An ideal design period would be defined as the time period in the near future during which supply will exceed the demand in order to achieve a surplus.
A Design Period is the period commencing with the Commencement Date of Design and Construction Works, up until and including the date and time that the Design is completed at the end of the Design Period.
In other words, it can be said in a simple way that the number of years that will pass before a given facility is attainable to satisfy the demand in the near future. This surplus allows for more efficient use of resources and better overall performance of the facility in the long run.