PoW coins are not backed by energy, this is a total misonception. PoW currencies are secured by their spent energy, while PoS coins aren't really secured by anything - if you hold enough coins, you can attack the network as much as you want, compared to PoW where it would always cost you money to do so. But simply having PoW does not guarantee security, you need a decent amount of energy involved, otherwise it's still practical to attack such coin, which is what you can see with coins like ETC or BTG which were successfully attacked in the past.