After skimming this paper this is one of those times.
Whilst there is a large literature on working hours and the contrast between old Europe and the US, I have never before seen working hours related to energy use.
These is one table of regression results and some interesting descriptives but otherwise I am still head scratching.
Are Shorter Work Hours Good for the Environment? A Comparison of U.S. and European Energy Consumption by David Rosnick and Mark Weisbrot
This is footnote 4.
The adoption by the EU-15 of a U.S.-style work and consumption pattern would increase total EU and U.S. energy consumption by 7 to 16 percent; and a switch by the United States to European work and consumption patterns would lower total EU and U.S. energy consumption by 8 to 14 percent. Thus, there is a difference of 15 to 30 percent in energy consumption between the two scenarios. Applying these estimates to the entire world, including developing countries, leads to a difference of 1 to 2 degrees Celsius in global climate change.
Of course it all hinges on whether working requires more energy than not working which in turn depends on what one does with ones leisure time. As in all economics papers there are some heroic assumptions but an interesting lighthearted read nonetheless.