Calculating wellbore energy. At least some of the illustrative embodiments are methods including: calculating an expected wellbore energy for a planned wellpath, the calculating by a computer system; calculating an expected tortuosity of the planned wellpath; calculating a first value indicative of probability of occurrence of a drilling event for drilling along the planned wellpath, the first value based on the expected wellbore energy and the expected tortuosity; and then drilling a wellbore along at least a first portion of the planned wellpath if the first value indicates the probability of the drilling event is less than a predetermined threshold.