The sun is a stubborn on-again-off-again partner in our solar energy relationship. With no way to store excess solar energy, solar homes are forced to return shamefacedly to the electrical grid each evening, not to mention in moments of cloud cover and/or rain.

Tesla Motors offers a solution to this dilemma, or claims to. Powerwall is billed as a home battery system that reserves extra solar energy during the day for use when the sun goes down. Powerwall is purported to revolutionize solar energy. The first systems will be installed in a matter of weeks. The Powerwall technology is impressive and even beautifully packaged. But Tesla’s global ambitions are perhaps far-fetched and over-hyped.    

Elon Musk, founder and CEO of Tesla Motors, and a bona fide rock star in the green energy movement, has said that his goal “is to fundamentally change the way the world uses energy.” He believes that solar power “can and will become the world’s predominant source of energy within our lifetimes.” Powerwall, and the revolution it will bring, has been eagerly anticipated since Musk’s announcement. The technology is critical to Hillary Clinton’s recent promise to install, within her first term, a half billion solar panels across the country.

Initial interest has surpassed Tesla’s most optimistic estimates. Thirty-eight thousand Powerwall units have already been pre-ordered. Based on the sale of these introductory units, Musk intends to expand globally in the hopes of selling some two billion batteries.

There are several snags in Tesla’s plans for world domination, however. The most interesting and fundamental objections come from Daniel Nocera, Harvard professor and inventor of artificial photosynthesis. He predicts that no matter how solar energy develops, the United States made its choice between fuel and batteries a hundred years ago. Simply put: the U.S. has invested too much to leave the grid now. 

“In the United States, the economics don’t make sense. If we could calculate all that you’ve invested in your energy infrastructure - all the wires, all the power plants, we’re talking 70 hundred trillion dollars which you’ve already paid off," Nocera said, exaggerating for effect, "You’re not going to walk away from that,” Nocera explained in an interview with the Washington Post.

Nocera is also unsure of Tesla’s chemistry. The larger 10killowatt-hour version of the lithium Powerwall battery uses a mixture of nickel and manganese, while the smaller 7killowatt-hour version relies on a mixture of nickel and cobalt. “Lithium has to be paired with other metals, so it’s paired with cobalt, and nickel. But on the global scale you run into materials issues, there is not enough of these metals,” said Nocera.  

Tesla is restrained both by these external realities, and by a slew of problems with the product itself. Powerwall can only deliver 2 kilowatts of power continuously. Reportedly, Tesla’s partner, Solar City, will not install the smaller 7killowatt-hour battery when it launches. Mass production will make the battery system cheaper, but Tesla has yet to complete its $5 billion dollar Nevada factory. Not to mention, Powerwall makes real consumer sense in only three regions: Germany, Austria, and Hawaii.   

Powerwall is a genuinely exciting advancement and in time may make solar energy profitable and efficient. But Elon Musk and Tesla must resolve these practical limitations to advance the revolution. Solar energy is coming, but don’t cut down your power lines just yet.   

Grant Wishard is an intern at The Weekly Standard.

This post has been corrected.