Last week, we covered the reborn Fisker Karma (now the Karma Revero) re-introduction, and there was one detail in that reveal I don’t think I paid enough attention to: the solar roof. Karma says it will “create enough energy to power the car.” That’s bullshit, but many sites are reporting it not just as true but revolutionary. Yes, that was the BBC calling it “A Car Powered by its Roof.” Let’s go into why this is incorrect.

To be fair, the BBC article does hedge and suggest that the roof likely wouldn’t power the car very long, but other sites say the Revero is “entirely powered by rooftop solar panels” and Karma certainly isn’t doing anything to dissuade this idea.

Karma started all this with this vague but very compelling claim from their press releases and on their website:

The claim that the “solar roof will create enough energy to power the car” not only literally violates the First Law of Thermodynamics, but even if we’re not literal and take the statement for what they seem to mean, that the solar panel can collect enough solar energy to power the car, while it may be technically true, in any practical circumstance, it’s ridiculous bullshit.

I’ll go into why, with some numbers and math and fun stuff like that.

First, let’s get our baseline specs in order: the Karma Revero has a primary battery rated at 20.1 kWh. The solar roof is rated at 200W. So, the question is not if the solar panel can charge the battery, because of course it can, but how long will it take for the Revero’s solar roof to charge the battery to “power the car”?

First, I called a local solar energy company, NC Solar Now, to get some information. I was told that a solar panel around Raleigh, North Carolina (a good city to use because, according to a Ford engineer I spoke with, is just about in the middle for the amount of sunlight received in the U.S.) makes right about 283 kWh/year.

So, if we divide that by 12, we get 23.6 kWh/month, and if we divide that by 30, we get roughly 0.79 kWh/day. That means, assuming the panel azimuth and angle are in the ideal position, and there’s not too much unexpected cloud cover, and you’re willing to keep the panels as clean and dust-free as possible, you should be able to charge your Karma Revero with just the solar panels in, oh, right about a month.

Now, if we don’t trust those numbers, we can try to figure it out for ourselves. So, a 200W solar panel is making 0.2 kW, to start. Then, let’s use the U.S. Department of Energy’s map on how many PV/watts of energy from the sun a solar panel of average efficiency (as in, not built by NASA for space probes) can collect. To keep things consistent, let’s stay in Raleigh, which gives us 1600 PV watts/year.

We then need to multiply that number by 0.78 to account for all the losses in the system, the inverter losses, wiring resistance, etc. That gives us 1248 PV watts.

So, let’s then multiply 1248 by the size of the panel, 0.2 kW, which comes to 249.5 kW/year, pretty close to the solar company’s figure. Let’s divide that by 365 to give us 0.68 kW per day, which means it’ll take 29.4 days to get to just about 20 kWh for the main battery.

So, again, about a month. A month of non-stop solar charging, to get you a full battery, which Karma says will take you a whole 50 miles.

And, of course, that’s being generous and assuming the car stays in a good angle to the sun and the days aren’t unusually cloudy and dust and leaves and bird shit don’t obscure any of the solar cells. Leave your car outside for a month without driving it and tell me how clean it looks afterwards.