if the wattage will go down because of a better efficiency, then more miners will be added = the same result as today(with more hashpower, but same consumption), you can't escape from that equation

the only thing to reduce it would be that bitcoin will remain at current price(after the halving), therefore miners need to shut down 50% of the total hash, unless they manage to double their efficiency in 1 year

so there is a percentage of possibility that nothing will change with the reward(besides the difficulty), highly unlikely though