You misunderstand what peak demand means. Peak demand is simply the time of greatest electricity usage. So it is when the usage reaches a maximum. For a source I will simply reference logic. If demand is not always constant then there will be times of lesser and greater usage. The time of greatest usage is the peak. If you want to learn more about what peak demand is and why it is important go read this book -> https://www.amazon.com/Grid-Fraying-Between-Americans-Energy/dp/1608196100