Even for a large amount of RAM that you’d find in a big server, it’s a few dozen watts at most. Here’s some charts showing the jump from DDR3 to DDR4 on a 16GB stick:
DDR5 dropped the voltage from 1.2V to 1.1V compared to DDR4, which tends to make it even more power efficient. Not quite as dramatic as DDR3 to 4, but in any case, it’s better still.
Even for a large amount of RAM that you’d find in a big server, it’s a few dozen watts at most. Here’s some charts showing the jump from DDR3 to DDR4 on a 16GB stick:
https://forums.macrumors.com/threads/ddr3l-vs-ddr4-power-consumption.2012014/
DDR5 dropped the voltage from 1.2V to 1.1V compared to DDR4, which tends to make it even more power efficient. Not quite as dramatic as DDR3 to 4, but in any case, it’s better still.