Solar and wind generation themselves are seasonal and don't match the seasonal patterns of demand. So you need to time shift across seasons if you don't have the instantaneous (base load) capacity available all the time.
You might say, well, just build more windmills or solar farms. Doesn't help when it is dark and calm. Your "overbuild" is useless in that situation. So you need storage (or other base load generation, fossil or nuclear).
In this study, it is estimated that Germany and California both need about 25TWh of storage to time shift energy supplied by intermittent sources to other parts of the year. The study claims $5 trillion to purchase batteries to store that much energy.
http://euanmearns.com/the-cost-of-wind-solar-power-batteries...