Electric vehicles (EVs), which are becoming increasingly popular in the transportation sector, are expected to bring significant changes to the power sector, in terms of power storage. Our previous research has focused on public transit buses, which had the advantages of certainty and centralized control. The moving characteristics of EVs are expected to enable more flexible use of renewable energy by charging at multiple locations. However, it is difficult to evaluate the power absorption adjustment potential, which is the range of the increase or decrease in charging at a specified location and time. Therefore, this study proposes a method for evaluating the potential of a group of electric buses from a spatio-temporal flexibility perspective using open-format bus operation data. A simulation example of a real city quantifies its potential and visualizes spatiotemporal flexibility. We have proposed a general-purpose method that can be used in other regions using the same format.