What dramatic effect did this switch have?

Ethereum's switch cut its energy use by roughly 99.9%. The network moved from proof-of-work, where miners run huge banks of specialized computers competing to solve puzzles and win the right to confirm transactions, to proof-of-stake, where validators instead lock up, or 'stake,' their own coins as collateral and are chosen to confirm transactions based on that stake rather than raw computing power.

Doubling the network's energy use is essentially the opposite of what happened, since removing the computational mining race is precisely what produced the massive drop in electricity consumption, and claiming transactions became permanently free is inaccurate, since users still pay network fees to process transactions after the Merge.

Because proof-of-stake replaces energy-intensive mining hardware with financial collateral as the basis for network security, the Merge is often cited as one of the largest voluntary reductions in electricity use ever achieved by a major piece of global computing infrastructure.