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B-3 Shaping of powerplant Releases - Daily Basis Generally, just as there is a seasonal pattern to Western's customer load demand, so also is there a daily pattern. During the night ("off-peak" hours), electrical demand is low. As people go to work in the morning, business and industrial demand for electricity increase. In the summer, this increase reaches a peak in the late afternoon. In the winter, there is also an additional peak around 10:00 a.m. As people return home in the evening, residential load begins to increase, displacing industrial load. Finally, electrical demand drops off sharply by midnight. The ability to meet these demands is highly dependent on the volume of water available for generation. During less than average release years the water supply is insufficient to meet contract peak demands, thus optimal use of this limited supply reduces the cost of purchases. As a result, scheduled hourly powerplant releases usually follow this demand for electricity. During offpeak hours powerplant releases are typically reduced to minimum release requirements whenever possible to save water for on-peak generation, and energy is purchased to serve off-peak loads. Hydrogeneration is therefore scheduled within release constraints against the hourly load to maximize the value of the limited energy supply. However, uncertainty in hourly load and other factors (e.g., downramp restrictions) prevent the optimal scheduling of the hydropower resource. Changes in forecasted inflow and release volumes introduce uncertainty in monthly available hydropower. In addition, Western's daily power scheduling must also address the full range of other events within each month or season which typically require changes in powerplant releases. These include changes in customer demand, generating unit outages, emergency requests for generation assistance, and unscheduled deviations from pre- scheduled activities. For each weekday, 24-hour advance hourly schedules of expected energy demand are required from SLCA/IP firm power customers. Western schedulers then determine total hourly firm requirements for the upcoming day. For weekends, advance schedules may cover longer periods, up to 4 days during holiday periods and are particularly difficult to predict. In addition, customers may vary their daily schedule throughout the month. As a result, Western often schedules hourly releases in average or typical patterns, particularly on weekends. In these cases less than full use is made of generation capacity, primarily to avoid violation of release constraints caused by real-time changes in demands. Thus the hourly operation of the powerplants is less than optimal and does not exactly follow the generation that might be predicted by an "after the fact" analysis using perfect knowledge of hourly demands.