What counts as the first solar car
Like many “firsts,” this one depends on the definition. If it means a vehicle that carried a person and ran on sunlight, the earliest examples came in the 1970s and 1980s. If it means a small demonstration vehicle powered by solar cells, the usual answer is a model built in 1955. Both are documented, and neither was a practical car.
The 1955 model
In August 1955, General Motors showed a small model car called the Sunmobile at an automotive show in Chicago. It was designed by William G. Cobb, and it was around 15 inches long. A group of selenium photovoltaic cells on the top produced enough electricity to turn a tiny motor that drove the rear wheel. This was a demonstration of the principle at a time when solar cells had been public for only about a year. Early silicon cells were extremely expensive, and nobody expected a full-size version.
Experiments in the following decades
From the late 1950s, engineers experimented with putting solar cells on larger vehicles, including small electric cars and carts, but the cells at that time delivered very little power per square meter. Some projects in the 1970s and 1980s, often by universities and enthusiasts, built one-person vehicles with large flat panels. They traveled slowly, and they depended on clear weather.
The races
The turning point for solar cars was competition. In 1982 the Australian adventurer Hans Tholstrup and the racing driver Larry Perkins drove a solar-powered car called the Quiet Achiever across Australia, from Perth to Sydney. It was a long trip with a vehicle built for efficiency rather than comfort, and it caught public attention.
Tholstrup then helped organize the World Solar Challenge, first held in 1987, in which teams race roughly 3,000 kilometers from Darwin to Adelaide across the Australian outback. The first winner was the GM Sunraycer, designed by a team that included engineers from General Motors and AeroVironment, which finished well ahead of the competition. Similar events later followed in other countries, including long races across the United States for university teams.
These races helped develop a class of ultra-efficient vehicles with very low aerodynamic drag, light carbon-fiber bodies, efficient electric motors and careful energy management. Teams used strategies such as slowing in clouds and driving faster in strong sun.
The arithmetic of a sunlit roof
The main reason solar cars stayed experimental is a matter of simple numbers. In bright sunshine, a square meter receives roughly 1,000 watts of solar power, at best. A car-sized surface of perhaps four square meters, covered with cells that are around 22 percent efficient, would give roughly 1,000 x 4 x 0.22 = 880 watts, or under a kilowatt, and only in ideal conditions. Typical cars need many kilowatts of power to travel at highway speeds, which is more than that surface could supply, while a race car is built to need very little power.
Race cars get around this by being long, thin, flat and extremely light, with very small tires, and by carrying minimal equipment. Those traits are the opposite of what a family wants from a vehicle, such as seats, luggage space, air conditioning, crash protection and a roof tall enough to climb under.
Where solar helps cars today
Battery cars were already around in the age of early electric cars, and the arrival of lithium-ion batteries made it far easier to store energy gathered from the sun. Modern electric vehicles can charge from solar panels on a house or from the grid, which changes the picture: the sunlight is captured on a large roof and stored in a battery for later use. Some production cars have offered small roof panels for modest extra range or for running fans and accessories, but these add only a limited amount to daily driving. Solar-assisted concept cars and a few small manufacturers have tried to push further, and results so far have been mixed.
Beyond the road, solar power has done well on vehicles that can be large, light and slow, such as solar-powered boats and some unmanned aircraft. For ground transport, the usual pattern is that sunlight arrives through a house or the electric grid rather than directly on the car.
What the history shows
The first solar car was a toy, and the best solar cars are racing prototypes. They have been valuable as engineering challenges, and some lessons carried over to ordinary vehicles in areas such as efficient motors and lighter structures. As everyday transport, though, sunlight falling on a car body is simply not enough on its own.