Heterotrophic cultivation can be a preferred option for commercial microalgal biorefinery owing to its significantly higher productivity and lipid content leading to high value products. However, limited modeling works have been carried out on heterotrophic culturing of microalgae so far. In this research, a mathematical model is developed to describe microalgae's metabolic behavior under heterotrophic conditions with their internal components classified into nine types (nucleotides, amino acids, proteins, polysaccharides, monosaccharides, membrane lipids, neutral lipids, and pigments including carotenoid and chlorine families). The modeling is intended to be general as the objective is to build an overarching model capable of describing microalgal behavior for a wide variety of microalgal species. However, the general model turns out to be nonparsimonious, containing a very large number of parameters, which complicates parameter estimation and optimization in its applications. To assist in applications, a model reduction procedure is suggested and demonstrated using two example cases. Therefore, the result is an overarching general model, which can be tailored to specific types of applications at hand, circumventing the need to develop a new model from scratch for each new type of application. The model can be used to improve the design and operation of a microalgal cultivation process with respect to specific targeted products, thereby contributing to the enhanced economic feasibility and sustainability of microalgal biorefinery.