by Michael Sivak, Sivak Applied Research.
This brief analysis examines the relative cost of the energy from milk and gasoline. The unifying approach involves the use of a common measure of energy—British thermal units (Btu).
A cup of whole milk contains 150 Calories of energy. Thus, there are 2,400 Calories in a gallon of milk. This translates to 9,518 Btu per gallon of milk. The average price of conventional (not organic) whole milk in September 2020 was $3.56 per gallon. Therefore, the cost of 1,000 Btu from conventional whole milk was $0.37.
Currently, a gallon of gasoline (with ethanol) contains 120,286 Btu of energy. The average price of regular gasoline in September 2020 was $2.183 per gallon. Therefore, the cost of 1,000 Btu from regular gasoline was $0.02.
The above analysis considered the available energy in milk and gasoline, and the logic there is straightforward. However, when considering usable energy, things get more complex, as highlighted in the following paragraph.
Current gasoline-powered vehicles lose about 70% of the available energy through engine losses (mostly through exhaust heat). In turn, not all of the available energy in food is digested. Examples of the factors affecting energy absorption include the preparation of the foods (more energy from cooked foods than raw foods) and the degree of processing (more energy from highly processed foods than less processed foods). Also, the digestion of proteins requires more energy from the body than the digestion of carbohydrates or fats (the thermic effect of nutrients). This results in the effective energy loss of 25%-30% for proteins, compared with 6%-8% for carbohydrates and 2%-3% for fats.
Michael Sivak is the managing director of Sivak Applied Research and the former director of Sustainable Worldwide Transportation at the University of Michigan.