A typical weekly load curve is shown in Figure 2.1: it represents the demand from the Electric Reliability Council of Texas (ERCOT) during three weeks in 2005. In Texas the peak load occurs in summer, around mid-day (orange curve), while demand in winter and spring ranges between 35% and 70% of the peak value. The timing of the peak demand depends on geographical location, meteorological conditions and the degree of industrialisation relative to activities in the commercial, service and residential sector of an area/country.