Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…come from PVWatts, a publicly available engineering tool that predicts solar electricity generation for different locations and solar system configurations.47 I construct estimates of lifetime solar generation based
46This calculation of external benefits builds on Gillingham and Tsvetanov (2017).
Category: