One thought which occurred to me recently: if the renewable energy is produced decentralised (i.e. "Bürgerwindparks" in Germany, rooftop Solar PV, etc...) then there should be no extra grid cost at all since not more power is transferred than without renewable. The energy fed decentralised into the grid then merely lessens the power transferred from utility owned centralised big powerplants!

And since power demand has always fluctuated greatly during each day fluctuation should not be an issue. If there is enough energy supply today and demand doesnt rise too much (in Western Europe, say) then all further decentralised renewable energy should cause no extra grid or back up cost at all. It will only cause less use of gas to balance.

Always interesting, Spain's electricity grid operator's website with real time supply and demand curves incl. archive. You can see that gas and hydro practically balance all wind, solar and demand fluctuation!

lazy link

http://www.ree.es/ingles/operacion/curvas_demanda.asp