About Energy
The joule (J) is the SI unit of energy. Kilowatt-hours (kWh) are used for electrical energy billing, while calories measure food energy.
1 food calorie (kcal) = 4,184 J — the "calorie" on nutrition labels is actually a kilocalorie.
Pro Tips
Energy (J) = Power (W) × Time (s) — a 100 W bulb running for 1 hour uses 360,000 J = 0.1 kWh.
BTU is widely used in HVAC — 1 BTU is approximately the energy to heat 1 lb of water by 1°F.