From the transcript of Insurrection:
ARTIM
Don't you ever get tired?
DATA
My power cells continually re-
charge themselves.
So he is powered by "power cells" that continually recharge themselves, therefore apparently he does not have to plug into the Enterprise to recharge.
Indeed there is also the episode where his head is stuck in the cave (Time's Arrow) for over 500 years, and it still works fine afterword.