Revised Frequency Analysis Study of AGC-Calculated and ESI-ID Aggregated Loads By Weather Zone

1. RevisedFrequency Analysis Study ofAGC-Calculated and ESI-ID Aggregated Loads By Weather Zone May 9, 2005 By Bill Boswell Load Profiling Group ERCOT

2. Revision of AGC-Calculated Load • The AGC-calculated load values used in this study are calculated by the EMMS AGC system for each weather zone. • In March 2004, ERCOT Operations made a change to the AGC load calculations. In this update, the UFE statistics are recalculated using the revised load data. • The revised loads are the values published to the ERCOT website. • The loads are found on the ERCOT website at URL: http://mospublic.ercot.com/ercot/jsp/ca_load_hist.jsp • The revised date range used is: 03/06/04 – 10/14/04. Recalculations will be made when there is a year’s worth of data available.

3. The purpose of this study was to determine if there is a close match between AGC-calculated load and load calculated through the aggregation of ESI-ID IDR’s and load profiles during settlement runs. The ESI-ID aggregated load (LSegTL) used in the comparison includes ESI ID kWh + NOIE kWh + Losses assigned to a weather zone. This is required because AGC-calculated load represents total load in weather zone as defined by the AGC calculation. AGC_Load by weather zone is calculated as a small control area (∑Gen – ∑Interchange = Load) with generation assigned to a weather zone and metering (interchange) points defined as the boundaries between weather zones. The following charts are frequency plots of the following variable: Percent = (AGC_Load – LSegTL) / AGC_Load * 100. Purpose and Definition of Study

4. Summary Statistics

5. Frequency Distribution – North Weather Zone Date Range: 03/06/04 – 10/14/04 Median = -27.7% Std Dev = 32.5

6. Frequency Distribution – Far West Weather Zone Date Range: 03/06/04 – 10/14/04 Median = -7.9% Std Dev = 3.2

7. Frequency Distribution – South Central Weather Zone Date Range: 03/06/04 – 10/14/04 Median = -2.0% Std Dev = 3.0

8. Frequency Distribution – Coast Weather Zone Date Range: 03/06/04 – 10/14/04 Median = -0.8% Std Dev = 3.4

9. Frequency Distribution – North Central Weather Zone Date Range: 03/06/04 – 10/14/04 Median = 2.8% Std Dev = 3.5

10. Frequency Distribution – West Weather Zone Date Range: 03/06/04 – 10/14/04 Median = 4.8% Std Dev = 5.7

11. Frequency Distribution – South Weather Zone Date Range: 03/06/04 – 10/14/04 Median = 5.9% Std Dev = 3.8

12. Frequency Distribution – East Weather Zone Date Range: 03/06/04 – 10/14/04 Median = 11.7% Std Dev = 7.9

13. Results and Conclusions • There are significant variations (positive and negative) between the AGC-calculated loads and the ESI-ID aggregated loads. • Possible causes of the variations: • Assignments of generation and selection of interchange (meter) points to create AGC weather zones do not match the zip code assignments to create ESI-ID aggregated weather zones • Incorrect assignment of ESI ID’s to zip codes which translates to incorrect weather zone assignments. • The AGC-calculated and ESI-ID aggregated loads are not close enough to be used for further studies.