Solar power generation has increased exponentially in the last decade. It is expected to play an essential role in generating renewable energy in the United States and the world in the future. One of the critical points for the return on investment of photovoltaic systems is to maximize energy generation. Yet, a problem exists in that solar energy systems do not have a monitoring system for all components, so there is no way to know, in real-time, if the solar energy system is performing as expected. This study proposes to evaluate the effectiveness of the PVWatts prediction calculator compared to actual real-world data obtained from the Solar Energy Research Institute (Keshena, WI). The results show that the PVWatts prediction of solar energy generation is statistically significantly different from the actual solar energy generation. Moreover, the actual solar energy generation varies year by year, and the theoretical vs. actual analysis demonstrates that this system is not working according to warranty expectations. Therefore, there is a need for a real-time monitoring system or another type of real-time preventive maintenance to know the performance of solar systems.