The first describes something that does not happen at regular, predictable times.

第一个意思指一些事情不在常规的、可预测的时间发生。

You can burn a specific amount of coal at a set time to deliver exactly the amount of electricity you want in a predictable time window.

您可以在设定的时间燃烧特定数量的煤炭，以便在可预测的时间窗口内准确地提供您想要的电量。