Energy Converter

About energy units

Energy is the capacity to do work or transfer heat. Its SI unit is the joule (J), named after 19th-century physicist James Prescott Joule, equal to one newton-meter. For human nutrition, calories are standard: note that the food 'Calorie' printed on packaging is actually a kilocalorie (kcal) — 1,000 times larger than the small calorie used in chemistry (1 small cal = 4.184 J, so 1 food Calorie = 4,184 J). Electricity is billed in kilowatt-hours: 1 kWh = 3.6 megajoules. Heating in the US is often quoted in British Thermal Units: 1 BTU is the energy needed to raise 1 pound of water by 1°F, approximately 1,055 joules.

History & origin

James Prescott Joule experimentally established the mechanical equivalent of heat in the 1840s, showing that mechanical work and heat are both forms of energy. The calorie predates joules — it was defined by French chemist Nicolas Clément around 1824 as the heat needed to raise 1 gram of water by 1°C. The kilowatt-hour emerged in the late 19th century alongside commercial electricity generation; Edison's Pearl Street Station (1882) was among the first utilities to bill customers by energy consumption. The British Thermal Unit originated in Victorian-era steam engineering.

Tips

The calorie versus Calorie confusion trips up anyone reading nutrition labels or fitness apps. A food label showing '200 Calories' means 200 kcal = 836,800 J — not 200 small calories (which would be trivial, less than the energy in a grain of sugar). When a scientific paper says 'calorie' (lowercase), it almost certainly means the small calorie (4.184 J). When a food label or dietitian says 'calorie,' they mean kilocalorie. The safest approach is to write kcal explicitly.

Common questions

What is the difference between a calorie and a Calorie?

A small calorie (cal) is the energy needed to raise 1 gram of water by 1°C, equal to 4.184 joules. A food Calorie (Cal, or kcal) is 1,000 small calories = 4,184 joules. Nutrition labels in most countries use the kilocalorie, but the word 'calorie' is used loosely to mean kilocalorie in everyday speech.

How many joules are in a kilowatt-hour?

Exactly 3,600,000 joules (3.6 MJ). One watt is one joule per second, so one kilowatt-hour = 1,000 W × 3,600 s = 3,600,000 J. A typical kettle uses about 0.1 kWh to boil a full litre of water.

What is a BTU and when is it used?

A British Thermal Unit is the energy needed to raise one pound of liquid water by 1°F at its maximum density. It equals approximately 1,055 joules. BTUs are still widely used in the United States for air-conditioner and furnace ratings, and for natural-gas billing — contexts where joules and kilowatt-hours are rarely seen.