How Battery Storage Works
A home battery system can serve a similar purpose as a traditional fossil fuel generator, but unlike a traditional generator, there is no fuel to be purchased and stored during an outage. When there is a utility grid outage, a battery storage system can isolate some or all of the household electricity needs (called loads) from the utility grid and continue to provide power to those loads. When utility grid power returns, the backed-up loads automatically reconnect to the grid.
Additionally, if paired with a solar electric system, the battery can be recharged by solar energy. Our Solar + Storage article provides more information about the benefits of pairing a solar electric system with battery storage.
Powering an entire home with a battery system is still relatively expensive, which is why a homeowner may elect to install a smaller battery system to power pre-designated critical loads. For commercial-scale customers, battery storage can provide savings by mitigating the higher costs of using energy at times when electricity from the grid is expensive (high “time of use” rates), or when a commercial property is using significant amounts of power in a short period of time (demand charges). Currently, in Massachusetts, residential customers are largely not impacted by demand charges or time of use rates, though this could change in the future.
Difference between DC and AC coupling
Similar to a solar electric system, a battery system produces direct current (DC), while the home’s electrical system and appliances operate using alternating current (AC). An inverter enables your household to use the energy stored in the battery by converting that energy from DC to AC; it may be physically integrated into the battery or may be a separate piece of equipment. If you are powering your battery with solar, the configuration of your battery may depend on whether you have solar installed before the battery, or at the same time:
- DC Coupling: Battery storage systems that are installed at the same time as a solar electric system installation may share a single inverter and be DC coupled, where the solar electricity is fed directly into the battery system. As part of this configuration, the battery is generally only charged by solar electricity (as opposed to electricity from the utility grid). This can be a factor for certain incentive eligibility.
- AC Coupling: If a battery storage system is installed as a retrofit to an existing solar electric system, it will often be AC coupled. The solar electric system and the battery system are independent and connected to two separate inverters (the original inverter tied to the solar electric system, and a separate battery inverter). This is generally done to allow the solar electric system to maintain the existing inverter and wiring. Electricity generated from the solar electric system typically enters the electrical subpanel where it is used to meet the load, and excess electricity not used is then stored in the battery. Note that this configuration can make it easier for the storage system to be charged from both the solar electric system and the grid, which can be a factor for certain incentive eligibility.
Battery types and components
There are currently two common battery chemistries used residentially: lithium ion and lead acid.
- Lithium Ion: These are the most common batteries used residentially. Although the upfront cost of lithium-ion batteries is higher than lead acid batteries, prices for lithium-ion batteries are decreasing. They have a higher density of energy (more energy per unit of space), and can be cycled more often during their lifespan. This longer lifespan (around 10 years) may lead them to have a lower lifetime cost than a lead acid battery. As a newer product that is growing rapidly, recycling options are still being created for lithium-ion batteries.
- Lead Acid: Lead acid batteries have been available for decades and are most commonly offered as “sealed lead acid” batteries. Depending on how often they are used (or “cycled”), they can last 5–10 years. There is an established market for recycling lead acid batteries.
Additional components may include an electrical sub-panel that includes all “critical loads,” and an automatic transfer switch that will isolate the critical loads from the main house’s electrical panel.
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