In a fairly typical summer weather pattern, high temperatures moved out of the Midwest into the population centers of the Mid-Atlantic and Northeast regions during June 19-22, 2012.
Heat waves like this one drive up air conditioner use and electricity demand. Electric systems are designed to maintain electric supply under these conditions.
Extensive operating and market information available on Regional Transmission Organizations' (RTOs) websites lets market participants track the effects of the heat wave in real-time and examine in more detail after the fact.
The three RTOs in the Mid-Atlantic and Northeast are the PJM Interconnection (PJM), the New York Independent System Operator (NYISO), and the New England ISO (ISO-NE). The publicly available operating and price information provides a fairly detailed picture of the level of electric system stress during heat waves.
Forecast and Actual System Demand: Electric systems use weather forecasts and other information to forecast electricity demand. Some RTOs post their hourly demand forecasts for up to the next seven days. This information provides earlier notice of upcoming heat wave-related system events. RTOs also post their actual hourly demand soon after the end of the hour. This allows monitoring whether actual demand is growing more quickly or more slowly than expected during the day.
Day-Ahead Prices: High RTO hourly day-ahead prices are another indicator of potential electric system stress. Electric systems typically commit generating units the day ahead to be available for the next day. Six of seven U.S. RTOs do this by optimizing bids submitted by generators and load-serving entities to produce hourly prices at hundreds or thousands of pricing locations on their system, known as locational marginal pricing (LMP). (In the Southwest Power Pool RTO, member systems perform this unit commitment function.)
With LMP, prices may be different at different locations. Price differences reflect transmission congestion—or the inability to deliver lower cost power to certain places on the system. Price differences may also indicate a potential inability to deliver needed supply. These prices reflect modeled system conditions based on supply and demand bids and the operating status of grid elements. These unit commitment-related prices are available in the afternoon of the day before.
Real-Time Prices: An indicator of actual system stress on the electric grid is high and rapidly changing RTO five-minute real-time prices. System operators must maintain a balance of supply and demand continuously. As demand rises and then falls over a day, operators bring on and take off committed generators to match demand. System conditions rarely match the model results from the day before, so the RTO runs an optimization every five minutes to handle needed adjustments. This model produces real-time LMPs.
High or low (even negative) prices can indicate an imbalance between supply and demand. A price spike over the five-minute interval, up or down, may indicate a short-lived imbalance due to generator outages or transmission congestion or outages. A sustained high real-time price indicates a supply shortfall. Locational differences in real-time LMPs indicate transmission congestion and, during a supply emergency, an inability to deliver supply to certain locations.
PJM Interconnection (PJM)
PJM's actual hourly demand on Wednesday, June 20, was a bit below the forecasted peak in the hour ending 5 p.m. The actual peak occurred in the hour ending 6 p.m. Demand was between 2 GW and 4 GW above forecast in the hours just before and after the super-peak hours of 3 p.m. to 7 p.m.
PJM forecast that Thursday, June 21, would have the highest peak during the heat wave. However, during the super-peak hours actual demand was as much as 7.5 GW below forecast. Peak hourly demand on June 21 was 3 GW less than the day before.
New York Independent System Operator (NYISO)
As forecast, NYISO's peak hour demand on Thursday, June 21, was its highest during the heat wave. Demand was forecast to peak at 97% of NYISO's all-time record peak. Actual demand came in at 95% of the record; actual demand turned out to be 95% of the record.
Actual demand was slightly above forecast on Tuesday, June 20, and below forecast on June 21.
New England ISO (ISO-NE)
ISO-NE's actual peak demand was 8% over forecast of Wednesday, June 20. On Thursday, June 21, actual demand was 4% below forecast.