When Californians purchase solar power, one of the selling points is net energy metering. It’s a billing arrangement that allows a customer to get credit for the electricity fed into the grid and pay only for the electricity he is taking from it above and beyond what he makes on his own roof. A customer who contributes 3,650 kilowatt hours a year to the grid, for example, and uses 4,000 kilowatt hours would only pay for 350 kilowatt hours.
http://latimesblogs.latimes.com/greenspace/2010/02/california-solar-power-use-it-or-lose-it.html