Negligible. Maybe 1–2 watts.

In an idle system, the power difference between 16GB and 32GB of RAM is basically a rounding error. You’re looking at maybe 1–2 extra watts at most. That’s less than a dim LED bulb.

Here’s why: RAM modules draw power based on the number of chips and their activity level, not total capacity. Idle RAM sits there refreshing its contents at a trickle, and adding more sticks or higher density modules inches that trickle up slightly. But it’s not like doubling capacity doubles power. The big power cost comes from active reads/writes, not from just having memory populated.

If you leave your PC on 24/7, that extra 2 watts costs about $2–3 per year (at typical US electricity rates). So unless you’re running a server farm or charging a laptop off a solar panel in the woods, don’t think about it. Get the 32GB if you need it. The performance headroom is worth more than the electricity.