Answer to Question 1
Black surfaces absorb the radiation incident on them. If the radiation is too
intense, it overwhelms the surface and the majority can be reflected. In this case, it can
appear white.
Answer to Question 2
Sometimes the Sun doesn't shine through clouds for days on end. This can
impact the output of solar hot water for household use. In addition, windmills depend on the
wind, which must be above a certain speed in order for the mill to generate electricity. If the
wind speed is below the threshold for substantial times, this can affect the availability of that
wind energy. In order to meet these exigencies, backup sources are necessary.