The increasing adoption of small-scale technologies for generating power represents a transition from centralized electricity systems to distributed energy resources (DER). This paper develops a multi agent-based model (ABM) to study the dynamic behaviors that influence DER investment decisions. The proposed model accounts for both local utilities (and load serving entities "LSE") and power generating plants. DER is introduced into the model by allowing LSE customers to invest in a particular photovoltaic systems. Consequently, the model forecasts electricity market outcomes and PV system adoption. Using an illustrative hypothetical case study, results show that customer demand in most LPCs cut in half, annual capacity factors at coal-fired power plants reduced by more than 60 percent, and sharp decreases in locational marginal prices. Ultimately, this research will result in a decision-support tool that will identify least cost strategies that utility companies can use to respond to increasing penetration of DER.