh
1 kcal 4184 J ≈ 0.001163 kWh ≈ 3.968 BTU
1 J ≈ 0.2390 cal ≈ 2.7778×10⁻⁴ Wh ≈ 9.478×10⁻⁴ BTU
1 eV 1.602×10⁻¹⁹ J (exact, SI definition)

Energy units and their relationships

Energy can be measured in many different units depending on the field: joules in physics, calories in nutrition, kilowatt-hours on electricity bills, BTU in HVAC, or electronvolts and Hartree in quantum chemistry. This converter keeps everything tied to the SI base unit, the joule.

Fundamental definitions used by this calculator

Base relationships

1 J = 1 J (by definition)

1 kJ = 1000 J

1 MJ = 1,000,000 J

Heat & chemistry

1 cal (thermochemical) = 4.184 J

1 kcal = 4184 J = 1000 cal

1 BTU (international table) = 1055.056 J

Electrical energy

1 Wh = 3600 J

1 kWh = 3.6×10⁶ J

Physics units

1 eV = 1.602176634×10⁻¹⁹ J (exact)

1 Hartree (Eₕ) ≈ 4.3597447×10⁻¹⁸ J

General conversion procedure

All conversions follow a simple two-step approach:

  1. Convert the input value to joules using the appropriate factor.
  2. Convert joules to the desired target unit by dividing by its factor in joules.

For example, to convert 2 kWh to BTU:

  • 2 kWh = 2 × 3.6×10⁶ J = 7.2×10⁶ J
  • BTU = J / 1055.056 ≈ 7.2×10⁶ / 1055.056 ≈ 6825 BTU

Worked example: 500 kcal to kWh

  1. 500 kcal = 500 × 4184 J = 2,092,000 J.
  2. Wh = J / 3600 ≈ 581.1 Wh.
  3. kWh = Wh / 1000 ≈ 0.581 kWh.
  4. In the calculator, set From: kcal, Value = 500. The kWh output shows approximately 0.581.

FAQ: using the energy unit converter

Which calorie definition does this tool use?

The converter uses the thermochemical calorie, defined as exactly 4.184 joules. This is consistent with most modern references and nutrition calculations. When you see kcal on food labels, it is simply 1000 of these calories.

Why are there small differences between different websites?

Differences usually come from rounding and from slightly different historical definitions of units such as BTU and the calorie. This tool uses widely accepted standard values and keeps good internal precision, while rounding displayed values to a reasonable number of significant digits.

Can negative energy values be converted?

Yes. Negative values appear naturally in many physics and engineering contexts (for example, work done by a system or potential energy changes). The converter treats negative and positive values in exactly the same way.

When should I use eV instead of joules?

Electronvolts are convenient whenever energies are at the atomic or sub-atomic scale: band gaps in semiconductors, binding energies in nuclei, photon energies in spectroscopy. Converting back to joules is straightforward using the exact factor built into this tool.