Before a child can understand the ways in which the sun can power everyday objects, she must have a handle on the basic concepts behind energy and the sun. Start by explaining what the sun is: a giant ball of gas that radiates heat. Explaining the concept of energy can be slightly more difficult, but pointing to easily understood examples of simple energy (turning a crank to operate a wind-up toy or explaining how electricity lights a bulb) will give the child an idea of how basic objects are powered.
If everyday objects need energy to work and the sun is a giant ball of heat, it makes sense that the heat of the sun can provide energy to objects that would otherwise require electricity. Show the child solar panels and explain how they capture heat which can be used in the same manner as traditionally generated electricity.
Children often learn best by seeing a concept at work. Try some experiments using solar energy. Simple experiments include making iced tea with the heat of the sun or creating a solar cooker. Children can also learn from examples of objects like solar-powered calculators or glow-in-the-dark toys.