The past post discussed why sustainable energy should be used, and that it is best to match the amount of energy generated as closely as possible to the amount of energy that is used.
However, renewable energy resources can be intermittent. Solar systems only generate electricity when the sun is shining, and will generate less electricity on a cloudy day or if shaded. Energy can be stored and then used later when it is needed, but when there is no or not enough renewable energy supplied, such as at night, or on cloudy days.
As of December 2013, energy storage for solar systems was not economically viable. [1, 2] But the cost of energy storage is coming down, so that batteries may soon become economically viable, if they haven’t already.
 G. Weiss, “Does ‘Affordable’ Solar PV really change everything?,” in Alliance to save energy, ed, 2013
 G. Weiss, “Could 2014 be the year of the battery?,” in Energetics, ed, 2013.