SYSTEM BOUNDARY 



purchased fuels 

 goods and services 



Figure 3. 

 and money. 



Energetics model of a farm illustrating the interaction of energy 



product for 1947-78 according to Odum et al . (1980). Energy to dollar ratios 

 are useful when evaluating urban energy flows because the dollar value of a 

 specific flow such as human labor often is the only data available. The 

 dollar to energy ratio gives an estimate of the quantity of fossil fuel and 

 natural energy required for the United States society to provide a specific 

 function. Note the drop in energy per dollar of the United States gross 

 national product. 



One final point to be made concerning the relationship between money and 

 energy is that the quantity of money flowing per unit of energy is constantly 

 changing, as plotted in Figure 4. Non-renewable energy, such as oil, is 

 recovered and processed for further use by human consumers. The consumers pay 

 the energy processors for providing the service. As the more easily recovered 

 fuels are expended, more energy must be used to recover the less accessible 

 fuels. The result is that the same expenditure of energy, measured in terms 

 of money, produces less usable energy, which causes inflation. Government 

 policies which expand the national money supply also contribute to the declin- 

 ing energy to dollar ratio. When using money flows to estimate energy flows, 

 the money-to-energy ratio will be dependent on the year that the data were 



283 



