star trek - Why don't Starfleet ships default to having their shields up most of the time?

It's an occurrence common enough to be routine:

"Captain, they have locked on phasers."
"Shields up! Go to red alert."

It makes sense that the captain would be the one who decides to go to red alert — and, for that matter, who makes tactical decisions regarding the shields. (As the answers to this question suggest, automated responses aren't necessarily dependable under all circumstances.)

My question is... why aren't the shields up all the time unless there's a reason for them not to be? Shields don't preclude maneuverability, communications, sensors, weapons use... pretty much anything except transporting, landing a shuttlecraft, and getting shot. And presumably there's a range of situations in which the ship could be damaged before the captain could get two syllables in, and before the security officer could get two LCARS pokes in.

The out-of-universe reasoning is obvious: ordering the shields to be activated allows the captain to inform the audience, in no uncertain terms, that s___ just got real. But is there an in-universe justification? A reason that shields on a starship should be raised as necessary, rather than lowered as necessary?

Answer

Presumably, to save power.

While how deflector shields work exactly is covered up in technobabble, in the Star Trek: The Next Generation Technical Manual, it states that shields do have a limit to how long they can be operated:

The deflector system utilizes one or more graviton polarity source generators whose output is phase-synchronized through a series of subspace field distortion amplifiers.... Heat dissipation on each generator is provided by a pair of liquid helium coolant loops with a continuous-duty rating of 750,000 MJ. Four backup generators are located in each hull, providing up to twenty-four hours of service at 65% of nominal rated power.

Emphasis mine.

And, pure speculation here: It may also be due to the same reason away teams don't wear protective gear, Starfleet is of peaceful exploration, it doesn't really help your case when if you're talking to a Klingon bird of prey that your shields are raised, like you're expecting them to suddenly attack.