When the proportionate change in supply is equal to the proportionate change in price. In this case the PES value will be equal to 1.
Below is a diagram to show the characteristics of a unit elastic supply curve:
The supply curve has the typical upward sloping relationship between quantity supplied and price, as a result of the greater profit incentives that arise from higher prices. But with a unit elastic supply curve, quantity supplied and price change by the same factor when moving along the supply curve.