Also if enough caffeine and flavor constituents are not extracted during initial use, perhaps the beans can be used again. If the same amount of water and energy is required to produce a good cup of coffee from used beans, then the bean consumption rate drops for the same coffee production rate. Also, a new type of bean may become available with greater caffeine and flavor content. To illustrate how the system can be described by a mathematical model, let the rate of coffee bean usage be written pb and the ratio of the weight of bean components in the coffee to the weight of beans used to make the coffee be written ab. The contingencies and details discussed above are hidden in B, W, pb, and pw, but if accurate values of these variables can be measured and if only the sustainability of the system as it currently operates is of interest, no further breakdown may be necessary.
Energy is a third resource.
The fraction of energy lost is very high in most coffeemakers since it is primarily converted to heat which is easily dispersed. Energy converted in this manner is not necessarily entirely lost, since on a cool day it may contribute to warming the temperature of a house or office, reducing heating energy required. However, on warm days, the heat from a misplaced coffeemaker may counter the effect of air-conditioning, i.e. impede a second process. Again, this is somewhat beyond the system of interest and does not need to be directly included in the analysis. The rate that energy is converted to product is the fraction of converted electrical energy present in the coffee when consumed, while the remainder is waste.