An IR image of heat escape from a Tesla battery pack.
This report for the Natural Resources Defense Council (NRDC), funded by a grant from the US EPA, looks at emerging residential battery systems that can provide backup power, store and reclaim excess solar energy, and offer reserve grid capacity to a utility or third party.
While these systems are in the very early stages of adoption, with only a few thousand units installed to date nationally, the number of installations is expected to skyrocket given decreasing purchase costs, the growing number of homes with roof top solar panels, and increasing interest in having round-the-clock access to electricity during/after extreme events such as hurricanes, forest fires, or earthquakes.
Storage systems can also help grid operators integrate higher fractions of renewable energy into their systems and help policy makers achieve zero-net energy goals for new homes.A typical residential battery system is about the volume of a file cabinet and is wall- or floor-mounted in a garage or utility space. A system that costs about $10,000–$15,000 can store 10 kWh of energy (enough to supply a typical home for a day), with peak power output of 10kW. Current costs are in the range of $1,000-$2,000 per kWh; cost reductions to $250-500 per kWh, as some project, would open up large-scale markets for home battery systems.
The research for this report took a high-level view of available products and trends, with a focus on overall efficiency and energy losses in standby and active modes. This report also reviews the status of test procedures and regulations for battery systems. The main findings include:
- Residential batteries form a nexus with solar PV systems and electric vehicles, with potential economic and performance benefits flowing from combined systems.
- There is currently a lack of official consensus on test methods and standards for residential battery systems, although national stakeholder groups are aware of the need to develop them.
- Long-term performance needs to be considered in battery selection, sizing, and operation, as system capacity degrades over time.
- Most grid-tied residential batteries are sold as backup power systems, creating the potential to harness unused capacity for grid services, such as peak load management, voltage support, and spinning reserve.
- The most common storage configuration is AC coupling (where all DC devices convert their power to AC), but DC coupling could improve efficiency. For example, sending DC power directly from solar panels to batteries (without converting to AC) reduces conversion losses.
- Residential battery systems can “consume” roughly 300-500kWh per year—about as much as a typical home refrigerator consumes annually. This consumption or energy loss occurs in two ways:
- energy lost during conversion from incoming DC power from the solar panel to the battery, and
- standby power losses from a fully charged battery.
- Variation in published round-trip (RT) efficiencies and standby power losses appear to be significant, but without standardized testing it is difficult to tell the difference between products, and harder still to tell how unit performance in the field will compare to claims.
- Opportunities to support the development of residential batteries include:
- Coordination of standards and test methods by working through IEC TC120 and other forums
- Field testing and measurement of real-world cost and performance
- Building on European and Australian experience, where thousands of systems have been operating for a year or more
- Inclusion of energy storage into building, energy, and electrical codes. Measures could include safety and sizing requirements, efficiency minimums, and elements such as “storage-ready” electrical system design.