In ST:Insurrection, Data says that "my power cells continually recharge themselves". Sounds very much as if they're energy storage devices that draw external power, like the batteries of an electric car. Only, the recharging system is wireless.
Makes sense: Data is supposed to have been built in and for a UFP colony, and would be compatible with UFP recharging networks, just like any random PADD or tricorder or phaser or groundcar. And "continually" would not mean "continuously", allowing Data to not be recharging at the very moment he makes the statement - as he is on a non-UFP planet that shuns technology and thus probably lacks a compatible wireless network.
How long Data can survive without the power net, we don't know for certain. He has operated for weeks outside such on occasion ("Thine Own Self", say), but whether he could go on for years is unknown, because whenever he's awakened from a long slumber, the heroes fuss around him with UFP technology.
Based on the ST:INS formulation, though, I'd discount any actual power-generating system within him. If he were producing his own power, he'd not be droning about his batteries. And he has little reason to lie to the kid he's addressing there.
Timo Saloniemi
