People with solar power and grid tie inverters can sell power back to the power company in some areas. I've read that to do this, they need a special bidirectional power meter that can differentiate between power delivered to or from the house. I'm curious how these work. The only things I know to measure current are a torroidal current transformer or a hall sensor, and as far as I know, both of these would give you the same output no matter which direction the power was going. Anybody know how it's done?