Based on a 2,000-calorie diet, the human body generates about 330 BTUs per hour of heat. The same diet also inputs 7,900 BTUs of energy into the human body through food consumption. Because most of this energy is used by the human metabolic system, the heat that is radiated, conducted or convected away from the body can be measured as generated BTUs.
This number fluctuates depending on the conscious state of the person. For example, a human body may give off closer to 400 BTUs of body heat while active but only 315 BTUs or fewer while sleeping. However, even while active, the BTUs that a human body generates may rise or fall depending on how stressful the activity is. A human body that is performing light work may only generate about 650 BTUs of body heat per hour, whereas more difficult or strenuous work may generate about 2,400 BTUs of body heat per hour. For reference, if the hourly average of BTUs of heat generated by a human body could be captured, they could be used to power a 100-watt light bulb for an hour. The conversion of calories to BTUs to watts is described as 1 calorie per hour equalling 3.968 BTUs per hour, which is equal to 1.16 watts per hour.