An established methodology is one of the benefits of the Blue Angel ecolabel award criteria. We document the process in the KDE Eco handbook:
See these publications for information about the methodology.
"Sustainable software products -- Towards assessment criteria for resource and energy efficiency" (Sec 4.1)
https://doi.org/10.1016/j.future.2018.02.044
And this German Environmental Agency report, pp. 22-27 summary in English:
https://www.umweltbundesamt.de/publikationen/entwicklung-anwendung-von-bewertungsgrundlagen-fuer