Have you ever spent any time in or close to a large wind farm? Honestly, it's relaxing if you haven't been told to be mad about them existing- they're quiet, sustainable, and once they're built the cost of each new unit of energy they deliver doesn't come with the unit cost and environmental cost of acquiring and burning yet more fuel-- so in that sense, the marginal cost of each new bit of power from them really does approach zero.
Of course, this (that the resulting energy is so cheap) is why the coal/oil/gas folks are mad- they know they won't be able to compete once a grid with sufficient edge-caching/power storage is built out