Yes and no. It depends what kind of renewables we're talking about, and where they are. My favorite solar tech is solar-thermal (CSP power plants look really awesome), which effectively stores the collected solar energy as thermal energy and uses this to drive turbines. As a result, it can produce power to meet demand over a 24 hour cycle if you put it in a desert. Hydroelectric power is also renewable and can produce power whenever it is needed (assuming the reservoirs don't go dry). The problem comes from wind turbines, which produce fairly consistent power over a 24 hour cycle and don't have a way to store it at night when demand drops, and also from rooftop solar installations, which obviously only work during the day.