Integrating photovoltaic cells on vehicles is nothing new. In fact, solar-powered race cars have been around for more than 25 years, proving that the power of the sun can indeed provide enough energy to propel a car down the road. Of course, these cars are ultra-lightweight and plastered with solar cells on every conceivable surface, tasked with carrying just a driver at a constant speed. While not practical for driving as we know it, they are valuable engineering exercises that helped move the bar in developing electric vehicle efficiencies. Just one example is GM’s Sunraycer solar race car, built under the guidance of the renowned master of efficiencies, the late Paul MacCready of AeroVironment, which won the World Solar Challenge in Australia in 1987.


First, let’s examine the requirements. For “acceptable” travel at freeway speeds (30 m/s, or 67 m.p.h.), and the ability to seat four people comfortably, we would have a very tough job getting a frontal area smaller than 2 m² and a drag coefficient smaller than cD = 0.2—yielding a “drag area” of 0.4 m². Even a bicyclist tends to have a larger drag area than this! Using the sort of math developed in the post on limits to gasoline fuel economy, we find that our car will experience a drag force of Fdrag = ½ρcDAv² ≈ 250 Newtons (about 55 lbs).


Work is force times distance, so to push the car 30 meters down the road each second will require about 7,500 J of energy (see the page on energy relations for units definitions and relationships). Since this is the amount of energy needed each second, we can immediately call this 7,500 Watts—which works out to about ten horsepower. I have not yet included rolling resistance, which is about 0.01 times the weight of the car. For a super-light loaded mass of 600 kg (6000 N), rolling resistance adds a 60 N constant force, requiring an additional 1800 W for a total of about 9 kW.


What can solar panels deliver? Let’s say you can score some space-quality 30% efficient panels (i.e., twice as efficient as typical panels on the market). In full, overhead sun, you may get 1,000 W/m² of solar flux, or a converted 300 W for each square meter of panel. We would then need 30 square meters of panel. Bad news: the top of a normal car has well less than 10 square meters available. I measured the upward facing area of a sedan (excluding windows, of course) and got about 3 m². A truck with a camper shell gave me 5 m².


If we can manage to get 2 kW of instantaneous power, this would allow the car in our example to reach a cruising speed on the flats of about 16 m/s (35 m.p.h.). In a climb, the car could lift itself up a grade at only one vertical meter every three seconds (6000 J to lift the car one meter, 2000 J/s of power available). This means a 5% grade would slow the car to 6.7 m/s, or 15 miles per hour—in full sun. Naturally, batteries will come in handy for smoothing out such variations: charging on the downhill and discharging on the uphill, for an average speed in the ballpark of 30 m.p.h.


One of the more realistic ways in which that solar powered cars could become practical is to charge up their batteries when they are parked, during the day. So this dream of a family being comfortably hurtled down the road by real-time sun will not come to pass. Some Prius models offered a solar roof option, but this just drove a fan for keeping the car cooler while parked.