But the biggest thing is the amount of rejected energy. That's 60.6% of all energy that's being consumed by the US that's being lost due to inefficiency of some sort. 80% of this and maybe more is coming from two uses: electricity generation and transportation. What they have in common is that they're using the burning of something to convert heat into rotary motion.
Here's an example: a conventional car contains an internal combustion engine, which works by spraying a mix of gasoline and air into a cylinder and setting it alight. The resulting explosion pushes the cylinder against a crankshaft which connects to the wheels of the car to make it move. This conversion is at best about 33% efficient--most of the energy--and with it the pollution, CO2 and other stuff, is lost to heat and noise, without producing forward motion. The same thing is happening when burning coal or gas to produce electricity: it heats something (either boiling water or the natural gas exhaust itself) so that it can be used to power a turbine, which is connected to a generator. The theoretical peak efficiency of such a process is about 60% and the reality is a lot worse: the very best electric plants operate at about 40% efficiency. The rest is waste heat.
If, on the other hand, the heat is the ultimate product, for example burning stuff to make heat in a furnace, it is much more efficient. Modern furnaces are over 90% efficient. The blast furnaces used in steelmaking are similarly efficient.
Wind and Solar suffer different inefficiencies, but the waste doesn't end up in the atmosphere as CO2 or smog, and it doesn't consume a resource that takes a hundred million years to replenish. Electricity can be used to make rotary motion much more efficiently than the reverse, so it's useful for a large fraction of the needed activity in industry and transportation. according to the chart, .03% of electricity is used for transportation. This includes electric cars like a Tesla or Leaf, and Trolley Buses and Electric Trains like on the northeast corridor, but does not include hybrids like the Prius or a diesel-electric locomotive.
It seems to me this is fairly low hanging fruit. 60% of our energy production is lost to the inefficiency of using heat to make rotary motion. The second law of thermodynamics says we're doomed to lose some, but to get half of this back seems relatively straightforward. Converting half the cars on the road to electric will save around 10 Quads of rejected energy. Converting half the gas and coal power plants to wind and solar will save around the same.
