very low power

The do exist, kid of.

They consist of a parabolic reflector that concentrates the sun's rays to a focal point where the grill and food are placed.

They aren't a major commercial product, that's all.

Type "solar grill" into google and you'll get a couple thousand pages of them.Build it yourself or order it up. You can have one now. . . .

Solar cookers have been around for a very long time- both grills, and simple ovens. They're used in the third world. You can make a solar oven with some cardboard boxes, insulation, a piece of glass, and aluminum foil.