Tesla’s Home Backup Battery: Not Ready for Prime Time?

Last week Tesla Motors Inc. unveiled two battery systems that homeowners can use to store solar power or back up their regular electrical service. But neither is economical or event practical, says Bloomberg New Energy Finance.

The smaller 7-kWh Powerwall battery is designed for daily charging and discharging. Tesla touts the unit as a way to store power from the grid or a home's solar array and use it during demand peaks or in the evening.

But SolarCity, the solar panel supplier chaired by Tesla CEO Elon Musk that is marketing Powerwall batteries, says the 7-kWh unit "doesn't really make financial sense" for homes with solar panels because of regulations that allow them to sell the electricity they collect to the grid. Thus SolarCity isn't offering the $3,000 battery.

SolarCity is promoting the larger 10-kWh Powerwall battery ($5,000) for use as an emergency backup unit for homes. But Bloomberg notes the unit has a different electrical architecture and is not suited for daily charging and discharging.

Even the larger battery supplies only 2 kW of continuous output, insufficient to power a typical American home, according to Bloomberg. Tesla batteries are designed to be ganged for more output. But the cost of the system can quickly become five times that of a conventional piston-powered generator.