Enligt Terminator wiki
Skynet's Central Core was located deep underground within Skynet's main complex. From intercepted surveillance of video data feed, the Resistance was able to hypothesize that this Central Core was a form of cold fusion reactor, needed to supply Skynet with its tremendous energy requirements. This Central Core is the key to Skynet's ability to operate; Skynet would be rendered virtually disabled without it. It has therefore become the main target for the most skilled Resistance strike teams. The Central Core is protected by the T-1000000, and thus there has, as of yet, been no successful strikes against it.
Så, även om Skynet inte kunde stängas av, kan strömkällan slås ut.