The NAFA Guide to Air Filtration (1993) quotes Avery (1978): "A method has been suggested to find the static pressure which optimizes the owning and operating cost of an extended surface filter." Avery plots filter cost per hour and energy cost per hour against time; this is what the energy engineer can do to determine optimum final pressure drop. Avery stresses that
Energy costs are low when the filter is clean but the filter cost per hour is high. The longer the filter is used, the lower its cost per hour becomes. At the same time as the filter resistance increases, the energy costs go up. At any point, the total cost per hour of operation is the sum of both the filter cost and the energy cost.... The optimum pressure drop at which to change a filter is when the total cost is the lowest. Changing the filter before [it reaches its time] is uneconomical because filter life is not optimized. Changing the filter after [it reaches its time] is uneconomical because the cost of power is greater than the saving in extending the filter life.