Audi has inked a deal with solar panel expert Alta Devices to fit the energy-generating panels to the roofs of its future cars.

Alta Devices, based in California, is renowned for its highly efficient solar panels. It will work with Audi to develop thin, lightweight solar cells that can be integrated into the panoramic glass roofs commonly found on cars. The first prototype should be ready in late 2017. Later, the two firms will develop solar cells that will be able to line a car’s entire roof surface.

Audi will initially fit the solar roofs to its electric cars. The generated energy won’t be able to add range directly, however. Audi says the energy will only be used to power ancillaries like the air conditioner or seat heaters, thus saving that little bit of extra energy to increase range.

Eventually, though, the solar roofs will be able to directly charge the main battery, Audi promises. Such a system features on the Fisker Karma and its rebranded successor, the Karma Revero. We’re not talking much range. Given the limited space, a couple of miles of range will likely require at least a week of maximum sunshine.

Audi’s first volume electric car will be the e-tron SUV due in 2018. It will be joined shortly after by the coupe-like e-tron Sportback. Both models will be built at a plant in Belgium and offer a range of at least 250 miles. Audi will add one additional electric car by 2020, believed to be a city car targeting the BMW i3 and to a lesser extent Chevrolet’s Bolt EV.