In Solar ROI you can compare offers and the higher the ROI, the sooner you have recouped your investment.
However, the payback in years depends on several factors. In a project, you should compare the result with the situation if you do nothing. If you would not buy solar panels, will you leave your money on saving account and receive savings or are you going to spend it on something else? In the first case, you may also face property tax. The results are displayed in a table or in a graph. The tool is not a black box and all calculations are displayed and explained in datasheets.