With the rapid increase in Building Integrated Photovoltaic (BIPV) systems and the popularity of photo voltaic (PV) applications, a simple but accurate model to calculate the power output of PV modules is crucial,for evaluating systems. In addition, in the analysis of energy payback, two factors, the power output (maximum power output) model of PV Modules and the representative local weather data, affect calculations of the energy savings and the payback time of BIPV systems. Most studies take the efficiency of PV modules as constant when calculating the energy payback time of PV systems, and ignore the influence of solar radiation and temperature oil the results of the calculation. This study tries to develop one simple, practical, yet more accurate model for describing the characteristics of the power output of PV modules. It develops a model for describing the I-V characteristics of PV modules according to the equivalent circuits of solar cells, by which an accurate but complicated model of the maximum power output (MPO) call be achieved. Taking this MPO model as a benchmark, two other application models from other studies art evaluated and examined. One simplified application model for describing the maximum power output of PV modules is then derived from the results of the simulation. Once the solar radiation on PV panels and the ambient temperature are known, the power output of BIPV systems or PV systems call be calculated accurately and easily.