Right now, homes equipped with solar panels are almost always tied into the local electricity grid. When the power goes out, the panels go too. Allowing panels to back-feed onto the grid risks the safety of a linesman making repairs. Using the panels during an outage—or just saving their daytime juice for use at night—requires storing the electricity the solar panels collect. These days, the rare homeowners willing to invest in battery backup usually accomplish it with a gang of lead acid car batteries.
But Tesla has worked for years to optimize lithium-ion battery packs for use in electric vehicles. SolarCity has developed a program to install and lease photovoltaic panels to customers at cost-effective rates in 12 populous states. A partnership, if successful, could create the equivalent of a giant residential solar backpack: Tesla crams lithium-ion batteries into one huge pack that could be charged up by SolarCity's photovoltaic panels. It's that simple.