You will have to open the wall at the panel either to connect to it and then offset on to the surface, or go all the way up into the attic. Whether you run it in the attic or on the surface is immaterial. Since you do not want to cut the wall to run the wires to a junction box in the wall at the heater, just bring it down the wall to the J'box. The wire from the box to the heater. Why are you going electric when you have gas there?