These days, most solar PV panels installed in the U.S. are part of a so-called "grid-tied" system. That is, the system is interconnected with the electrical grid. When the solar panels' electricity production exceeds on-site demand, current flows out into the grid, crediting the owner's utility account. (If your neighbor is bragging about how cool it is to see his meter run backwards, this is what he's talking about.) When production falls short of demand, the flow reverses, and power is drawn from the utility's supply like usual.