I heard a lot of urban myths about ZFS, but this one is new....
Actually, RAM mostly doesn't care if it is under load or not, the differences in read/write/refresh power usage are miniscule compared to the "baseline" power draw of the module and single DRAM package. Even the total size of the module isn't an indication, but the actual number of DRAM packages - a 32GB module with 8x4GB chips will draw roughly the same power as a 16GB module with 8x2GB chips (for DDR4 usually roughly ~2-3W per module). So if you have a lot of 16GB or especially 8GB modules, you can save *a lot* of power if you replace them with 32GB or even 64GB modules while keeping the same total memory size (but don't underpopulate the available memory channels; the performance penalty will lead to an overall higher power draw for the same workload).
micron released very detailed technical notes on this topic which can be easily found. They are usually named in the form of 'TN-12-34' (TN for 'Technical Note').
I can only join
gpatrick with his statement about Brendan Gregg - his books and blog articles are some of the most detailed and first and foremost tichnically founded you will find. He never makes assumptions - everything is found by actual real-world tests and comparisons and followed through to the end, even if it involves some major changes to the kernel or software. If you can get your hand on the first edition of "Systems Performance", it contains a lot of DTrace examples which also work on FreeBSD, but all of that is gone in the more linux-centric second edition. But as the second edition contains a lot of new and additional material, it is also worth a read - so basically you'll want both...
its more the other way around: linux has always been used on a lot of
junk small, 'exotic' and old systems and without much actual workload, hence it was desirable to have *very* aggressive power saving in place. For larger systems the idle or low-load power draw is of zero concern; what matters is the actual performance per watt for a system that is fully utilized - any resource that isn't used is wasted money and/or power, so tweaking the system to perform well at or near its maximum capacity is far more critical than some idle power savings by mocking with each and every special sleep-mode of any hardware component one might plug into a dekstop computer...