Does selling refurbished equipment as new to the electricity company, after the client becomes aware that it is not new, make this sale unlawful and the profit from it unlawful gain?

It is not permissible to repair a machine for someone whom it is known will use it for cheating and deception, due to the Almighty's saying: "And do not cooperate in sin and aggression." As for repairing it for someone whose condition is unknown, or for someone who intends to use it for a permissible purpose, there is no harm in that.