Humans are constantly on the move. On a daily basis we are walking, jogging, biking, swimming, hiking, or doing some type of activity powered by our bodies without giving it any thought. Because of this, human power is commonly thought of as a “free” energy source. However, this line of thinking is not accurate because human power requires an energy input (i.e. food), and the efficiency with which humans can digest food to produce power is extremely low. On average, producing the fuel source human power consumes eight times as much energy as it produces.
The daily energy output of the average person is 600 watt-hours. At its most efficient, the calories of food energy required to produce this can be provided by eating 4.6 lbs of corn which requires 2,300 Watt-hours of energy to produce or 1.5 lbs of beef which requires 43,000 watt-hours of energy to produce. The conversion of food calories to energy by a human is between 1.4 and 26% efficient and by no means impressive.
The largest muscles in the human body are in the legs. Therefore, the most effective means of generating human power is to power a bicycle or similar device. A well-conditioned professional cyclist can generate 400 watts of energy for each hour of cycling. However, the average person can produce less than about 125 watts for each hour. In comparison, operating a hand crank generator for one hour produces about 33 watts of energy.
To put things in perspective, one barrel of oil contains about 1,700,000 Watt-hours, which is equivalent to 22,000 hours (ten years) of human work. The average human consumes 50 barrels of oil per year, so the deficit is substantial.
Human power is very useful if it can be delivered without the need to consume additional food calories. Something as simple as taking the stairs instead of an elevator or walking instead of driving are easy ways to save energy and can have a significant impact on reducing non-renewable energy consumption when millions of people participate.