Electricity is an essential component of our economy and our personal lives. That is why electric system reliability is so important to PNW. The company has numerous programs in place to help ensure the efficiency and reliability of our transmission and distribution system.

	In the past 10 years, APS has added 300,000 new customers while reducing the number of outages by 38 percent and the duration of outages by 44 percent

 APS follows the Institute of Electrical and Electronics Engineers (IEEE) Guideline 1366 when measuring its reliability and when it benchmarks with other utilities.

IEEE 1366 defines an interruption as a loss of power for more than five minutes. Anything less then that is considered a momentary interruption. The company uses three popular measures of reliability that are defined in the IEEE Guideline. Within a one-year period, throughout the entire APS system, APS measures the:

• System Average Interruption Frequency Index (SAIFI): the total number of sustained customer interruptions divided by the total number of APS customers
• System Average Interruption Duration Index (SAIDI): the total number of sustained customer interruption minutes divided by the total number of APS customers
• Customer Average Interruption Duration Index (CAIDI): the total number of sustained customer interruption minutes divided by the total number of sustained customer interruptions

For each of these indices, the lower the number, the better.

The largest reliability benchmarking effort we know of is performed by the Edison Electric Institute (EEI). In its most recent benchmarking survey, which measures reliability for 2004 and contains data from 75 utilities:

1. For SAIFI, APS was in the middle of the second quartile with a value of 1.17. The U.S. average is 1.47.
2. For SAIDI, APS was in the first quartile at 95 minutes. The U.S. average is 381 minutes.
3. For CAIDI, APS was in the first quartile at 81 minutes. The U.S. average is 187 minutes.

As the below graphs show, between 1996 and 2004 (the latest EEI statistics available at the time of this report), APS' System Average Interruption Frequency Index (SAIFI) or the average number of sustained interruptions seen by the average customer on our system, improved (lower is better) by 38%, while the national average rose (got worse) by 9%.

Similarly, the APS System Average Interruption Duration Index (SAIDI) or the average annual duration of sustained interruptions seen by the average customer on our system improved (lower is better) by 44% while the national average rose (got worse) by 109%.

SAIFI Comparison

SAIDI Comparison

TransWest Express Project
A potential transmission project from Wyoming to Arizona that would provide Arizona and other southwestern states increased capability to access electricity generated from wind, coal and other resources in the Wyoming area.

New Technologies
New technologies are a critical factor in meeting increasing electric demands in the most reliable and sustainable manner. Examples of some of the new technologies we are incorporating into our electrical system are included in this section.