I've never been in prison nor do I hope to ever be.
However from what I've seen on TV on prison documentaries the prison cells I've seen on those shows do have electrical outlets.
I would assume that most prison cells do have electrical outlets because some prisoners have televisions and even electric fans to keep them cool.
So in that case the prison cells would have electrical outlets.
Now as for supermax prison cells or Solitary Confinement prison cells I would think that those prison cells would not have any electrical outlets in them.
But the prison cells I have seen on TV had electrical outlets.