During most of our interactions with the computer (emails, internet, office), the machine spends an eternity waiting in a low-power mode, only briefly waking up to process your input. This is the reason why we get such good battery runtimes to begin with. Almost everything in a computer, you can either turn off or power down significantly if its not being used. You can totally turn off the GPU logical units if there are no updates to the screen. You can even put your CPU in an ultra-low power mode between user's keystrokes (and modern OSes do that). If your activity monitor tells you that "10% of your CPU is being used", this roundly means that out of each 1 seconds, 9/10 of this second the CPU spends sleeping since it has nothing to do.
So the thing about RAM RAM, you can't really turn it off. It needs to be supplied with power constantly or you'd lose your data. You can put it in a lower power state, but AFAIK that doesn't save you that much energy. So of course, if you are doing something intensive (number crunching, video editing) with your machine, RAM power consumption is negligible. But when you are doing office stuff (and this includes things like writing papers, programming, doing research on the internet, answering emails), RAM becomes a significant factor. Laptops such as 15" MPB use around 12 Watts of power while idling. At the same time, 32GB of DDR4 RAM alone uses around 12 Watts, as measured by independent tests. And even if one can cut this down with some smart utilisation of low power modes etc., it is still a very significant part of idle power consumption.
Bottomline: if you care about battery life, RAM is probably one of the worst offenders nowadays, since it can't be turned off, consumes power constantly, and battery life is mostly about idle anyway.
