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http://en.wikipedia.org/wiki/Peak_demand
...Peak demand, peak load or on-peak are terms used in energy demand management describing a period in which electrical power is expected to be provided for a sustained period at a significantly higher than average supply level. Peak demand fluctuations may occur on daily, monthly, seasonal and yearly cycles. For an electric utility company, the actual point of peak demand is a single half hour or hourly period which represents the highest point of customer consumption of electricity. The daily peak demand usually occurs around 5:30 pm. At this time there is a combination of office, domestic demand and at some times of the year, the fall of darkness...