Energy units (BTU) are constant, they do not change with position or time. $ units are constantly changing. To compare BTU to $s requires a function with a time variable. Graph# 12 shows that relationship. In 1975 one dollar could have bought, on average, 42,348 BTU; by 2010 a dollar would only have bought 6,946 BTU. The graph is used as $-energy costing is employed in the study.