Energy Converter

Using the Energy Converter

1. Pick the source unit. The selector lists joule (SI base), kilojoule, megajoule, calorie, kilocalorie (food Calorie), kilowatt-hour, British Thermal Unit (BTU), therm and electronvolt - the units you meet in physics, nutrition, HVAC, electricity bills and particle physics respectively.
2. Type the amount. The result recomputes on every keystroke because the handler is wired to onInput rather than a submit button.
3. Pick the target unit. Both directions use the same joule-pivoted factor table, so swapping "From" and "To" is cosmetic, not algorithmic.
4. Swap with the arrow button to reverse direction.
5. Copy the output to the clipboard via the Clipboard API.

NIST kilowatt hour to joule: why 1 kWh = 3.6e6 J

The factor 1 kWh = 3.6e6 J is exact, not measured. The derivation, from the SI Brochure (BIPM, 9th edition) and NIST Special Publication 811 (the canonical US guide for SI units), uses only definitions. The watt is the coherent SI unit of power, defined as one joule per second: 1 W = 1 J/s. A kilowatt is therefore 1000 J/s. The hour is a non-SI unit accepted for use with the SI, defined as exactly 3600 s. Multiplying power by time gives energy: 1 kW × 1 h = 1000 J/s × 3600 s = 3 600 000 J. Written in scientific notation that is 3.6e6 J, equivalently 3.6 MJ. There is no temperature correction, no purity assumption, and no measurement uncertainty - everything in the chain (watt, kilo-, hour, second) is exact by definition in the post-2019 SI, where the joule, the second and the kilogram are all anchored to fixed numerical values of fundamental constants. The energy converter on this page uses the literal 1 kWh = 3.6e6 J factor in its joule-pivoted table; any value you enter in kWh is multiplied by 3.6e6 to land in joules, and any joule value is divided by 3.6e6 to land in kWh. NIST SP 811 Appendix B lists the same factor in its energy-and-work conversion table, with the kWh marked as exact.

What the Arithmetic Actually Does

Energy is a coherent SI derived quantity measured in joules (J = kg m^2 / s^2), a point CGPM fixed in 1948 and the 2019 SI redefinition inherited without change. Every unit in the picker is stored as a factor relative to the joule. The factors are: 1 cal_IT = 4.1868 J exactly (IT calorie, the International Steam Table standard used in engineering); 1 kcal = 4184 J (thermochemical kilocalorie used on food labels, IUPAC); 1 kWh = 3.6e6 J exactly (watt-second is joule, kilowatt-hour is 1000 * 3600 watt-seconds); 1 BTU_IT = 1055.05585262 J (ISO 31-4); 1 therm = 10^5 BTU = 1.055057e8 J (US EIA); 1 eV = 1.602176634e-19 J (exact since the 2019 SI redefinition fixed the elementary charge e). The conversion is result = value * factorFrom / factorTo, performed in V8 / SpiderMonkey / JavaScriptCore as a single float multiply-divide.

When You Reach For This

• Translating a food label's kilocalorie count into joules for a biochemistry homework problem.
• Turning a monthly electricity bill (kWh) into joules to reason about solar-panel output or thermodynamic efficiency.
• Reading HVAC or gas-appliance specs quoted in BTU or therms and cross-checking against a kWh-based tariff.
• Converting a particle physics energy in MeV or GeV into joules to compare with mechanical or thermal quantities.
• Estimating how much kinetic energy a moving object carries - 1/2 m v^2 comes out in joules when m is in kg and v in m/s.
• Sizing a battery bank: ampere-hours times volts is watt-hours, and this converter moves watt-hours to joules to pair with a thermal load.

Pitfalls

Calories have three definitions in the wild. The thermochemical calorie is 4.184 J (used for reaction enthalpies and food labels in the US); the IT calorie is 4.1868 J (steam tables, engineering); the 15-C calorie is 4.1855 J (mercifully rare). A food Calorie - with a capital C - is a kilocalorie, so "200 Calories" on a candy bar is 200,000 small calories, or 836,800 J. This converter uses the thermochemical calorie for "cal" and kcal, matching most nutrition and chemistry work. Other traps: BTU has an IT, thermochemical and 59-F variant; the therm comes in US (100,000 BTU_59) and EC (10^5 MJ) flavors. The page uses BTU_IT and US therm. If your document specifies a different variant, expect drift in the fourth decimal.

Where the Units Came From

The joule was named for James Prescott Joule, whose 1840s paddle-wheel experiments established the mechanical equivalent of heat. The calorie predates it, coined by Nicolas Clement in 1824 as the heat needed to raise one kilogram of water by one degree Celsius - later split into "small calorie" (1 g of water by 1 K) and "large calorie" (1 kg, i.e. a kilocalorie). The BTU crystallized in the late 19th century for coal and steam-engine work. The kilowatt-hour became a unit of utility billing after Edison and Westinghouse popularized electric service in the 1880s. The electronvolt was defined around 1912 for atomic-physics convenience, and the 2019 SI redefinition fixed it exactly by fixing the elementary charge at e = 1.602176634 x 10^-19 C (SI Brochure, BIPM 9th edition). NIST SP 811 is the canonical reference for all the numeric values in this converter.

Alternatives

For a single lookup, Google or Wolfram Alpha will answer faster than navigating here. For chained calculations (energy times a rate gives power, divided by volume gives energy density), Python\'s scipy.constants plus pint preserves units through the whole expression and catches errors. Engineering thermodynamics work that needs state-dependent behaviour (enthalpy of steam as a function of pressure, fuel heating values) should use IAPWS or NIST REFPROP rather than a scalar converter, because the right answer depends on temperature, pressure and composition, not just a unit factor. This page shines when you want nine common units at your fingertips, no server round-trip and no dependency to install.