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When a contractor accidentally snipped a cable at a Tucson Electric Power Co. substation April 16, what ensued was an experience that is becoming more familiar to Tucsonans.

Traffic lights didn't work, causing at least one major crash. Some grade-school as well as college classes were canceled or simply suspended in progress. People couldn't work without their computers. Buildings became stuffy without functioning coolers — and that was only in April.

Though the cause and size of the April 16 outage were rare, TEP outages themselves have become more common over the last three years.

The number of outages per customer, and their duration, spiked last year, the third year in a row of increases. However, that came after a one-year sharp decline in 2004.

Last year, customers were twice as likely to have an outage as in 2003, and the outages lasted an average of 12 minutes longer.

"We had a pretty busy storm season last summer, and that contributed to the outages we had last year," said Joe Salkowski, a TEP spokesman.

In addition, he said, customer demand has increased not only from new growth, but from more people using bigger electronics in their homes.

"We have seen an ongoing increase in customer demand. So I think we're seeing the effect of that. We have a number of system improvements that we're looking to make. The numbers suggest the time for those improvements has come."

By comparison, Arizona Public Service Co., the state's biggest utility, also had a steady increase in outage frequency and duration since 2003. But by the end of last year, a TEP customer was twice as likely to have an outage, and outages were lasting an average of 20 minutes longer than those of APS customers in metropolitan areas.

"They've had more resources to invest in their system, since they've had more rate increases over the last 10 years," Salkow-ski said.

TEP's rates have been frozen over that period.

He also noted that APS is large enough that it separates its metropolitan and rural statistics, and it's easier to restore power in urban areas. While TEP serves a mostly metropolitan area, its numbers do include information on rural customers as well, Salkowski said.

The years between 2003 and 2007 tell their own story of improvement followed by decline at TEP. After experiencing 3,608 sustained outages — that is, outages lasting longer than five minutes — in 2003, TEP got that number down to 2,503 the very next year. But in 2005, it bounced up to 2,956, and in 2006 it continued creeping, ending up at 3,079 sustained outages for the year.

And then last year brought 3,400 of them.

TEP has scheduled $1.4 billion in capital improvements over the next five years, Salkowski said. The company is also pursuing a rate increase — and has come to a tentative agreement with state regulators for a 6 percent overall increase.

Every spring TEP surveys the system, looking for the weakest links that can be replaced in time for the storm season and hopes it has upgraded at the right points, Salkowski said. "It's pretty much impossible for us to predict where a storm might hit."

Luckily, the April 16 outage proved more inconvenient than anything else. There were no deaths in the car crash. And people stayed fairly calm, said Laura Andrade, 35, whose teenage daughter was in school at Tucson High that afternoon.

"She was just excited to not have to do anything for that period of time," Andrade said of her daughter, whom Andrade reached by cell phone when she found out about the outage. "It was a nice little break, I guess, for them."

For three of the last five years, TEP was roughly in line with other regional electric utilities as far as the average number of interruptions that customers experience, but the company has generally had a longer average outage duration, according to industry-standard benchmarking statistics provided by TEP, APS, Salt River Project and Public Service Co. of New Mexico.

But it's also hard to compare to other utilities on an apples-to-apples basis.

The Edison Electric Institute, a national trade association of electric utilities, prepares benchmarking reports for its members, but those reports are confidential, said Ed Legge, an institute spokesman.

And even if all the reports were placed side by side, some utilities count outages differently from others, said A.D. Patton, a consulting engineer and retired university professor in Texas who researched electric reliability for more than 30 years.

TEP's Salkowski pointed out an important difference in that some companies exclude "major event days" from their numbers, while TEP includes everything.

Regardless of how TEP compares to other companies, it's clear that outages are becoming longer and more frequent, based on its own data.

Though it's widely assumed that our electricity is more reliable than in parts of the country that endure snowstorms, earthquakes and tornadoes, the fact that we have prolonged heat waves year after year can affect the electric system, said Vijay Vittal, an electrical engineering professor at Arizona State University. Vittal also is the head of the Power Systems Engineering Research Center.

Normally, equipment gets to cool down at night, he said. When the air doesn't cool off at night, the equipment doesn't get that break, he said.

Preliminary data from Southern California, which had a hot summer last year, shows that the system experiences more pressure and more transformers fail as a result, he said.

Here in Tucson, the summer storms and additional household consumption don't help, Salkowski said.

"We replace and repair what we can during the springtime," he said. "We will never be entirely successful in preventing outages. They are a necessary inconvenience when you operate a system as complex as ours. We can only minimize the impact of outages on customers."