Energy — The Capacity to Do Work

Energy is the capacity to do work or produce heat. The SI unit is the joule (J): one joule is the energy needed to move 1 kilogram a distance of 1 metre against a force of 1 newton. In practice, different fields use different units — kilowatt-hours (kWh) for electricity, kilocalories (kcal) for food, and BTU (British Thermal Units) for heating and air conditioning. All of these can be converted to joules, which makes it possible to compare the energy in a battery with the energy in a meal or a tank of petrol.

Energy in Everyday Life

Lifting an apple one metre off the floor takes about 1 joule. A food calorie (kcal) is 4,184 joules. A AAA battery stores roughly 5,000 joules. A litre of petrol contains about 34 megajoules (MJ). A lightning bolt releases roughly 1–5 gigajoules — but over such a tiny time span (microseconds) that it delivers very little usable power. A 1-megaton nuclear weapon releases approximately 4.2 petajoules (4.2 × 10¹⁵ joules). These comparisons illustrate how enormously energy density varies between everyday objects and extreme phenomena.

kWh, Electricity Bills, and Food Calories

One kilowatt-hour is the energy used by a 1,000-watt appliance running for one hour — equivalent to 3,600,000 joules. A typical US household uses about 900 kWh per month. Running a 60 W light bulb continuously for 24 hours consumes 1.44 kWh. The average electric vehicle uses 15–25 kWh per 100 km. On the food side, what nutrition labels call a "calorie" is actually a kilocalorie (kcal) — the energy required to raise 1 kg of water by 1°C. Fat delivers 9 kcal per gram, while carbohydrates and protein each provide 4 kcal per gram. That is why fat is so energy-dense: gram for gram, it contains more than twice the calories of sugar or starch.