First of all, when you project wind incursion into the grid, you also must project what kind of grid will then exist. The various possibilities in the projected grid would have widely varying associated 'backup' costs. There are renewable based projected grids which need minimal backup costs, as the evolution of the grid itself provides it's own backup.

furthermore, there are already huge advances in technologies used to "store" windpower, such as compressed air. so predicting what future costs might be is completely dependent upon what scenario one chooses.

There are scenarios with no backup costs involved, though they may still be two decades in the future... but then that's the time frame we're investigating.

PS. In many parts of the wind world, wind incursion remains paltry. US is still 2% or so, China even less. This means the discussion of backup is premature.

the US still needs 300,000 MW to reach 20%, so let's keep the horse in front of the cart. Let's not forget that the "backup" issue is one created by those conventional players with the most to lose.

"Life shrinks or expands in proportion to one's courage." - Anaïs Nin