Last fall, Elon Musk suggested that the Tesla Model 3 might have a solar roof, allowing the electric car to recharge its batteries in sunlight. It looks like he's having second thoughts about that, as he just announced that the company is scrapping the idea … for now. But why?
Let's Do the Math
What part of the car's surface should be covered with solar panels? To get as much sunlight as possible, solar panels should face directly toward the sun, so it makes no sense to put panels on the sides. Sure, they'll get some energy, but not enough to justify the cost. So we'll focus on the top-facing surfaces: roof, trunk, and hood. That gives us about three square meters of surface area, which, with today's PV technology, is roughly 400 watts worth of panels.
On a good day, the car would be parked (or driving) in direct sunlight all day, but the panels are always oriented directly up, so unless you're living near the equator, you won't get the maximum sunlight. Let's assume twelve hours of daylight with a typical value of five Peak Sun Hours per day. That's a fairly generous estimate because I'm establishing a best-case scenario. If it doesn't work out in the best conditions, then it definitely won't make sense under normal circumstances.
400 watts x 5 PSH = 2000 watt-hours = 2 kWh per day
The Model 3 is expected to have a battery capacity of around 60 kWh, giving the car a range of roughly 215 miles (345 km). That means that a full day in the sun would replenish about 3% of the battery's total capacity, increasing the EV's range by a paltry 7 miles (11.5 km). For someone with a short commute to work, that would result in a net-zero energy vehicle on sunny work days. Even then, is it worth it?
… And the Economics
While technically feasible, the solar option has to make economic sense, and for a relatively low-end EV, that's where the problem lies. At today's average utility rates, two kilowatt-hours of electricity costs just under $0.25, so, a driver could save about $1.00/week on electric "fuel" costs. By itself, a 400-watt solar panel costs $200 - $300, plus the added circuitry to wire it into the charging system, and the extra cost of integrating it onto the car's surface. Let's say all that adds $1000 to the retail price of the vehicle. By saving $1/week, the added solar charging system would have a payback period of nineteen (19) years. Ouch.
The Day May Come, but It Is Not This Day
Musk is a visionary - some might even call him a Pollyanna - but he's also a realist. At this time, the technology for a solar-assisted electric vehicle is available but pricey. Remember when he launched the first Tesla vehicles - the goal was to sell enough high-end cars so the company could refine its design and manufacturing processes enough to eventually sell more affordable EVs to the middle class. That's what the Model 3 is: an EV for the middle class. Add too many bells and whistles and it's priced out of range for the target market.
But there's hope! Innovative student groups like Team Eindhoven with its new Stella Vie, PrISUm and its Penumbra, and Sunswift with its record-breaking eVe, are working on practical solar-assisted EVs. Some of that technology will make its way to existing car companies, including Tesla. Maybe in a decade or so we'll see a lower-end Tesla sporting a photovoltaic topside.
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