You Can't Plug in a 2 with a 6

S1 E9: It's planting time in Hooterville and Oliver is shocked that the locals make their crop choices based on aches and pains. Back on the farm Oliver assigns every electrical device a number from one to seven in an effort to keep their creaky generator from blowing. All gadgets up to a total of seven can be plugged in at the same time but Lisa can't quite grasp the concept.