Green Energy

If you convert the chemical energy in a unit of coal to heat (burn it) you can calculate how much energy exists in that coal, measured in appropriate units (e.g. kWh). That is evidently what this author is trying to dumb down as "invested energy". The amount of energy extracted as electricity is typically 40% of that.. the rest ends up as heat which is much less useful than electricity.

I agree that this is not particularly useful in discussing the merits of different energy sources because good design tends to do as well as is practical and the supply of fuel and negative impacts of that process can't vary dramatically.