This is today's, 5/10/09, electricity demand curve from Ontario electric power company (IESO), updated daily here. (You can look at updated curved from California or France.) As you can see the demand is lowest at the middle of the night bottoming at about 11,200 MW's. As the people of Ontario wake up after 6am the electricity useage increases quite steeply to 14,400 at noon at which point it levels off. The green line stops at 15:00 (4pm) because that is what time it is now. The demand is projected to fall steeply at 9pm as people go to bed. IESO anticipates this rise and fall in demand and matches their power output to the grid to meet the demand. This as a typical daily electricity demand curve. The power below 11,200 MW is called base power and there will be either nuclear or coal plants running full time at an efficient output to meet this base demand. The power needed from 11,200 to 14,400 in many cases will be supplied by what are known as cyclic load following generators. These are flexible power plants, burning coal or natural gas and they can follow or anticipate the demand by adding fuel to the fire, which increases power and generates more electricity. In general the cyclic, load followers are smaller, less than 1,000 MW, but are more expensive to operate, with the wholesale fuel costs being twice as much as the large and more efficient base plants.
In the next blog post we will consider the implications of adding a wind plant to the grid. Basin Electric Power Cooperative in North Dakota posts the weekly output of their wind farms here and their chart will serve as an excellent live example because wind industry states "North Dakota has the greatest wind energy potential of any state in the country."