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Energy Converter

Convert energy units: joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU, and electronvolts.

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How to use Energy Converter

  1. Enter an amount of energy in its row — joules, kilojoules, calories, kcal, Wh, kWh, BTU, or eV.
  2. All eight rows recompute through the joule, so they stay in lockstep with your figure.
  3. A label says “Calorie” with a capital C? Type that number into the kcal row, not the cal row — they differ by a factor of 1,000.
  4. To compare gas heating (rated in BTU) with electric (rated in kWh), convert both to the same unit here, then price each by its own per-unit rate before deciding.

Energy converter: joules, calories, kilowatt-hours, and BTU

This converter links eight energy units that span physics, nutrition, electricity, and heating: joules, kilojoules, calories, kilocalories (food Calories), watt-hours, kilowatt-hours, BTU, and electronvolts. They all measure the same quantity — energy — so any value translates exactly into any other. Below are the precise factors and where each unit shows up.

The joule is the base unit

Every conversion here runs through the joule (J), the SI unit of energy. One joule is small: it is roughly the energy to lift a 100 g apple one meter. Because real-world energy comes in vastly different sizes, the table also offers kilojoules, kilocalories, and kilowatt-hours for human-friendly numbers.

Key energy conversion factors

  • 1 kJ = 1,000 J
  • 1 cal = 4.184 J (the thermochemical calorie)
  • 1 kcal = 1 food Calorie = 4,184 J = 4.184 kJ
  • 1 Wh = 3,600 J
  • 1 kWh = 3,600,000 J = 3,412 BTU
  • 1 BTU = 1,055.06 J
  • 1 eV = 1.602 × 10⁻¹⁹ J

Nutrition: reading food energy

Food labels show energy in both kJ and Calories (kcal). To convert, multiply kcal by 4.184 to get kJ. A label reading 500 kJ is 119.5 kcal. The capital-C "Calorie" on US labels is always the kilocalorie — the small calorie is only used in chemistry.

Electricity and appliances

Your power bill is in kilowatt-hours. To estimate an appliance's daily energy, multiply its power in watts by hours used and divide by 1,000. A 60 W bulb left on 5 hours uses 0.3 kWh. Converting that to joules (1,080,000 J) or BTU (1,024 BTU) is rarely needed for billing, but it is essential when comparing electric and gas heating, where gas is rated in BTU.

Heating, cooling, and BTU

Air conditioners and furnaces are rated in BTU per hour. A 12,000 BTU/h unit (one "ton" of cooling) moves about 3.5 kW of heat. To compare it with an electric heater rated in watts, convert: 12,000 BTU/h × 0.293 = 3,517 W. This converter handles the energy side; divide by time to get power.

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Frequently asked questions

Calorie vs kilocalorie — which is on food labels?
Food labels use the kilocalorie but write it as "Calorie" with a capital C. So a 250 "Calorie" snack is 250 kcal = 250,000 thermodynamic calories = about 1,046 kJ. The small calorie (cal) is 4.184 J.
What is a BTU?
A British Thermal Unit is the energy needed to raise one pound of water by one degree Fahrenheit — about 1,055 J. It is the standard unit for heating, air-conditioning, and gas appliance ratings in the US.
How many joules are in a kilowatt-hour?
Exactly 3,600,000 J (3.6 MJ). A kWh is the energy of one kilowatt sustained for one hour, and it is the unit your electricity meter bills in.
How do I convert kWh to BTU?
Multiply kWh by 3,412.14. For example, a 1.5 kWh heater run delivers about 5,118 BTU of heat. To go back, divide BTU by 3,412.14.
What is an electronvolt used for?
An electronvolt (eV) is a tiny unit — 1.602 × 10⁻¹⁹ J — used in physics and chemistry for the energy of single particles, photons, and chemical bonds. Visible light photons carry roughly 1.5 to 3.5 eV.
How much energy is in food versus electricity?
They use the same physics. A 2,000 kcal daily diet is about 8,368 kJ, or 2.3 kWh — roughly what a small space heater uses in just over two hours. Energy is energy; only the unit changes.
Why do gas bills sometimes show therms?
A therm is 100,000 BTU ≈ 105.5 MJ ≈ 29.3 kWh. Convert BTU to therms by dividing by 100,000, then to kWh if you want to compare gas and electricity costs on the same scale.

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