Twenty years ago, the problem with rooftop solar was that customers needed a large collection of lead acid batteries to store their daytime energy and use this energy at night. But simple net metering rules made it possible for the electric grid to function as a 100 percent efficient storage device. Unfortunately, utilities are doing everything they can to eliminate net metering so they can maximize their profits. So the compelling need for battery storage linked with rooftop solar has re-emerged.
Although lead acid batteries are an inexpensive and mature technology, they are not well-suited for home energy storage applications. Fortunately, lithium-ion batteries have become much more reliable and inexpensive — primarily because of production volumes required by the automotive industry. That availability comes just in the nick of time as net metering is constrained in some states, and time-of-use electric rates shift towards late afternoons and evenings.