RAM size doesn't affect battery life much.

Not noticeably.

The difference in power draw between 8GB and 16GB of laptop RAM is tiny — typically well under a watt. Modern DDR4 or DDR5 sticks use something like 2–3 watts per module at full speed, and idle is even lower. Going from one stick to two (or from one 8GB to one 16GB) won’t change your battery life by more than a few minutes.

The bigger battery drain is always the screen, the CPU, and the GPU. If you’re running out of RAM and hitting the swap file on an SSD, that actually uses more power than having extra RAM. So 16GB might even be slightly better for battery if it keeps your system from paging.

Don’t worry about power — upgrade for performance.