In an interview with Forbes, Charles Hoskinson claims that Cardano is 1.6 million times more energy efficient than Bitcoin due to its more advanced Proof of Stake consensus algorithm.
"The primary difference between proof of work and proof of stake is that first stage-the picking of somebody to be in charge. With a proof of work system, it’s a meritocratic MINING process; basically, you’re mining hashes, and those are like lottery tickets. You just keep going until you hit the lucky numbers. Then you say, “I have a golden ticket.” And that gives you the right to make a block. The higher the value of the asset, the more competition you get, the more energy expenditure. The advantage of proof of stake is that you don't need expensive equipment and lot of energy to decide who will mine the Block. Avoiding extreme mining competition can save lot of energy and help environment."
-Charles Hoskinson (Forbes Interview)