The solar car was parked at a diesel show

On August 31, 1955, General Motors demonstrated what is often called the first solar-powered automobile. The Sunmobile was 15 inches long.

William G. Cobb had fitted a balsa-wood model with 12 selenium photovoltaic cells. They supplied a 1.5-volt motor, which drove the rear wheels through a small transmission. It was a working solar vehicle in the same sense that a model railway is a working railway: the mechanism was real, while several expensive problems had been removed by scale.

That scale was not a cute detail. Make a model ten times as long in every dimension and its surface area grows by a factor of 100, but its volume—and roughly its mass—grows by 1,000. The sunlight available from a vehicle’s own roof does not keep pace with the vehicle built beneath it. The Sunmobile showed that photovoltaic cells could propel something shaped like a car. It did not show that enlarging that thing would produce transportation.

The setting supplied a second qualification. GM unveiled the model at Powerama, a million-square-foot exhibition beside Chicago’s Soldier Field devoted mainly to the company’s command of diesel and industrial power. Exhibits included locomotives, submarines, an oil-drilling rig, military equipment and a 50-ton dump truck. In a stage show, bulldozers danced the mambo. Solar power occupied considerably less floor space.

Later accounts can make the event sound like an early corporate glimpse of the post-petroleum automobile. It was also a science demonstration tucked inside a $7 million celebration of engines that already moved enormous objects. GM subsequently sent the Sunmobile around the country with its educational “Previews of Progress” program; the little car was useful as a visible proof of photovoltaics, not as a product proposal.

Powerama displayed both technologies at their persuasive size. Diesel got the locomotive. Solar got something that could sit on a table.