California’s mandated push to at least 33% renewable energy by 2020 faces supply problems for six months each year when the sun sets earlier, says the California Independent System Operator. Their ‘Duck Chart’ shows the problem. More power is needed in late afternoon / early evening, when people return home from work. However, for six months each year, that’s precisely when solar power production drops. And if wind isn’t blowing at wind farms, the problem gets worse. If more renewable energy is added, then there will probably be excess capacity during the other six months.
Of particular concern to the ISO are three-hour periods when the sun goes down in January, February and March and in October, November and December of each year (called “shoulder months” by the ISO). In a future state energy system that is half green, it would no longer be the peak hot months of the summer or an unusual winter cold snap that would present the risk of blackouts or rolling brownouts. Instead, it would be the sunset period of each day during moderate temperature months that is likely to present a critical challenge to the reliability of electricity for customers.