Its technically very simple but politically very difficult. The point I am making is that the power usage is not directly tied to transactions. Not making a transaction on the blockchain will not save X watts because that power is burned regardless of what transactions are going through.
An example would be that me not driving to the beach saves some fuel but bitcoin is just a train constantly driving to the beach and back regardless of how much I use it. You could grab the number of trips and the power usage and calculate a power usage per trip but its not an entirely useful metric since the power usage does not scale up as more people use the service.