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Solar panels have one job: They collect sunlight and transform it into electricity. But they can make that energy only when the sun is shining. That’s why the ability to store solar energy for later use is important: It helps to keep the balance between electricity generation and demand. Lithium-ion batteries are one way to store this energy - the same batteries that power your phone.
The main is the energy storage technology, usually a battery. A battery is a device consisting of one or more electro- chemical cells that converts stored chemical energy into electrical energy through chemical reactions.
Each cell in a battery has a positive terminal (cathode), a negative terminal (anode), and an electrolyte. The electrolyte allows ions to move between terminals, which generates a current that can flow out of the battery to perform work. While there are many different varieties of batteries with different chemical compositions, the two most common technologies for solar+storage applications are lead-acid and lithium-ion batteries.
Lead-acid batteries have been around since the mid-1800s and remain the workhorse of PV storage applications, though this may change as other technologies continue to evolve. Most people are familiar with the lead-acid batteries found in automobile engines; however, these are not the same as those used in energy storage systems. Car batteries are designed to be almost always at or near full charge, whereas those required for solar storage must be able to withstand frequent deep discharge. These are known as deep-cycle lead-acid batteries. Some of the disadvantages of lead-acid batteries for energy storage applications, such as lower energy density and shorter battery life, are now being addressed with the next generation of advanced lead-acid battery technologies.
Lithium-ion batteries are a much newer and still devel- oping technology. First used for consumer products like laptops and mobile phones, lithium-ion batteries have a far greater energy density than lead-acid batteries, which means that a lithium-ion battery can weigh less and require less space while storing the same amount of energy as a lead-acid battery. The term “lithium-ion” actually refers to a wide array of different chemistries, all of which transfer lithium ions between electrodes during charging and discharging reactions. Primarily due to their use in electric vehicles, the cost of lithium-ion energy storage technologies has been decreasing at a rapid pace in recent years.
Each battery technology has its advantages and disad- vantages. Deep-cycle lead-acid batteries are a proven technology that is widely available and relatively in- expensive. On the downside, they are quite large and heavy and tend to have a shorter lifespan than lithium- ion batteries. Lithium-ion batteries are more compact and lightweight and are better suited for frequent cycling. Lithium-ion batteries also typically perform better at low temperatures than lead-acid batteries.
For some applications, the increased lifespan and more robust cycling capabilities of lithium-ion batteries will make them a more cost effective choice. As the costs of lithium-ion batteries continue to drop, they are likely to become increasingly cost competitive. Hybrid battery systems are also being deployed that combine the use of lead-acid and lithium-ion batteries to capture the benefits of each technology.
Speedometer scoring high speed, but without indicators numbers.

Why Lithium
There are many ways to store energy: pumped hydroelectric storage, which stores water and later uses it to generate power; batteries that contain zinc or nickel; and molten-salt thermal storage, which generates heat, to name a few. Some of these systems can store large amounts of energy.
Lithium is a lightweight metal that an electric current can easily pass through. Lithium ions make a battery rechargeable because their chemical reactions are reversible, allowing them to absorb power and discharge it later. Lithium-ion batteries can store a lot of energy, and they hold a charge for longer than other kinds of batteries. The cost of lithium-ion batteries is dropping because more people are buying electric vehicles that depend on them.
While lithium-ion battery systems may have smaller storage capacity in comparison to other storage systems, they are growing in popularity because they can be installed nearly anywhere, have a small footprint, and are inexpensive and readily available—increasing their application by utilities. Growth in the electric vehicle market has also contributed to further price decreases given that the batteries are an essential component. In fact, more than 10,000 of these systems have been installed throughout the country, according to "U.S. Energy Storage Monitor: Q3 2018" from GTM Research, and they accounted for 89% of all new energy storage capacity installed in 2015.
What’s a solar-plus-storage system?
Many solar-energy system owners are looking at ways to connect their system to a battery so they can use that energy at night or in the event of a power outage. Simply put, a solar-plus-storage system is a battery system that is charged by a connected solar system, such as a photovoltaic (PV) one.
In an effort to track this trend, researchers at the National Renewable Energy Laboratory (NREL) created a first-of-its-kind benchmark of U.S. utility-scale solar-plus-storage systems. To determine the cost of a solar-plus-storage system for this study, the researchers used a 100 megawatt (MW) PV system combined with a 60 MW lithium-ion battery that had 4 hours of storage (240 megawatt-hours). A 100 MW PV system is large, or utility-scale, and would be mounted on the ground instead of on a rooftop.
What's a megawatt-hour?
A megawatt-hour (MWh) is the unit used to describe the amount of energy a battery can store. Take, for instance, a 240 MWh lithium-ion battery with a maximum capacity of 60 MW. Now imagine the battery is a lake storing water that can be released to create electricity. A 60 MW system with 4 hours of storage could work in a number of ways:
So you can get a lot of power in a short time or less power over a longer time. A 240 MWh battery could power 30 MW over 8 hours, but depending on its MW capacity, it may not be able to get 60 MW of power instantly. That is why a storage system is referred to by both the capacity and the storage time (e.g., a 60 MW battery with 4 hours of storage) or—less ideal—by the MWh size (e.g., 240 MWh).