Energy units — joule, calorie, kWh, BTU and eV
Nine common energy units, one physical quantity. Rounding errors compound across conversions — do all math in joules, present in whatever the audience reads.
Every field kept its favourite energy unit. Physicists use joules and eV. Utilities bill in kWh. Food labels use kcal (calling it Calorie with a capital C). HVAC keeps BTU. Nutrition sometimes reverts to plain calorie. Nuclear engineering slips into eV even in macroscopic calcs.
The exact factors
1 J = 1 N·m = 1 W·s > 1 kWh = 3.6 × 10⁶ J = 3.6 MJ > 1 cal = 4.184 J; 1 kcal (Cal) = 4184 J > 1 BTU = 1055.06 J > 1 eV = 1.602 × 10⁻¹⁹ J
Cross-scale examples
Chocolate bar: 250 kcal = 1.05 MJ = 0.29 kWh. Home daily electricity: 30 kWh = 108 MJ. Gallon of gasoline: 33.7 kWh = 121 MJ = 115 000 BTU. 1 eV per particle × Avogadro ≈ 96 kJ/mol.
Worked example — heating water
Raise 100 L water from 20 to 60 °C. Q = mcΔT = 100 × 4186 × 40 = 16.7 MJ = 4.65 kWh = 15 875 BTU = 4000 kcal. All four numbers describe the same job — utility bill vs equipment nameplate vs food-science reference just pick different names.
Recap
Work in joules. Present in the audience's unit.
Try the converters mentioned in this article
FAQ
Is 'Calorie' with a capital C the same as calorie?+
No — food 'Calorie' is 1 kcal = 1000 physics calories. It's a permanent source of confusion in nutrition literature.
Why is eV used for macroscopic energy?+
Convenient when scaling from single-particle physics — 1 eV/atom × N_A ≈ 96.5 kJ/mol.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.