Not every solar power system has a solar battery attached. In fact, only about 4% of residential solar installations had a battery backup in 2020, though the numbers are climbing quickly. Many solar power systems instead send electricity into a home without any on-site solar energy storage. At that point, through a connection to the conventional power grid, the solar power system "sells back" extra energy to the power company. At night, these grid-connected systems just draw on conventional electricity, buying it from the power company.
Adding a battery system allows for two main benefits. First, it gives homeowners the ability to draw on battery power during hours when the power company is selling electricity at a higher rate, called peak hours, if they use "Time of Use" pricing (PDF).
Peak hours are often the weekdays when the most people are home and using electricity, often around 8-10 a.m. and 6-8 p.m. on weekdays. Secondly, a battery backup allows you to avoid interruptions in electricity if the power grid is out or at night.
While battery systems add to the cost of a solar power installation, these benefits can also defray the costs. To learn more about whether solar batteries are a good fit for you, it helps to understand how they work.
How solar batteries work
When your home doesn't need all the energy being produced by your solar panels, the excess energy is stored in the battery. When the solar panels aren't generating enough energy to fill the power needs of the building, the battery discharges reserve power to make up the difference.
Solar batteries typically contain two materials, which react inside the battery in order to both store and discharge electricity -- lead acid, an older and cheaper type, and lithium ion, the most popular type of solar batteries currently.
The way you configure your solar battery system affects how it works. You have three main options:
- Connect your solar panels to only a solar battery system, filling and discharging it frequently in response to the times without sun that you still want electricity. This is useful when in an area that is "off-grid," away from electrical utilities.
- Connect your solar panels to a solar battery system and to the grid (PDF). You can choose to fill your battery and just keep it as a backup, rarely discharging it, and mostly selling and buying excess electricity from the electric company.
- If you live in an area where electricity rates vary with the time of day, you can configure your system to use battery backup at the most expensive times and to buy electricity from the grid at the .
Eventually, batteries lose their ability to efficiently react and store energy for future use. Solar battery manufacturers often offer warranties or guarantees to build your confidence in how long these products will work, but their lifespan can vary widely depending on different factors.
A bit about inverters
Inverters play an important role in how the battery stores and converts solar energy. While solar panels generate electricity in direct current, or DC, the electric grid and homes generally use alternating current, or AC. An inverter can convert AC to DC or vice versa, and most solar batteries include an inverter to store the energy in DC form, as well as an inverter to convert it back into AC to be used in the grid or the home.
Because inversion of current is not perfectly efficient, battery producers are always experimenting with how to invert less often and increase the efficiency of the battery. As a result, some batteries will not have inverters for both input and output included in the system. Talk to your solar installer about the battery system you're considering so that you can make sure you have all the external inverters you need.
Solar power is only available for part of the day. Adding a solar power storage battery system ensures you always have power when the sun isn't out. Solar batteries are designed to work for many years, but they do eventually lose efficiency and stop working, usually before the lifespan of a solar panel, which often last 25-40 years.
A professional solar electricity system installer can help you select a battery that works well with your goals, whether you want to be entirely off-grid, have an emergency store for outages or minimize your costs from the electric company during peak hours.