How much more power does 32GB of RAM use compared to 16GB in an idle system?

The difference is negligible—maybe 1–2 extra watts at most, less than a dim LED bulb. Idle RAM draws power based on chip count and activity, not capacity, so doubling capacity doesn’t double power. That extra 2 watts costs about $2–3 per year, so unless you’re running a server farm, get 32GB if you need it—the performance headroom is worth more than the electricity.