The variations of the demand for electric power described in Equations 2.11 and 2.12 are ideal models of what the demand can be. This variation is quite different in real power pools. There are only a few power pools whose summer and winter peaks have exactly the same magnitude. In the southern U.S., the summer peak is significantly larger than the winter peak while, in the north, the opposite is often true. In the north, the urban areas may have a summer peak while the suburban and rural areas may have a winter peak. In all areas, the daily peaks during the weekdays are significantly higher than the peaks during Saturday and Sunday. In a limited study it is not possible explicitly to take into account all the possible load variations that can occur and only idealized power demand curves can be considered. However, the difference in peak demand between weekdays and weekends can easily be allowed for. The probability of not meeting the power demand is a dimensionless number. The probable number of days per year when the load will not be met is obtained by multiplying this probability by the effective number of days in a year. If there is no reduction in power demand during the weekend, this number is 365. When the daily peak demand during the week is significantly less than that during the weekend, the effective number of days in the year is 261. This implies that the peak demands during the weekend are so low that if there is a 99.95% chance of meeting the weekday peaks, the probability of meeting the weekend peaks is 100%. This approximation is often used
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