Over capacity and in the US Hydro can easily cover most issues. Other areas may use peaking power plants or grid storage just like today.
PS: If you build more wind than you need it does get slightly more expensive, but the wasted peaks are a fairly low percentage of energy generation so there not that important. ie. If you build 5% more wind than you need the cost only increases by 5% but you need to time shift far less power. Considering how much cheaper wind is than Nuclear you can have a lot of extra capacity factor.
PS: If you build more wind than you need it does get slightly more expensive, but the wasted peaks are a fairly low percentage of energy generation so there not that important. ie. If you build 5% more wind than you need the cost only increases by 5% but you need to time shift far less power. Considering how much cheaper wind is than Nuclear you can have a lot of extra capacity factor.