You've invested in solar panels to harvest energy from the sun, but without daisy-chaining an array of car batteries, you've got no way to store it. With the Powerwall, Testa used its expertise manufacturing lithium-ion batteries for electric vehicles to provide a sleek alternative. The 210-pound wall-mounted battery can store surplus energy from solar panels. But it can also automatically bank electricity from the grid during off-peak times, when rates are lowest. That way you can power your home-or charge your Model S-for pennies even when electricity is in highest demand. The company makes a 7 kWh battery for daily use, and a 10 kWh version that can double as a backup generator.
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