By design, the TDP slider on the Steam Deck will put a hard limit on how much power the CPU/GPU can draw from the battery. You can't get much more direct battery savings than that.

Valve has already issued a fix for a Steam client update that caused boot loops and broke TDP and GPU clock speed sliders. Valve led a bad patch through beta, but fixed it fast. Skip to main content ...

Low TDP (15W): The Steam Deck OLED demonstrates superior efficiency in low-power scenarios. For example, in Forza Horizon 5, it achieves 65 FPS, slightly outperforming the Legion Go S’s 63 FPS.

The Steam Deck OLED has a better screen, battery life, and Wi-Fi module. But power-wise, it’s identical to the original. Running a game on the Steam Deck OLED will be the same as on the Steam Deck.