Using the sun’s energy to power green vehicles seems like a no-brainer. There’s a lot of sunlight in this world—why not harness it in order to reduce our dependence on both fossil fuels and the electrical grid? Well, the main problem is, it’s not as easy as you think.

There are a few solar concepts out there. We have the Batmobile-esque , which has been evolving for over half a century and will be released this year, the company says—though the solar power can only get you about 40 miles on a clear day. There’s the , a luxury sedan that integrates solar panels onto its hood and is said to get up to 500 miles on a charge. There was the C-MAX Solar Energi Concept, designed in a partnership between Ford and Georgia Tech. Instead of needing to be plugged into an electrical grid to charge, its battery would have been fed by solar power. There’s even a Formula SAE program designed around building a solar-powered car.

But if you’ve ever been wondering why solar-powered EVs just haven’t caught on, Jason Fenske over at is here with a whiteboard full of math to tell you exactly what you need to know about what makes this prospect so challenging.

I’m not the kind of person who’s going to be able to explain math to you in a way that makes sense—you’ll have to watch Fenske’s videos for those details. What I can do, however, is paint a few broad strokes to give you the overview you need.

Basically, to harness the 173 trillion kilowatts of power that hit the Earth at any given second is no easy task. The small size of a car can only harness about 12 kilowatts of that power, but that translates to about 62 mph when you convert that power into speed—and you could travel 62 mph indefinitely, as long as the sun is out. In those same ideal circumstances, you could also charge a 75 kW battery in just over six hours. Not bad, right?

That’s because those numbers are in a perfect world, which is not the world we live in. Sunlight is reflected back into the atmosphere. It’s dulled by clouds and tall buildings. And the Earth is curved, so some parts of the planet are going to get more sun than others. And Fenske’s example car was entirely covered in solar panels; in his next calculations, he factored in windows and glass.

Now, instead of 12 kilowatts of power, only 0.375 kW are hitting the solar panels. That translates to a constant speed of 19 mph under pretty ideal conditions. It also means it would take over eight days to charge your battery. And that’s only if you’re not using it.

That’s not ideal. It’s not even really worthwhile to slap a solar panel on there to try to absorb some extra power, since that power would be incredibly minimal. It would be the equivalent of putting a batch of cookies out in the weak sunshine on the off chance they might get cooked a little bit.

Once again, it all comes down to efficiency. In the future, solar panels may be more efficient, but right now, we’d be depending on a pretty finicky natural resource whose ideal performance could only be achieved at a certain location at a certain time of day during a certain time of year and under certain weather conditions. That’s not exactly something you want to rely on for your daily commute.