The costs involved in the manufacture of a product at any single point in time are exactly what I can make assumptions about from the price of the product, nothing simplistic about it, given that prices are a result of the negotiation of a lot of details, including whether the optimal industrial input is some raw mass of coal that's burned in some managed way or Watt-seconds of directly supplied electricity. It's not going to be a perfect signal (demand matters, and any single estimated or registered price reflects a certain degree of imperfect judgment/optimization), but energy inputs are going to be a bounded factor.

If what you're saying is the actual energy expenditure may be what we're concerned about if we're speaking about environmental impacts and might be more properly modeled by something more complex, that's a worthwhile point. But it's going to be much less a matter of 1000J from a given mass of coal vs 1000 J of directly supplied electricity -- this factor will disappear behind whatever market allocations/optimizations are available -- and much more a matter of general industrial energy costs circa 1995 vs 2015, combined with the relative efficiency of manufacturing processes at both points in time.

What would we expect on those two fronts? Personally, I'd expect energy prices to rise with economic growth and occasionally fall with recessions, absent some large new source coming online or state-imposed costs for use. I'd also expect process efficiency to increase as well. Which would lead me to, again, see energy expended as reasonably estimated by some bounded factor of a final product price.