Yes, CAPACITY is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, P 3, A 1, C 3, I 1, T 1, Y 4.

Is it a valid word?

CAPACITY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

capability to perform or produce
the susceptibility of something to a particular treatment
the amount that can be contained
the maximum production possible
a specified function
(computer science) the amount of information (in bytes) that can be stored on a disk drive
an electrical phenomenon whereby an electric charge is stored
the power to learn or retain knowledge; in law, the ability to understand the facts and significance of your behavior
tolerance for alcohol

Also: electrical capacity, mental ability.