One of the main attractions of electric and plug-in hybrid vehicles is that they allow owners to avoid gas stations. However, it can be difficult to know how much each mile driven actually costs, since local electricity rates can fluctuate. Enter General Motors’ new OnStar EcoHub app, which calculates charging costs for the Chevrolet Volt.

When a Volt is plugged in at home, EcoHub collects data on total home energy use from the electric company and figures out how much of that electricity is used to charge the car. Some people might find GM’s use of their home energy consumption information a little Big Brother-ish, but the General says EcoHub will allow consumer to compare their Volts’ energy use to other household appliances.

GM says the average cost to charge a Volt is between $20 and $30 per month. The cost varies based on local electricity rates, when owners charge their cars (off-peak electricity is usually cheaper) and how often they use their cars’ gasoline engines.

EcoHub tallies the amount of time a Volt spends running on electricity or gasoline. A “Ticker” display shows total electric miles driven versus overall miles, and tells owners how much gas they’ve saved while in electric mode.

GM is testing EcoHub in Pecan Street neighborhood of Austin, Texas. Pecan Street is a prototype smart grid community, so constitutes a sort of best case scenario as far as efficient EV charging infrastructure is concerned.

GM wants to make EcoHub available to all Volt customers, although there is no set timeline for its release. The obvious hurdle involves working with various power companies to gain access to home energy use data. GM hopes to partner with the Green Button Initiative, which allows electricity customers to access their home energy use data with smart meters, to make EcoHub work.

Any Volt, or other future GM EV, needs OnStar for this app.

In addition to the prototype EcoHub app, the Volt gets a modified battery pack for the 2013 model year. That gives the 2013 Volt an electric-only range of 38 miles and an EPA rating of 98 MPGe, compared to 35 miles and 94 MPGe for 2012 models.

Drivers of conventional vehicles have the advantage (or disadvantage, depending on how you look at it) of a gas pump ticker, which tells them exactly how much they are spending to fuel their cars. With EVs recharging off the power grid along with phones, refrigerators, and other devices, things get a bit more complicated.

EcoHub may give GM access to people’s energy use data, but it should give those people a better idea of how much they’re spending to drive their cars.