Like analog TV and Marshall Tucker fans, solar power is a holdover from the Carter administration. Yet, for modern businesses like Google and General Motors, it’s a promising alternative energy source. So far, “promising” is as far as it’s gotten: the density in data centers and in the typical office complex — lots of demand in a small area — turns solar arrays into a pipe dream. At Google HQ, for example, nearly every rooftop is covered with solar panels, and they have plans for more coverage, but the array can only provide for about 30 percent of peak power usage.

That’s not stopping GM from building the largest solar array in the world at their factory in Zaragoza, Spain. With 85,000 modules on the roof (compared to only about 10,000 at Google) covering an area as big as a small town (about 44 acres), GM plans to pump 10 megawatts of power into the assembly plant, which builds 480,000 vehicles per year. Partnering with Clairvoyant Energy (a company that designs photovoltaic solar devices) and Veolia Environmental (an energy management company based in Paris), the array will be completed by October 1.

So, you might wonder: how are they getting around the density problem? For starters, the assembly plant, a one-story building, is a good fit for solar power.–And, while big tech companies like Intel and Microsoft give every employee a computer and an LCD screen (and maybe a laptop), assembly plants are not nearly as power-intensive per employee.

According to Dale Sartor, a team leader at the Lawrence Berkeley National Laboratory who oversees research on energy efficiency in high-tech buildings, companies like GM overcome another solar energy problem: they will produce excess power when the sun is bright and sell energy back to the local utility company, then buy it back at night. Yet, even for high-tech companies, better energy efficiency inside an office building, good power management, and a little ingenuity can make solar arrays more tenable. “If the office building is multi-story or the energy intensity is high (e.g., a data center), then the solar array can expand beyond the building. Anything is possible with enough space and budget,” says Sartor.

Now if we can just make one small enough to power the family sedan.