Please turn off the lights when you leave the room.

Literal

Room [object-を] exit [when-とき] [topic-は] electric-light [object-を] turn-off [please-てください].

A polite request to switch off the lights when leaving the room. Uses 電灯 (electric light) — slightly more specific than 明かり (light/illumination) or the colloquial 電気 (electricity).