Why are solar cells not used effectively?

101 Why is the sun very hot than any other hot body and how does it continously emit solar energy? Justify. 102 Give the purpose of mirror and glass sheet in solar cooker. 104 Give the limitations in using commercially wood as fuel, over windmills. 105 A student constructed a box type solar cooker. He found that it is not working efficiently. What could this be due to? Give any four possible mistakes that he could have made?