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KAI RYSSDAL: We can only hope you're listening to this from the comfort of an air-conditioned car or some other cool place. Because it's a pretty good bet that wherever you live it's hot. One hundred and four at my house over the weekend. At least in the 90s in many other parts of the country today. The National Weather Service has come out with excessive heat warnings for parts of Illinois, New Jersey and Pennsylvania. And cities like Chicago and New York have set up official cooling centers. All that heat is, of course, taxing the power grid along with our patience. Marketplace's Amy Scott reports it might get worse before it gets better.

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AMY SCOTT: As temperatures hit the triple digits in parts of the country, utilities and grid operators asked customers to conserve.

PJM Interconnection manages a huge chunk of the power grid from Illinois to Maryland. Spokesman Ray Dotter expects today's demand to break last year's record by about 6,000 mega-watts.

RAY DOTTER: One mega-watt is enough power to run about a thousand homes. The increase that we expect to see in our peak usage today would be like adding an entire city to the area.

A city of air conditioners. California's grid operator also expected to shatter previous records today. It's asked power companies to put off any repairs until demand cools. Stephanie McCorkle is a spokeswoman:

STEPHANIE MCCORKLE: Murphy's Law is, you know, you change out a lug nut or something, and something else goes awry. So what we're saying is, if you don't have approved maintenance work through us, don't start any.

McCorkle says there's no threat of the kinds of rolling blackouts California endured five summers ago. She says the state has added about 13,000 megawatts of capacity since then. Boston University finance professor Mark Williams says other parts of the country aren't so lucky. New England would need a new power plant every year to keep up with demand. But Williams says there isn't much incentive to build them.

MARK WILLIAMS: The cost of coal in some regions has more than doubled in the last 18 months to two years. But the price of power that someone who's generating electricity can get in the open market has not. So economic incentives have not been there.

Williams says political pressure only lasts as long the heat.

In New York, with a high of 97, I'm Amy Scott for Marketplace.