Supply and Demand is a graphical representation of the ERCOT system’s current power supply and demand using Real-Time data, as well as projected power supply and demand from hourly forecasts.
The graph’s solid purple line represents the historical committed capacity (the amount of power that was available from on-line generating units). The graph’s turquoise line represents the historical system demand (the amount of power used). The purple shaded portion represents the amount of quick start capacity forecasted to be available in the near future. Quick start capacity is provided by generating units that can come on-line within 10 minutes of receiving ERCOT notice. These generating units are tested to ensure their quick start capability. The dotted turquoise line represents the forecasted future demand. These values are all provided in megawatts (MW).
Although supply should meet demand, the capacity displayed during each interval includes reserve power supplies. ERCOT procures reserves, which may be needed in case of sudden changes in operating conditions in order to help maintain reliability. For instance, reserves may be needed in an unexpected generation outage. Available capacity amounts are based on generating units' Current Operating Plans and can change over time due to variations in start-up times among generating units. When the actual demand crosses into these reserves, it indicates that ERCOT may need to bring all available generation on-line and notify consumers that conservation is needed.