Charles F. Kutscher & Jeffrey Logan in Undark:
Hailed as the greatest invention of the 20th century, our now-aging grid was based on fundamental concepts that made sense at the time it was developed. The original foundation was a combination of base load coal plants that operated 24 hours a day and large-scale hydropower.
Beginning in 1958, these were augmented by nuclear power plants, which have operated nearly continuously to pay off their large capital investments. Unlike coal and nuclear, solar and wind are variable; they provide power only when the sun and wind are available.
Converting to a 21st-century grid that is increasingly based on variable resources requires a completely new way of thinking. New sources of flexibility — the ability to keep supply and demand in balance over all time scales — are essential to enable this transition.
There are basically three ways to accommodate the variability of wind and solar energy: use storage, deploy generation in a coordinated fashion across a wide area of the country along with more transmission, and manage electricity demand to better match the supply.