Due to the presence of algebraic constraints as well as existing of different modes of operation, a standalone solar power system consisting of photovoltaic arrays, battery bank, electrical load, and a converter is becoming a complex system that can no longer be modeled using the conventional block diagram approach. While the block diagram approach is based on causal interactions between a chain of the ordinary differential equations (ODE), a more appropriate acausal approach solves a flat model of the system consisting of hybrid differential algebraic equations (HDAE). In effect, this paper proposes a nonlinear HDAE-based model of a standalone solar power system. The proposed model is presented using the Modelica language that allows object-oriented and acausal modeling of the multi-mode systems. Next, a general purpose solver is employed to simulate the system. The results of the simulation shows proper match with the information available in the com-ponents datasheet. It is shown that the simulation provides a sufficiently accurate prediction of all the system behaviors, which is vital for any model-based controller, including mode transitions.