If that was the case, why then supply them with power from Israeli power stations?
Three reasons:
1) The war is over. The civilians in Gaza are technically still under Israel's jurisdiction and it is obligated by law to at least LOOK like it's trying to prevent a humanitarian crisis in a time of non-hostilities
2) Israeli supplies only provide a fraction of what the Gaza plants do. I'm not seeing a more recent source, but there are reports from 2008 and 2010 that a typical household in Rafah had electricity around 6 hours a day
3) Israel can just as easily cut off the power supply the next time they decide to "mow the lawn." Those GPS guided bombs are expensive, you know.