Animals cannot use solar energy directly but can use the chemical energy contained in plants or other animals. When energy is being used some heat is nearly always generated. Energy can be measured in either joules or calories. In practice, both units are used just as different units are used to measure liquids, eg pints, litres. One calorie is equivalent to 4.184 joules.
People use large amounts of energy so nutritionists use larger units.
The energy in the diet is provided by carbohydrate, protein, fat and alcohol. The amount of energy made available to the body by each of these varies . Fat is the most concentrated source of energy - weight for weight it provides just over twice as much as either protein or carbohydrate. Alcohol provides almost as much energy as fat. For some people alcoholic drinks form a large part of their energy intake.