Solar power installations become significantly more valuable to the user when the system includes sufficient battery storage to collect surplus energy generated during the day for use when the sun is not shining. The value of a battery installation increases immeasurably during power outages when it keeps the lights on while the neighbors go dark.
Although battery storage can add substantially to the upfront cost of a solar installation, it allows the user to benefit fully from all the energy generated.
For example, battery storage can save the user money by avoiding costly time-of-use (TOU) rates many electrical utilities charge during peak demand periods. It is not unusual for peak TOU rates to be double or more the cost of low-demand rates. Discharging the battery during peak periods when electricity rates are the most expensive, then charging the batteries from a solar source or the utility when rates are cheap, is a smart and cost-effective energy management strategy of which electrical utilities strongly approve.
For decades, lead-acid batteries commonly used in vehicles and boats have been the go-to electrical storage option for solar installations. Although a mature and reliable technology, lead-acid batteries are cumbersome, heavy, and take up a lot of space, often making them impractical for residential use. Further, they offer lower energy density and a shorter life span than more advanced battery technologies.
Lithium-ion batteries are the most popular choice for electrical storage for new home and business solar installations. Tesla's PowerWall and Li-ion options from companies like LG, Panasonic, and Sonnen are popular choices for today's buyers. They feature much higher energy storage densities than lead-acid batteries, making them considerably smaller and lighter. They exhibit a respectable life expectancy of around 8,000 charge/discharge cycles (roughly 15-20 years based upon a daily cycle) when the depth of discharge does not exceed 80%. Deeper discharging can reduce the life of Li-ion batteries by about 25%.
In contrast to lithium-based chemistry, flow batteries rely on a liquid electrolyte pumped across a membrane. They exhibit exceptional depth-of-discharge capabilities and suffer little storage capacity loss even after thousands of charge/discharge cycles. These advantages make flow batteries a popular choice for large-scale PV installations. However, given their relative complexity compared to Li-ion, they are not yet practical for the residential and commercial market.
Solar power installations integrating battery storage require a solar charge controller (also known as a solar regulator) to continually adjust the variable voltage and current from the solar panel array and prevent battery damage due to overcharging.
Basic charge controllers for small or micro solar installations shut PV panels off or choke down on the amount of energy delivered to the batteries through a process known as pulse-width modulation or PWM. PWM switches the energy supply from the PV array on and off rapidly, diminishing the amount of energy sent to the batteries.
For larger solar installations, more efficient controllers employ maximum power point tracking (MPPT) to reduce the energy delivered to batteries, depending on the status of the battery charge. MPPT controllers operate at 150 volts DC or higher, with amperage ratings up to 100 A. They are relatively inexpensive for solar arrays rated up to 6 kW, and they offer an efficient way to charge batteries should AC power shut down.
Another option to reduce or turn off the power from the PV array involves redirecting surplus free energy away from the batteries to some other beneficial use, such as heating air or water.
Just as solar panels require inverters to modify the direct current (DC) electricity they generate to alternating current (AC) electricity used by households, a battery storage system requires an inverter to change the DC electricity it stores and supplies to AC electricity.
Some batteries include an integrated inverter, which requires AC-coupling to a home's solar panel array and the network grid. With this equipment, AC power supplied from the panel inverters and grid is connected directly to the battery system. The integrated battery inverter provides storable DC power to charge the batteries. Integrated inverter batteries are ideal for retrofitting homes with existing solar panels as they can be AC-connected directly.
Other battery storage devices require an external third-party inverter. Separate inverters will take up more wall space, so the buyer must be aware of space limitations that an external inverter may impose.
Solar Battery System Types
Solar battery systems are grouped into four major categories:
- DC coupled systems
- AC coupled systems
- AC battery systems
- Hybrid inverter systems.
DC coupled systems are commonly associated with off-grid solar arrays (where no power from the utility is connected) and small automotive and marine applications. They typically use solar charge controllers, also known as solar regulators, to charge batteries directly from solar panels.
AC coupled systems employ solar inverters and battery inverters to manage the power coming from solar arrays or the grid or local generator to keep the battery charged. These systems are ideal for most modern grid-tied home systems and commercial applications.
AC battery systems contain an integrated module consisting of lithium battery cells, inverter/charger unit and battery management system. The Tesla Powerwall 2 is a recognized brand of AC battery. These systems are ideal for retrofitting energy storage to existing solar installations.
Hybrid converter systems are grid-connected and DC coupled solar battery arrangements. The term hybrid refers to modern inverters that combine higher voltage solar MPPT controllers and battery inverter/chargers in one package. Higher voltage batteries operate in the range of 120 VDC to 500 VDC whereas traditional battery systems operate at 48 VDC.
Combining both AC and DC coupling allows an AC coupled configuration without a backup generator to rely on backup DC battery charging directly from solar panels through a solar controller should AC power be lost.
Sizing a Battery Backup System
The key to properly sizing and pricing an optimal battery backup system for a home's solar panel array is identifying critical loads at home versus "nice to have" concerns.
- A battery system's energy capacity, measured in kilowatt-hours (kWh), and the inverter output, measured in kilowatts (kW), need to match the household demands at night when the battery is partially discharged from supplying power throughout the evening.
- A practical battery backup design will limit the number of circuits it supplies during an overnight outage when solar panels are not generating electricity. Critical loads may include refrigeration, lighting, entertainment, communications and convenience outlets. Identifying and adding up the load demands each critical load imposes on a battery will determine the appropriate size to install.
- The math behind sizing the battery system is straightforward.
If a battery is discharged over the evening down to 3 kWh of stored electrical energy, then it's capable of operating a 10-kW electric stove for 3 kWh/10 kW x 60 min/hr = 18 minutes. A large air conditioning unit drawing 5 kW of power will discharge the battery in 36 minutes.
In these examples, the battery cannot supply such large loads for a useful duration. The options for a homeowner are to either add more battery capacity or eliminate such power-hungry loads from the battery. For most, adding more battery capacity may be a non-starter due to budget and space constraints.
- Another restriction affecting the choice of critical loads is the limited power capacity of an inverter. An inverter will likely not supply the momentary startup current required by air conditioners and other large appliance motor loads (often several times the steady-state current draw), so such appliances will never start. Correcting this deficiency means installing additional inverters – another potential strain on available space and budget.
- The solar energy system should be of sufficient size to recharge the battery bank at least partially, even on a cloudy winter day.
Smart home technologies are now available that will automatically drop non-critical electrical loads upon detecting a power outage, leaving the battery backup to supply only those circuits deemed essential. And, combining available financial incentives with the downward-trending costs for batteries, inverters, and smart control technologies allow more homes to power up with battery storage to augment their solar energy systems.