Tesla's battery is better at doing what it was designed for, compared to coal plant(s) that are better at doing what they're designed for. In conjunction with one another, both technologies work incredibly well.

In addendum to this, I'd like to mention that things that are designed for a specific purpose are good for that specific purpose.

And in general, things that are made to work well together work better than things not made to work well together. This also applies to things that happen to work well together, despite not initially being designed to.