Humans are warm-blooded mammals and their body temperature is maintained constant at about 37C. Comfortable ambient temperature is much cooler at about 20C. We are constantly losing heat energy from the body at higher temperature to the surroundings at a lower temperature. The heat energy is lost by radiation, sweating and respiration. That is why when a lot of people are present in a room, it feels warmer.
The body temperature is maintained at 37C by compensating the energy lost to the colder surroundings by converting the food we eat into energy. Heat is generated in biochemical reactions in cells (we say that food is burnt to produce energy). Each gram of carbohydrate generates 4 Calories, while fat and protein generate 9 and 4.5 Calories respectively. In the end, all energy generated by digesting food is converted into heat which needs to be lost to maintain a constant body temperature.
Therefore, energy given off by a human body in a day is equal to the energy produced by the food we eat per day. We shall calculate this in the following:
Suppose, that every day one consumes 2000 Calories (Some eat much more and get obese!). We can calculate, using physics, the amount of energy produced in a day.
The Calorie used by nutritionists is actually equal to 1000 times the calorie used by physicist to measure energy. Note C and c in the two definitions of energy.
1 Calorie (used in nutrition) = 1000 calories or 1 kcal (used by physicists)
In physics the usual unit to express energy is a Joule (J), and 1 calorie = 4.18 J
So we consume 2000 Calories or 2000 x 1000 calories/day
= 2,000,000 calories/day
= 2,000,000 x 4.18 J/day
= 8,360,000 J/day
To make sense of Joule per day, it is better to talk about power that is J per second or Watt (W)
A day has 24 x 3600 second = 86,400 seconds
We produce 8360000/86400 J per second (W) = 96.76 W (nearly 100 W) and this is eventually given off to the surroundings as heat.
10 humans will give off 1000W - that is like one bar of electric fire switched on!
A human gives off heat equal to that of a 100 W incandescent light bulb (old style light bulbs) which actually converts most of the electricity to heat.
The new style LED lights are much more efficient in converting electric energy to light and that is why they do not get hot when in use. This also makes LED much cheaper to use for the same light output.
Your calorie consumption goes up if you are physically active, like running or climbing stairs than when you are sitting calmly.
In one hour, a 70 kg man would burn, approximately, 80 Calories sitting; 110 Calories standing; 320 Calories walking and 500 Calories jogging.
Or why you sweat and breath faster when jogging - you are burning extra Calories and producing heat faster. The body has to get rid of the extra heat by sweating and respiration.