* Photo: James Day * Problem It's high noon in July. At 90-plus degrees outside, the masses are jonesing for AC. But it's seriously expensive to keep the juice flowing when demand crests. Firing up turbines that sit idle 360 days a year can multiply electricity costs by a factor of 10. How to keep cool without stressing the grid?

Solution Pay big users to cut consumption when the need arises. Many utilities already do an ad-hoc version of this, an emergency practice known as demand response that has lately been promoted by Jon Wellinghoff, acting chair of the Federal Energy Regulatory Commission. Now there's an alternative: Call EnerNOC, a Boston-based company that gangs commercial users who are willing, for a quarterly payment, to trim back operations on 30 minutes' notice. EnerNOC micromanages consumption at 3,400-plus locations from Maine to California. Between dimming lights, adjusting thermostats, and suspending industrial activities, the potential cuts top the output of a large nuclear reactor. And the savings can be huge. EnerNOC's cofounder, Tim Healy, points out that 10 percent of all US generating capacity exists to meet the last 1 percent of demand. Utilities paid EnerNOC $100 million last year simply to stand at the ready—insurance, in effect, against the inevitable days when every AC unit is humming.

Illustration: Lamosca

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