Peak Period Forecasting in electric grid
I have never worked in forecasting but, if I were given the problem, I would consider using one the following methods:


Specify what your peak capacity is. Set a warning threshold - 90%, for example. Any forecast value above that value may be considered "peak".
Find the actual peak value. Set any value > 95% (for example) of that peak as "peak".


It all depends on what you want the system to tell you but you haven't specified this.