Tesla's New Battery Doesn't Work That Well With Solar

Even Elon Musk's SolarCity, the biggest supplier in the U.S., isn't ready to install Tesla's home battery for daily users

Here's Why Tesla Is Building Batteries for Your Home

Tesla Chief Executive Elon Musk introduced a new family of batteries designed to stretch the solar-power revolution into its next phase. There's just one problem: Tesla's new battery doesn't work well with rooftop solar—at least not yet. Even Solar City, the supplier led by Musk, isn't ready to offer Tesla's battery for daily use.

The new Tesla Powerwall home batteries come in two sizes—seven and 10 kilowatt hours (kWh)—but the differences extend beyond capacity to the chemistry of the batteries. The 7kWh version is made for daily use, while its larger counterpart is only intended to be used as occasional backup when the electricity goes out. The bigger Tesla battery isn't designed to go through more than about 50 charging cycles a year, according to SolarCity spokesman Jonathan Bass.

Telsa CEO Elon Musk points at the sun.
Telsa CEO Elon Musk points at the sun.
Photographer: Tim Rue/Bloomberg

Here’s where things get interesting. SolarCity, with Musk as its chairman, has decided not to install the 7kWh Powerwall that’s optimized for daily use. Bass said that battery "doesn't really make financial sense" because of regulations that allow most U.S. solar customers to sell extra electricity back to the grid.1

For customers of SolarCity, the biggest U.S. rooftop installer, the lack of a 7kWh option means that installing a Tesla battery to extend solar power after sunset won't be possible. Want to use Tesla batteries to move completely off the grid? You'll just to have to wait. “Our residential offering is battery backup,” Bass said in an e-mail. 

Musk said in a quarterly earnings call on Tuesday said that demand for the batteries has been "crazy off the hook," with 38,000 reservations for the Powerwall. While storing residential power with the Powerwall is still more expensive than grid power, he said, "that doesn't mean people won't buy it." Demand for the new batteries, including those for businesses and utilities, has been so strong that the company may need to considerably expand its $5 billion battery factory that's under construction in Nevada.   

The Economic Case for Tesla's New Battery Gets Worse

SolarCity is only offering the bigger Powerwall to customers buying new rooftop solar systems. Customers can prepay $5,000, everything included, to add a nine-year battery lease to their system or buy the Tesla battery outright outright for $7,140. The 10 kilowatt-hour backup battery is priced competitively, as far as batteries go, selling at half the price of some competing products.

But if its sole purpose is to provide backup power to a home, the juice it offers is but a sip. The model puts out just 2 kilowatts of continuous power, which could be pretty much maxed out by a single vacuum cleaner, hair drier, microwave oven or a clothes iron. The battery isn’t powerful enough to operate a pair of space heaters; an entire home facing a winter power outage would need much more. In sunnier climes, meanwhile, it provides just enough energy to run one or two small window A/C units. 

For more demanding applications, Tesla made its Powerwall batteries so they can be attractively stacked, side-by-side. It looks like this:

But SolarCity doesn’t offer a discount for multiple batteries. To provide the same 16 kilowatts of continuous power as this $3,700 Generac generator from Home Depot, a homeowner would need eight stacked Tesla batteries at a cost of $45,000 for a nine-year lease. "It's a luxury good—really cool to have—but I don't see an economic argument," said Brian Warshay, an energy-smart-technologies analyst with Bloomberg New Energy Finance.  

Yes, Tesla's Powerwall is cool technology with massive disruptive potential. As battery costs continue to fall and electricity regulations continue to evolve in the U.S.,2 it's going to make ever more sense to own a home battery. SolarCity said in its earnings call on Monday that it plans to offer an off-grid package next year in Hawaii, where electricity prices are almost triple the U.S. average.  

And the home-battery system is just one offering in the new lineup of Tesla batteries. The company is also doing business with big companies like Wal-Mart, Amazon and even with electric utilities like Southern California Electric and Texas-based OnCor. The economic argument for the commercial systems is straightforward in states with the right incentives, including battery subsidies and expensive electricity charges during peak hours.3 Tesla now has a clear pricing advantage against its battery competitors.

But the Powerwall product that has captured the public's imagination has a long way to go before it makes sense for most people. Even in Germany, where solar power is abundant and electricity prices are high, the economics of an average home with rooftop solar "are not significantly enhanced by including the Tesla battery," according to an analysis by Bloomberg New Energy Finance. 

That won't stop homeowners from buying Tesla's new batteries. Germans are already buying storage systems by the thousands at significantly higher prices. In the U.S., the product's launch prompted a record day of inquiries from prospective new customers, according to SolarCity's Bass. "There's a tremendous amount of interest in backup power that's odorless, not noisy and completely clean," he said.

Tesla is probably making very little profit on the home batteries at this point and might even be selling them at a loss, according to research by BNEF. Both Tesla and SolarCity are just getting started, trying to get some traction before Tesla's massive $5 billion battery factory begins production next year. That's when the battery market really gets interesting.

Update, 6:25 p.m., May. 6: Added Elon Musks's comments from Tesla's quarterly earnings call

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  1. 1 One of the reasons Tesla's Powerwall batteries don't make sense for many U.S. customers is the policy called net metering. These laws require utilities to buy back excess solar power sent from rooftop systems to the power grid. Net metering is one of the biggest government incentives for installing rooftop solar—and it's also one of the biggest reasons not to use Tesla's batteries for storage. 
  2. 2 There are 43 states with net metering policies to allow rooftop solar customers to sell power back to the grid, according to the Solar Energy Industries Association. At least 12 states are weighing legal disputes against the rules, according to BNEF. If net metering goes away in states with expensive electricity, the argument for pairing solar with home battery storage will improve. 
  3. 3 Peak demand charges in California can account for as much as half of a commercial electricity bill. Those costs can be greatly reduced by storing electricity from cheaper hours to be used in more expensive hours—a practice known as peak shaving. California also offers subsidies for commercial battery projects that can cover up to half of the eligible costs.