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Energy Converter

Swap joules, calories, kilocalories, watt-hours, kilowatt-hours, and electronvolts accurately.

About Energy Converter

Energy appears in a bewildering variety of units depending on context: physicists use joules, nutritionists use kilocalories, electricians use kilowatt-hours, HVAC engineers use BTUs, and particle physicists use electron-volts. The word "calorie" alone means two completely different things — a single value of "250 cal" versus "250 kcal" differs by a factor of 1,000. This converter links all the major units in one shared input: J, kJ, MJ, cal, kcal, kWh, Wh, BTU, and eV. Type in any field and every other unit updates instantly using NIST-exact factors. Real-world anchors — 1 kWh = 3.6 MJ, 1 food Calorie = 4.184 kJ — help ground unfamiliar conversions. Whether reading a nutrition label, auditing an electricity bill, sizing a heating system, or teaching physics, all the units you need are a single input away.

Why use Energy Converter

Common & Scientific

Bridges the gap between everyday energy units (kcal, kWh, BTU) and scientific ones (J, kJ, eV) in a single interface. No more juggling separate tools for nutrition, electricity, and physics energy values.

Food vs Physics Calorie

Explicitly distinguishes the small gram calorie (cal = 4.184 J) from the food kilocalorie (kcal = 4,184 J). The tool labels them clearly so you never accidentally mistake a nutrition value for a physics value or vice versa.

Live Conversion

Every unit field updates on every keystroke with no submit button needed. Watching how 250 kcal translates into kJ, kWh, and BTU simultaneously makes the relative scale of energy units immediately intuitive.

Real-world Anchors

Quick-reference callouts like '1 kWh = 3.6 MJ' and '1 food Calorie = 4.184 kJ' are displayed near the relevant fields to help sanity-check conversions without needing to memorise the exact factors.

Engineering Precision

Six-decimal-place output with NIST-exact conversion factors ensures no rounding surprises when converting large energy values for power-system design, laboratory measurement, or financial energy auditing.

Privacy First

All calculations happen locally in your browser. No data leaves your device. There is no account, no tracking, and no server involved — type freely without concern about where your numbers go.

How to use Energy Converter

  1. Select the source unit from the dropdown or click directly into any unit field.
  2. Type the energy value you want to convert.
  3. Read all other unit fields update simultaneously with NIST-exact conversions.
  4. For nutrition contexts, use the 'kcal (food Calorie)' field — not the small 'cal' field.
  5. Adjust decimal precision for scientific work requiring more significant figures.
  6. Copy any result with its unit label for paste into a report, formula, or spreadsheet.

When to use Energy Converter

  • Converting a food-label kilocalorie value into kilojoules for a European nutrition database.
  • Calculating how many BTUs a space heater produces when the spec is given in watts.
  • Translating a monthly electricity consumption in kWh into joules for an energy-density comparison.
  • Understanding the energy in an electron-volt for an introductory atomic physics assignment.
  • Cross-checking whether a nutrition app's kJ display matches the kcal on a packaged product.
  • Estimating the energy stored in a battery (Wh) relative to a food Calorie for a science class demonstration.

Examples

Food calorie to kJ

Input: 250 kcal (food)

Output: 1,046 kJ = 0.291 kWh = 992 BTU

Electric bill

Input: 1 kWh

Output: 3,600,000 J = 3,600 kJ = 859,845 cal = 3,412 BTU

Tiny scale

Input: 1 eV

Output: 1.602 × 10⁻¹⁹ J

Tips

  • On food labels, '1 Calorie' (capital C) always means 1 kcal = 1,000 small calories — the distinction matters for any calculation.
  • 1 kWh = 3.6 MJ is a fundamental conversion worth memorising for energy-system and electricity-bill math.
  • BTU is primarily used in US HVAC and gas-appliance ratings; most of the rest of the world uses kJ or kWh for the same specs.
  • Power (W) × time (s) = energy (J) — never add watts and joules as if they were the same unit.
  • Electron-volts are convenient for atomic-scale energies because the joule values would require scientific notation with large negative exponents.

Frequently Asked Questions

What is the difference between calorie and Calorie?▾
A lowercase calorie (cal) is the gram calorie equal to 4.184 joules — a small amount of energy used in physics and chemistry. An uppercase Calorie (Cal or kcal) is the kilocalorie equal to 4,184 joules and is the unit used on food nutrition labels.
How many joules are in 1 kWh?▾
Exactly 3,600,000 joules (3.6 MJ). One watt equals one joule per second; one kilowatt-hour is 1,000 watts running for 3,600 seconds, giving 3.6 million joules.
Why do food labels use kcal but say 'Calories'?▾
Food 'Calories' (capital C) have always meant kilocalories, a convention dating to 19th-century nutritional science. The European Union now mandates both kcal and kJ on labels to reduce this ambiguity.
What is 1 BTU in joules?▾
1 BTU (British Thermal Unit) = 1,055.06 joules. It was originally defined as the energy needed to raise 1 pound of water by 1 °F. BTUs are still common in US HVAC ratings, gas appliance specs, and air conditioner capacity.
How is energy different from power?▾
Energy is the capacity to do work, measured in joules or kWh. Power is the rate of energy transfer, measured in watts (joules per second) or kilowatts. Energy = Power × Time. A 1 kW heater running for 1 hour uses 1 kWh of energy.
What's an electron-volt used for?▾
An electron-volt (eV) is the kinetic energy gained by one electron accelerating through a 1-volt potential difference, equal to about 1.602 × 10⁻¹⁹ joules. It is the standard unit in atomic, nuclear, and particle physics because joules produce unwieldy small numbers at those scales.
How do I read my electricity bill in joules?▾
Electricity bills report consumption in kilowatt-hours. Multiply by 3,600,000 to convert to joules, or by 3,600 to get kilojoules. A typical household using 300 kWh/month consumes about 1.08 × 10⁹ joules (1,080 MJ) per month.
What's the SI unit of energy?▾
The joule (J) is the SI unit of energy, defined as one kilogram times metres squared per second squared (kg·m²/s²). It is also equal to one watt-second and one newton-metre.

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Glossary

Joule (J, SI Energy)
The SI derived unit of energy equal to one Newton-metre or one watt-second (kg·m²/s²). All other energy units are defined as exact multiples or fractions of the joule, making it the universal reference for energy conversion.
Calorie (cal)
The gram calorie, equal to exactly 4.184 joules. It represents the energy needed to raise 1 gram of water by 1 °C at standard pressure. Not to be confused with the food Calorie (kilocalorie).
Kilocalorie (kcal / 'food calorie')
Equal to 1,000 gram calories or 4,184 joules. Nutrition labels list energy in kcal, often written as 'Calories' (capital C). The European Union additionally requires kJ on packaged food labels.
Kilowatt-hour (kWh)
A practical energy unit equal to exactly 3,600,000 joules (3.6 MJ). Used universally for electricity billing and battery capacity. One kWh is the energy consumed by a 1,000-watt device running for one hour.
British Thermal Unit (BTU)
A legacy energy unit approximately equal to 1,055.06 joules, defined as the heat needed to raise one pound of water by one degree Fahrenheit. Widely used in US HVAC ratings, boiler capacities, and natural-gas billing.
Electron-volt (eV)
An energy unit equal to the kinetic energy acquired by a single electron accelerating through a 1-volt potential difference (~1.602 × 10⁻¹⁹ joules). The standard unit in atomic, nuclear, and particle physics where joule values are impractically small.