1. A computer-implemented method for modeling and/or improving operational performance of a power plant, said method comprising: providing a graphical user interface configured to allow a user to manipulate equipment icons into an energy system model representation; using modular, function-based equipment models to simulate behavior of a plurality of physical components represented by the manipulated equipment icons, wherein the equipment models are piece-wise linear functions that simulate non-linear behavior of the plurality of physical components; generating at least one script that defines a mathematical relationship between the plurality of physical components as a function of at least one rule governing behavior of the plurality of physical components; using the at least one script to generate an optimized solution to the energy system model using the equipment models, wherein said generating an optimized solution comprises performing a mixed integer linear programming optimization on the equipment models to determine a first set of said plurality of physical components to operate and a second set of said plurality of physical components to not operate, wherein the first and second sets represent an optimized mode of operation of the power plant; and displaying the solution of the energy system model.