Solar panels contain solar cells. Most solar cells are made of some form of silicon. Panels also contain metal conductor strips and antireflective coating. Wiring and hardware to attach panels to the roof are also part of your entire solar system. All of these components do not add an unbearable weight load to your roof.
Today’s solar panels don’t weigh much at all -- typically about four pounds per square foot.
That being said, if your roof is damaged or in disrepair before installation, it doesn’t take much additional weight to cause cracks or leaks.