of energy per unit of time. The table shows that there are various units to measure power depending on the context. Electric power is measured in kilowatts. Heating power is measured in BTUs per hour. Automotive power is measured in horsepower (HP). Later in this section, we will see the definitions of those terms. For now, just become familiar with their use.
Notice the examples of the farn1 (agriculture), hospital (healthcare), and the light bulb. They are different from the others, because they are not described by an output measure. A farm's size of 160 acres is a mea sure of one input to the production process, land. A hospital's capacity of 100 beds measures the number of people treated per day, which is a throughput measure, whereas the number of people cured per day would be a strict output measure. In the example of the light bulb, the com mon way to describe a bulb is by the amount of electric power it draws, measured in watts, which is an input measure, not an output measure. The correct capacity measure for a light bulb, lun1ens, is a measure of light output. However, the lumen is not a measure of light per tin1e but of light intensity (candlepower) falling on one square-foot of area, one foot away from the light source. This example shows that capacity, even as an output measure, is sometimes calculated in relation to a variable other than time.
Capacity Measures for Power Technologies
A power technology transforms energy from one form into another. The capacity of a power technology is therefore expressed in units of energy per unit of time. However, different systems of measuring energy have evolved in different contexts, and the student of sustainable technologies needs to learn their definitions and uses.
Water Heating Technologies (BTU)
A furnace burns fuel to create heat. In scientific terms, it is a technology that transforms the chemical energy in a fuel into heat energy through the process of combustion. Heat energy is traditionally tneasured in BTUs. One BTU is defined as the amount oenergy ne ded to heat 1 pound of