During Chol Ha'moed, it's definitely allowed to shut a light off if it'll help you sleep. And I assume it's also allowed to shut lights off in the evening, since a gradually-darkening environment may help the body to prepare for sleep. Finally, I assume it's probably also fine to shut off a shul's lights, since many shuls have huge expensive banks of bright fluorescent lights.
But I wonder if you can shut off your house's indoor lights during the daytime, to save electricity.
Maybe, if you left all your house's incandescent lights on all day during all of Chol Ha'moed, it would cause a significant financial loss. Or maybe not. I dunno.
A friend of mine in Israel suggests that, if it's okay to do a melacha on Ḥol Ha'moed (e.g. turning a light on), perhaps it's implicitly allowed to do the reverse (e.g. shutting the light off).
In the end:
A) Is it allowed to shut off your house's indoor lights during the daytime of Chol Ha'moed, to save electricity?
B) [Edit:] And, if so, why is it allowed?