Hodson, H. July 25, 2015. Power to the people. NewScientist.

Demand for electricity varies every second, minute, hour, day, and season.  Production is easy to provide from always-ready natural gas and coal. But as we move towards a 100% renewable electric grid as oil, natural gas, coal, and uranium decline, wind and solar will have to provide most of the power, and they are extremely unpredictable, unreliable, and seasonal.  When demand and supply of electricity don’t exactly match, a blackout can occur.

The only way to get around this in the future will be energy storage, mainly from batteries since pumped hydro and compressed air energy storage are limited geologically.

In March 2015 Germany experienced this problem when 66% of solar generation failed during a solar eclipse.  Since this was predictable, utilities had already ramped up goal, natural gas and hydroelectric power to compensate.  If really large utility-scale batteries had existed, that could have also compensated.

Storing energy for the entire grid is a much bigger challenge. “The scale is unimaginable,” says Dahn, whose lab signed a five-year research contract with Tesla in June. He calculates that storing the output of just his local utility company, Nova Scotia Power, for 24 hours would take the energy storage capacity of every battery made worldwide this year – and then half as much again.