The productive capacity of an economy is closely connected to the quantity and nature of its energy sources. In the pre-industrial world the available sources of energy set quite clear and quite low limits to that capacity as the following overview will show. A substantial amount of total energy consumption was still provided by the muscle power of humans and animals. Their calorie requirements as ‘input’ depended primarily on their height and weight and their labour power as ‘output’ in turn depended primarily on that calorie intake. Let us begin our analysis with information on human calorie- intake. Currently health organisations advise a daily calorie intake of 2,000 to 2,600 calories for men who are not very physically active; 2,200 to 2,800 calories for men who exert a certain amount of physical activity and 2,400 to 3,000 calories for those who lead a very active lifestyle. For women the figures are 1,600 to 2,000 calories for a sedentary lifestyle, 1,800 to 2,200 calories for a fairly active lifestyle and 2,000 to 2,400 calories for a very active lifestyle. Most of the energy humans normally take in, some eighty per cent, is needed to simply support the basic functioning of the body's vital organs and to maintain a steady body temperature. This is the so-called basal metabolic energy. The net energy efficiency of humans thus is some twenty per cent. An adult human being needs to consume at least 1,500 calories per day to maintain her or his basic metabolic functions and to resist infection. This means that with calorie intakes as low as they often were in the pre-industrial world, even a small reduction in intake could have dramatic consequences in the long term. In a weakened condition because of a reduced calorie intake stress on the body and disease can easily be fatal. Next to that basal metabolic energy we can in this context distinguish two other types of energy: thermal energy (energy in terms of heat and heat transfer) and kinetic energy (energy in terms of motion and power).

How many calories did people consume in the early modern period? Most of the estimates that we have - which indeed are estimates even if the data for some periods and places are better than for others - are rather low, especially considering the hard, physical labour that many people had to perform.