This would never happen...

This isn't necessarily true. During the 1977 NYC blackout there was looting, and I believe that may have lasted only a day or two. Like the title of the movie, it's a trigger effect. All it takes is for one person to start acting crazy to get the ball rolling for disaster!

The movie really doesn't explain why or how the power went out. But realistically speaking, when there is no electricity and on top of that, phones and radio don't work, there will be chaos within a few hrs, because people depend on one or the other as a lifeline to the outside world. And if you have none of those things, you are totally cut off, with no news or anything and you panic. We don't know how far the blackout reached in the movie because there is no radio or tv to give the news...just random word of mouth.

they are implying that power outage was planet-wide. unusual solar activity could in theory cause this.

it all depends on the community that it happens in.
power goes out for a week in a small town where everyone knows each other people band together to "get through it". power goes out in a vast metropolis the city burns.

As always, Typing on a Dvorak

"i have been in 3 power outages in my life, one lasting 7 days. nothing like that even remotely happened to my town. very little looting, no deaths were reported. they made it seem that after one day of no power the entire town went into chaos...it would be at least a week for it to get that bad"

You probably knew why the the power was out - hurricane, earthquake, ice storm, etc. - and knew that it was not out in the rest of the country. Some radio stations were probably still on the air to keep you informed. Also, if the blackouts you describe happened recently, there was probably cell phone service and, thus, at least some internet access.

These people had none of that. They had absolutely no source of information about what was happening in the outside world. They were completely cut off and, furthermore, had no knowledge of why the power went out. It would be a terrifying situation, psychologically much worse than for people who lose power after a big storm or even in one of those weird widespread outages (like SoCal in 2011) that seem to happen for no reason (at the time).

Furthermore, the film focuses on the events involving the three main characters. Maybe everyone else on their street just stayed home, waited it out, and were fine.

Well, it's possible, and certainly someone would be as irrational and scared as these people -- but yes generally life would go on and only be a little more dangerous due to dumb people like this. Corporations will figure out how to keep selling us stuff.