You'd need a whole bunch of panels, probably costing well over $1000 for all of them. You might as well get a solar oven, although it may take a couple hours to get the rice done that way. Using the sun's energy directly to cook, rather than going through the inefficient conversion, is much better. (Unless you want to cook at night, then you'd need the solar panel to charge a battery during the day.) Or, use small stove that takes sticks for fuel... there's a number of them out there. You don't need a rice cooker... any pot that'll boil water will do. I've done it and it comes out just as well if not better.