Update from 2021:

https://www.sciencedirect.com/science/article/abs/pii/S01676...

Edit to add sci-hub link to full paper: https://sci-hub.st/10.1016/j.scico.2021.102609

This appears to be a web page where the authors have posted links to their research, data, updates, etc:

https://sites.google.com/view/energy-efficiency-languages/