Energy Converter

Converts energy quantities between joules, calories, kilowatt-hours, BTU, and electron volts. Conversion factors used: 1 cal = 4.184 J (thermochemical), 1 kWh = 3.6 MJ, 1 BTU ≈ 1055.06 J (IT definition), 1 eV ≈ 1.602176634×10⁻¹⁹ J.

Food calories (the kind printed on nutrition labels) are kilocalories, equal to 1000 cal.

The internal pivot is the joule, the SI unit. Each non-SI unit is mapped to joules with a fixed conversion constant, then to the requested target by the inverse factor. The electron volt constant is exact by SI redefinition (since 2019, the elementary charge e is defined as 1.602176634×10⁻¹⁹ C, making the eV-to-J factor exact). The kilowatt-hour is also exact: 1 kWh = 1000 W × 3600 s = 3,600,000 J. The calorie has multiple definitions that disagree at the 0.1% level, and the BTU likewise; the converter uses the thermochemical calorie (4.184 J exactly) and the IT BTU (1055.05585 J) by default and exposes the alternates as toggles.

A worked example: a 2000 kcal daily diet equals 8,368,000 J or about 2.32 kWh. A typical electric kettle running for 5 minutes at 1500 W consumes 450,000 J, or 0.125 kWh, or roughly 108 kcal of electrical energy. These conversions show that human metabolic energy and household electrical energy live on the same scale once units are reconciled, a fact that becomes intuitive only after seeing the joule equivalence.

Limitations come from definition ambiguity, not arithmetic. The 'calorie' on a US nutrition label is the kilocalorie; mixing the two by a factor of 1000 is the most common conversion error. The BTU has at least four definitions (IT, thermochemical, mean, 39°F, 59°F, 60°F) within a 0.5% spread, and HVAC industry conventions vary by country. Electron volts and joules differ by 19 orders of magnitude, so converting between them produces values in scientific notation; users typing decimal-only inputs may overflow the precision of double-precision floats at the extremes.

For everyday energy comparisons, the relevant landmarks are: 1 J ≈ lifting an apple 1 m, 1 kcal heats 1 g of water by 1°C (close enough for kitchen use), 1 kWh runs a 100W bulb for 10 hours, and a gallon of gasoline contains about 33.7 kWh of chemical energy. Calibrating intuition against these anchors prevents most order-of-magnitude mistakes.