Ethereum moved to proof-of-stake in 2022, and it was initially expected to use only 1% of the energy it used when it was proof-of-work.
Turned out they were under-estimating the savings. According to studies by the Cambridge Centre for Alternative Finance and the Crypto Carbon Ratings Institute, Ethereum's 2022 switch to Proof-of-Stake reduced its global network energy consumption by over 99.98% overnight.
As far as I understand, this was made possible because the Ethereum protocol was set up to be made extensible like this (even though big changes like these are hard), while Bitcoin on the other hand, is stuck on proof-of-work forever, since its protocol does not allow for changing it.