Rooftop panels spread faster than most forecasts allowed for, and the reason was rarely the technology itself. Panel prices fell steadily, but what turned a falling price into a purchase was the billing arrangement: households were credited at the retail rate for every unit they exported to the grid. That credit made the arithmetic simple enough to explain on a doorstep, and installation companies grew around it. Where the credit was withdrawn, installation rates fell within a year, even though panels had never been cheaper.