You can't upgrade soldered RAM anyway.

No, and you probably can’t do it anyway.

Most laptops with soldered RAM aren’t meant to be upgraded. Desoldering and reballing new memory chips is risky, expensive, and only worth it if you really know what you’re doing (or have a shop that does). Even then, the power difference between factory RAM and a compatible upgrade is negligible — usually within the margin of measurement error. Higher capacity might draw a fraction more power, but you won’t notice it in real-world battery life.

If you’re worried about battery life, you’re better off lowering screen brightness or disabling background apps. That’s where the real savings are.