Images: A visit to 'negawatt' central

Inside the "Noc" of EnerNoc, one of a handful of new companies in the energy-efficiency business. The network operations center is where employees monitor the power grid and customers' sites. When a utility or power generator gives EnerNoc a signal, technicians in the operations center alert their customers and then dial down their energy usage using the Internet.

The changes can be relatively simple, such as turning down the lights in a hotel lobby for 15 minutes. But added together, they can lighten the load significantly on the grid during peak times, and potentially obviate the need to build more power plants. Utilities also want to avoid turning on expensive and dirty "peaking plants" that get turned on at peak times, such as the middle of a hot summer day, when air conditioners are on high.