It isn't entirely separate either. Often increases in efficiency direct increases in consumption. This observation was made by the English economist William Stanley Jevons and is known as Jevons paradox[1]. More recently, the economists Daniel Khazzoom, Leonard Brookes, and others have analyzed this relationship[2].

[1] https://en.wikipedia.org/wiki/Jevons_paradox

[2] http://www.iaee.org/en/publications/ejarticle.aspx?id=1091