The findings in this study refute mathematical equation from existing research that has been determining the optimum tilt angle of the solar panel with a subtropical location perspective. This research found that the influence of degrees latitude (Y) and longitude (X) to the optimum angle of solar panel installation in the territory of Indonesia (tropical location) can be represented by the equation 0.0093 X + 1.3042 Y, with an RMSE value of 1.88 and an R squared value of 0.928. However, mathematical equations for optimum tilt angle based on the coordinates of locations in 9 different prefectures of Indonesia resulted in better RMSE and R squared values. The maximum potential economic benefit to be gained from the installation of solar panels at the optimum angle in Indonesia, assuming a feed-in tariff in Indonesia of US$ 0,25 and a solar panel capacity of 1 MW and production life of 20 years, is US$ 740.839,66.