Our society demands more energy at certain times of day: these times are called peak hours or hours of high electricity consumption. In winter, these peak hours are between 11:00am and 12:00pm (midday) due to the activity of companies/services and households (the use of hobs and ovens), or between 7:00pm and 8:00pm due to the confluence of business activity and people being at home. However, in summer the peak hours occur in the middle of the afternoon, coinciding with higher temperatures.
During these hours, it is more expensive to produce electricity because it is necessary to use the more expensive power stations, which are also those that produce more CO2 emissions. In addition, the whole electricity system must be dimensioned in order to be able to deal with the demand in this reduced time-span. For example, it takes about 4,000 MW, the equivalent of ten 400 MW combined-cycle power stations or four 1,000 MW nuclear power stations are necessary to meet the demand of the 300 peak hours or hours of highest consumption that occur throughout the year.
The hours of lower consumption are known as valley hours (off-peak) and correspond to the night-time hours, coinciding with a period of reduced activity in all consumer sectors.
On any given day, the start of the working day, the closure of businesses at lunchtime or an increased number of people being at home in the evening, explain why demand is not identical at the different times of day. During the night-time hours is when the daily minimum demand occurs. At these times, only the industrial demand maintains an important consumption level. This is due to fact that large factories consume electricity 24 hour a day, also taking advantage of the night-time hours when energy can be contracted at a cheaper tariff. During these hours, some services also stay in operation (lighting of public spaces, hospitals, computer equipment, etc).