This is a story about technology, science and goodwill coming together in a way that benefits everybody and costs nobody. Sound improbable? Well, it gets better. The architect of this arrangement is, if you can believe it, a municipal utility.

It's Con Edison—New York City's electric company.

Con Ed is offering its customers an Internet-connected thermostat. It's smart, simple—and you can control it online or via a smartphone.

For example, you can adjust your home's heat or air-conditioning as you return from a trip to make it comfortable when you arrive. Or if you forget to turn off the AC, you can do it with a couple of taps on your phone.

You don't even need home Internet service for this setup. The thermostat communicates with the Internet on a frequency reserved for those old pocket pagers. Sneaky!

But what if, like most New Yorkers, you have a window air conditioner instead of central air?

For you, Con Ed offers a new kind of thermostat. This smart AC kit, from a company called ThinkEco, resembles a short extension cord. Once you plug your AC into it, you can use the included remote control within your apartment. (The remote also measures room temperature.) In addition, the kit comes with a USB transmitter stick that plugs into your computer and borrows its Internet connection for the air conditioner's use. This allows you to program your window unit from a Web site or a phone app.

The best part, though, is that all of this is free. Actually it's better than free: sign up and use either of these thermostats, and Con Ed also gives you a $25 gift card.

Have they lost their ever loving minds?

Not quite. It turns out that there's a beautiful catch.

You're not the only one who will have control over your AC. During what industry insiders call “peak usage events”—the most sizzling hot days—Con Ed can dial back your AC from its office.

Yes, it sounds like Big Brother is warming you, but it's not as bad as it sounds. Con Ed adjusts your temperature by only two or three degrees. (And last year it did so only twice.) Plus, it warns you in advance by phone, e-mail or text message. Most reassuringly, you can override the override. If you don't like how Con Ed tweaked your temp, you can dial it right back again.

Isn't Con Ed in the business of selling electricity? Why would it go to such lengths to get you to consume less of it?

Imagine if thousands of customers installed these thermostats. If Con Ed can throttle back their AC en masse, even just a notch or two, New York might avoid a brownout or a blackout.

For Con Ed, there's a bigger payoff: infrastructure cost savings. As the population grows and demand rises, Con Ed has to install more equipment. More substations, more cables. If Con Ed can delay those expenditures by two or three years, this free-thermostat program will have been a shrewd investment.

Con Ed and other utilities have offered similar deals to commercial customers for years. “During a demand-response event, we can take one or two elevators out of service,” says Con Ed's Adrianne Ortizo, who runs these programs. “We can turn down common-area lighting. If the HVAC is connected, we can also cycle those systems on and off.” In exchange, building managers enjoy discounts for power and equipment.

Yet bringing this offer to individuals is an overwhelmingly powerful idea. Cutting back on power improves air quality. The utility delays those huge capital expenditures. The city avoids a blackout. And you get a cool-looking thermostat you can control with a free app.

Now, there are six million air conditioners in Con Ed's territory. So far 23,000 customers have signed up for the thermostat; Con Ed aims to distribute 10,000 more this year. Together they could save five megawatts—on a day when the city typically consumes 13,000 megawatts. That's the tiniest drop in a very big bucket.

But never mind. Every great idea has to start somewhere.

Three systems to control temp from afar: