1. A method of controlling the production of at least one liquid product produced by at least one plant over a forecast time interval, the at least one plant having a liquid storage capacity and consuming electrical power to produce the at least one liquid product, said method comprising: continually, upon the elapse of a calculation time interval equal to the forecast time interval: calculating a rate of change of liquid inventory of the at least one liquid product contained with the liquid storage capacity over a past time interval equal to the calculation time interval and ending upon the calculation of the rate of change of liquid inventory; inputting input linguistic values into an expert system controller employing fuzzy logic to apply the input linguistic values to three rule sets to obtain an output linguistic value and to convert the output linguistic value into an output numerical value of a forecasted differential production rate for the at least one liquid product; the input linguistic values comprising first and second of the input linguistic values related to the current liquid inventory and the rate of change of the liquid inventory, respectively, a third of the input linguistic values related to the projected demand requirements for the at least one liquid product and a fourth of the input linguistic values related to the projected unit power costs for the electrical power over a forecast time interval; the three rule sets and the input linguistic values defined such that the forecasted differential production rate tends to decrease as the rate of change and/or the liquid inventory and/or the projected unit power costs increase and the differential production rate tends to increase as the projected demand requirements increase and vice-versa; the first and second of the input linguistic values are applied to a first of the three rule sets to obtain first intermediate linguistic values; the third of the input linguistic values and the first intermediate linguistic values are applied to a second of the three rule sets to obtain second intermediate linguistic values; the fourth of the linguistic values and the second intermediate linguistic values are applied to a third of the three rule sets to obtain the output linguistic value; adding the output numerical value of the forecasted differential production rate to the current production rate to obtain a new production rate; and controlling the plant to produce the at least one liquid product at the new production rate during the forecast time interval.