。
Baseball serves as an outlet for excess energy.
Literal
Baseball [topic-は] energy [gen-の] outlet [to-に] becomes.
はけ口 ('outlet, vent') is a native Japanese compound — 捌ける ('drain, be disposed of') + 口 ('mouth, opening'). Metaphorically used for emotional/physical outlets. ~になる ('become, serve as') pairs naturally with functional roles. 精力 ('vigor, energy') is a Sino-Japanese word more specific than generic エネルギー.