Working for Ontario Power Generation, I understand a bit more about energy production and consumption now, and why really hot days are really hard on the system and the designers.
Since our power grid is connected across most of North America, we have a huge capacitance from all the transmission lines. Put simply, the lines themselves can hold a bit of power. If production outstrips consumption on a given day, the lines just increase their capacitance slightly, and obviously the same is true in reverse. This does not constitute a "battery" however, as there is no simple or compact way to store that much power. Power is (basically) generated and consumed at the same time.
You can compare of large-scale energy production and consumption to a car's electrical system. Your altenator, in a real car with a battery, just has to charge the battery. Imagine if there was no battery and your altenator had to be ready to deal with you cranking up the radio, a/c, and headlights at the same time. That altenator would have to be much beefier, and output much more power all the time. This would make the whole system inefficient.
Same goes for large-scale power generation. It is hard to justify upping the generation capacity just for those few days a year when everyone decides they NEED the inside of their house to be 20c/70f. I'm not talking unreasonable amounts of adjustment here, but if everybody would just be OK with 75 or 80, and turned their a/c off or down when they weren't home, we wouldn't have as many problems as we do. California especially - that state has to get some more generating capacity or they're going to be in serious trouble soon.