> I agree with you on your point that I don't see the connection between batteries and gas vs coal either. If anything, batteries are to stabilize energy demands when the Sun is down.

Because spinning a coal plant up or down is slower, the batteries let you make the gradient more gentle.

Suppose Alice's gas plant can go from 100MW output to zero in ten minutes. So if Sam's solar farm goes from zero to 100MW over ten minutes in early morning Alice's plant can mirror, between them Alice and Sam produce 100MW of power smoothly. Obviously real life is more complicated but this gets the point across

Now, Charlie owns a coal power plant, it can ramp from 100MW to zero, but it takes about an hour. This doesn't directly mirror the solar farm, Charlie and Sam can't really just work together to smoothly produce the same power

Bill's 100MWh BESS fixes this. An hour before the sun comes up, Charlie's coal plant begins shutting down, Bill's BESS puts out 100MW while consuming the gradually reducing coal power output and some of the gradually increasing additional power from the solar farm as that increases.