Energy is the capacity to do work; the SI unit is the joule (J = N·m = kg·m²/s²). A dietary kilocalorie (food Calorie, written with a capital C) equals 4,184 J. The average person requires about 8,700 kJ (2,000 kcal) of food energy per day. One kilowatt-hour — the unit on electricity bills — equals 3,600,000 J (3.6 MJ). One litre of petrol releases approximately 34.2 MJ when combusted; one kilogram of TNT releases ≈ 4.184 MJ.
Electronvolts (eV) are used in particle and atomic physics: the ionisation energy of hydrogen is 13.6 eV = 2.18 × 10⁻¹⁸ J. Visible light photons carry 1.8–3.1 eV depending on colour. Nuclear reactions release millions of eV (MeV) per event. The BTU (British Thermal Unit) — defined as the heat needed to raise 1 pound of water by 1 °F — remains the standard unit in US HVAC systems and building energy ratings.
| 1 kcal | = 4,184 J |
| 1 kWh | = 3,600,000 J |
| 1 BTU | = 1,055.06 J |
| 1 ft·lb | = 1.3558 J |
| 1 MJ | = 238.85 kcal |
| 1 kWh | = 3,412 BTU |