Oops: Tesla’s Green Battery Doesn’t Actually Work That Well With Solar

  • Date: 06/05/15
  • Tom Randall, Bloomberg

Even Elon Musk’s SolarCity, the biggest supplier in the U.S., isn’t ready to install Tesla’s home battery for daily users because the battery “doesn’t really make financial sense”

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. – 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. 

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. 

A Tesla spokeswoman, Khobi Brooklyn, said via e-mail that both sizes of the Powerwall battery would be available in the U.S. by late summer. She didn’t say which distributors will offer the smaller battery designed for everyday use. Tesla is set to report earnings later Wednesday, although the new line of home batteries will probably take a back seat to sales of the company’s battery-powered cars.

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.  

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