Every plug-in vehicle owner knows it’s cheaper to drive on electricity than gasoline. But how much electric driving actually costs is tough to ascertain from a monthly bill.

General Motors has developed a smart phone app designed to tell Chevy Volt drivers exactly how much car charging costs on a daily, monthly or yearly basis.

The EcoHub app is only available to people in the Pecan Street development, a community funded by federal research to explore how different clean-energy technologies can be integrated. But the company intends to make it available to other Volt drivers over time. And the app shows some of the potential of tying cars into the electricity grid.

People who opt in can have their Volt charge time monitored by GM’s OnStar service. Since homes in the Pecan Street development have smart electricity meters, the app can gather data on the entire home’s electricity usage as well. With that consumption data and electricity rates, drivers can see how much Volt charging compares to overall household demand

Plug-in vehicles are very much at the forefront of “connected cars,” or using mobile devices with cloud services to give consumers added features.

Volt drivers, for example, can remotely set charge times or unlock their cars through the OnStar network. The Nissan Leaf also has remote control features and the company is developing ways for the Leaf’s batteries to supply back-up power to a home. Ford and Microsoft, too, have a partnership to give Ford electric vehicle owners a way to charge at off-peak times for lower rates.

GM says that it intends to enhance EcoHub so that people can compare car charging to individual appliances or the heating and cooling load. But for those types of services, a household needs a smart meter or some other form of data gateway. “In the future with the right partnerships the app has the potential to actually check the status of the home as well as controlling devices from anywhere you use your phone,” said Paul Pebbles, global manager for electric vehicle and smart grid services at GM.

It’s not clear that consumers will turn to GM and use an OnStar app to monitor and control smart thermostats, for example, along with their plug-in car. But the Pecan Street project shows some of the benefits smart meters can bring electric vehicle owners.