The first plot is a very good outcome. It shows no grid usage. The home (blue) is exclusively being powered by battery storage (green) and solar (orange).  This pattern happens fairly often in the spring when household energy consumption is low and solar production is high. It also means no grid exports, even if the grid needs additional resources. The second case – contributing to the ramp – is disturbingly common. The battery starts charging as soon as the sun is up and is fully charged around mid-day, at which point, all of the solar comes online all at once. From a grid standpoint, it’s contributing to the ramps and is not helping absorb surplus solar. The last two scenarios are less common. The third plot shows some use during off-peak hours (I was charging a electric car), my intentional draw from the grid immediately before the 4-9 pm period, and use of the battery throughout that peak window (with a small amount of exports). It was also a very hot day. You see my AC unit cycling on and off. The last plot shows the full capability of the battery, close to 7 kW, which is rarely seen. The battery went into storm mode and drew power from the grid rather than charge only using the rooftop solar. When operated in default mode, the battery will almost never charge or discharge at its full capability. It means that behind-the-meter batteries are an under-utilized, untapped resource during periods from the grid needs resources the most and during period with excess generation on the grid.

If left to operate on their own, the batteries typically charge as soon as the sun comes up (the wrong time from a grid perspective), often don’t absorb surplus generation, and rarely, if ever, export to the grid when resources are needed most. By design, they operate with the customer in mind, which is an excellent objective. However, it is possible to lower customer bills, provide backup power, and also improve operations for the grid. As saying goes, “we can walk and chew gum at the same time.”

Why does this matter? Behind the meter battery storage is a growing, untapped resource, and the need for flexible, predictable resources is growing.  The below plot shows the growth in residential behind-the-meter battery storage in California. There are currently about 400 MW, but the magnitude of growing quickly. Roughly 8-10% of new solar installations are also install battery storage at the same time. And the share of solar sites electing battery storage is growing. What can be done to tap into this under-utilized resource? Clearly, it is not enough to have the batteries installed. It is necessary to operate them at the right times and to provide customers incentives to do so.