Hydroelectric power covers only two-thirds of our needs in the best water years.
Every utility buys some power because building enough power plants to meet all customer needs all the time would be too expensive. Utilities run their cheapest plants as much as possible, turning to their most expensive plants only for short periods when nothing else is available. Customer demand for electricity peaks seasonally during the coldest and hottest weather of the year, usually in January and July. Demand also peaks within the day. On a daily basis, customer demand peaks on weekdays from 6 AM to 8 AM and from 4 PM to 8 PM, when most consumers are at home.