That's a pretty big shift of the initial question, but it's an important shift!
If you look in the "most free" markets for energy in the US, where there is the most possibility for independent companies to come in and provide generation and other energy resources, we are looking at ERCOT in Texas and PJM in the north.
In these free markets, we are seeing huuuge amounts of storage resources in the interconnection pipeline over the next few years [1].
So where indecent investors are allowed to participate in energy, we are seeing massive influx of capital to build storage, from people that are profit-seeking. Meaning storage, at least this first chunk of it, is already likely to be economically efficient.
That said, when we talk about renewables, wind and solar are largely uncorrelated, and where we have good wind resources we tend to get good wind at night.
And when you look at load on grids, such as Texas' independent ERCOT grid, you see a sinusoidal pattern too [2]. And for ERCOT, with a sinusoidal amplitude of about 20GW, by sheer coincidence there are about 17GW of storage projects in the pipeline. And I'm pretty sure that doesn't include the solar projects that have storage directly on them, which is happening with a huge chunk of solar projects. And though Texas has been good at building wind resources, they have left their solar mostly untapped up until now. So as they start to deploy solar, they are going to get a ton of storage deployed with that too.
The challenge is not longer the technology or the costs, the future challenges will be entirely about regulation, and whether entrenched interests will use regulatory capture to prevent new entrants that can supply cheaper low carbon energy to consumers and industry.
[1] https://rmi.org/clean-energy-is-canceling-gas-plants/
[2] http://www.ercot.com/content/cdr/html/loadForecastVsActualCu...