The problem is you both seem to focus only on your own life experience in Modern, Fully Energized First World Cities.  So no wonder it's so difficult to imagine the collapse of a major urban center.

But consider looking to history for examples - both modern and ancient times - when resources could no longer support cities or countries...  

Or you could watch in Real-Time how First- and Second-World Cities are currently experiencing urban/economic decay (eg. India, Pakistan, S. Africa {www.allafrica.com]).  You can See the effects of the Power Shortages and interuptions on sewer and water services, the lack of reliable employment due to unpredictable power outages on industry, decling property values, and heavy tax burdens for those who try to stick it out... there are many, many symptoms and issues far beyond simple AC for the hot summer.  People may not want to leave their homes but might be Forced too - either because they lost their homes for foreclosure or because their city is becoming virtually unlivable.