Energy saving technology and energy service price
A problem with using energy service prices is that they might reflect competition conditions (e.g. monopoly/oligopoly, or the big six in the UK) rather than the true cost of energy to firms. 

A problem with using energy prices is that they might reflect demand and supply issues. For example, no one would say that the drop in oil prices from 140 to 50 was due to technological improvement. Instead, it was driven by a shale oil revolution, and perhaps geopolitical aspects.