How do you attack the power grid? Are you planning on blowing up every exchange station, every power pole and every inch of cable?? Even if you can do that, it will be repair pretty soon.
In a traditional power plant, you bomb the plant continously so they stop generating electricity, on average, Egyptian Power plant was bomb ed 3 times during Yom Kippur War and North Vietnam power plant were on average bombed 5 times during the entire 10 years of war, some plant were bombed up to 20 times.
You CAN'T do that on an active Nuclear Power Plant tho, that is the point.