There has been a lot of research that showed that this is possible with relatively little over provisioning for a place like the US or Europe. The thing that is still missing are highly integrated electricity nets, but that is improving raapidely mainly for economic reasons (with a very competitive market and integrated nets one can make quite a bit of money by selling from places we're generation is currently cheap). If we use hydro also for storage (pumping up water when electricity is cheap), you need even less overprovisioning.