Day-ahead energy refers to electricity scheduled in the day prior to the actual day of flow. The day ahead is the common point at which supply resources are scheduled to ensure that enough supply is available to serve loads. Since loads fluctuate across the day, day-ahead energy is scheduled in one-hour blocks. Day-ahead scheduling is done by balancing authorities typically using a least-cost security-constrained economic energy dispatch model which optimizes supply to meet load given system physical capabilities. The scheduling process considers the incremental costs of providing supply by each resource, resource operational capabilities, and forecast load requirements as it determines which units will be dispatched at each hour of the day.
In organized competitive wholesale markets, such as those facilitated by ISOs in the U.S. and PXs in Europe, day-ahead scheduling is done through a financial market where market participants buy and sell electricity for the following day at financially binding prices. The day-ahead market allows buyers and sellers to hedge against price volatility in intra-day or real-time markets.
Here is a simple example of a day-ahead energy schedule for supply resources (note: click on table to enlarge in new window):