Food energy is the amount of energy obtained from food that is available through cellular respiration.

Food energy is expressed in food calories (labeling: EU kcal, U.S. Calories) or kilojoules (kJ). Food calories, or the "Calorie" units used often in nutritional contexts, measure amounts of energy 1000 times greater than the units in scientific contexts known also as calories, or gram calories ("cal"). Food calories are thereby referred to less ambiguously in some formal contexts as kilocalories (kcal) or kilogram calories. One food calorie is equal to 4.184 kilojoules. Within the European Union, both the kilocalorie (kcal) and kilojoule (kJ) appear on nutrition labels. In many countries, only one of the units is displayed.

Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during respiration—this is often called 'food energy'. When nutrients react with oxygen in the cells of living things, energy is released. A small amount of energy is available through anaerobic respiration. Fats and ethanol have the greatest amount of food energy per mass, 9 and 7 kcal/g (38 and 30 kJ/g), respectively. Proteins and most carbohydrates have about 4 calories per grams (17 kJ/g) [but see below]. Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than 4 kcal/g.

Saurav Jha is a commentator on energy and security. His first book, The Upside Down Book of Nuclear Power, was published by HarperCollins in March 2010 to favourable reviews, and is currently being translated into several Indian languages. Jha is trained in economics from Presidency College, Kolkata, and Jawaharlal Nehru University, New Delhi, and has written for publications such as Le Monde Diplomatique, World Politics Review, The Diplomat, as well as national dailies such as The Daily Telegraph, Hindustan Times and Deccan Herald.

Currently Jha is writing a techno-thriller Ayni as well as a quirky travelogue, The Heat and Dust Project (along with wife Devapriya Roy) that chronicles the story of travelling through India on local buses “on a very very tight budget”. Both books are for HarperCollins India. The Heat and Dust Project is also a first-of-its-kind dynamic book as the couple used Facebook to document their journey even as it was underway.