Carbon dioxide production by the human body
The human exhalation volume is 6 to 12 liters, according to the activity intensity; the exhaled air contains about 5% carbon dioxide (CO2). For a 9 liter/minute exhalation we get about 240 m3/year CO2, containing about 500 kg CO2. For about 8 billion world population, that makes annually about 4 billion tons CO2.
Thus, the annual volume of CO2 exhaled by the people would be close to the CO2 emitted by the fossil fuel power plants.
Metabolic energy burned in the human body
The in 2018 was 26,700 TWh, of which about 60% was produced from fossil fuels, providing (1 year = 8760 hours: 26700 x 0.61%/8760 =) 1.8 million MWh.
Average daily dietary energy consumption per capita is approximately (cca) (kcal). For 8 billion (8x109) people we get (3000 x 8 x 109 =) 2.4 x 1013 kcal, which is (1 kcal is 4184 joul) cca 1017 joul/day, yielding annually (x 365) cca 3.7 x 1019 joul/year, providing (1 joul/s = 1 W, 1 year is 31 536 000 seconds) 1.2 million MW. Thus, the annual energy biologically burned in the people would be close to the electric energy supplied by the fossil fuel power plants.
Energy output of the human body
Some discrepancies can be observed in the world database relating to the energy production, but it seems that the world still-operating coal power stations produce 2 million MW (which is not perfectly consistent with the above average electricity generation of 1.8 MWh obtained differently and from different sources, but is not far from that).
It is known that power consumed in the human body makes about 85 W. The average human produces around 100 watts of power at rest, so for all levels of activities it may be up to (cca 1010 people x 150 W =) 1.5 million MW.
This way of comparison between the humankind bodies and world power plants would also show similar values.
Although it would be interesting to discuss the details of the above calculations, including for example the efficiencies of the energy conversions in the industrial processes or in the biological pathways, the bottom line here is the fact that the total human-breathed carbon dioxide and the energy output are of the same order as the total carbon dioxide emissions and the power output, respectively, of all power plants burning the fossil fuels.
It may be noted that the CO2 breathed out by the humans and animals, having been borrowed from atmosphere via plants’ photosynthesis, is merely returned to the same atmosphere. However, the present text aims at emphasizing the relatively low CO2 amounts of the broadly discussed fossil power plants in comparison to the CO2 breathed out by human and animals.
For example, human-reared cattle exhale 1.5 times more carbon dioxide than humans, based on the total mass of the species, and for another example, arthropods including termites and ants exhale 16 times more than humans, etc. So that the industrially produced CO2 can be in fact neglected in comparison with the amounts produced otherwise.