How Do Solar Batteries Work?

A solar battery is a battery energy storage system connected to solar panels. Electricity generated by converting sunlight into energy through solar panels can be stored in the battery for later use. Most solar batteries are lithium-ion, the same type used in electric vehicles, phones, and laptops. Advanced lithium-ion battery materials allow the storage of solar energy for use even when the sun is no longer shining. The demand for solar batteries has increased in recent years as homeowners seek energy independence and protection from power outages.

Solar battery technology is crucial in delivering clean energy transitions and protecting energy security globally, as reported by the International Energy Agency (IEA). Battery energy storage systems are set to contribute to the tripling of renewable energy capacity by 2030 as set out at COP28. Where a predicted sixfold increase in energy storage will need to happen to support this goal.

Working Principles of Solar Batteries

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Solar panels contain light sensitive materials that absorb sunlight and produce charged species. These charged species are transported to electrodes within the solar panel and then exported as current. Find out more about how solar panels work.

Solar panels generate direct current (DC) which is used to charge solar batteries. However, our energy grids use alternating current (AC) to power our homes. In order to integrate solar batteries into our local grids we must consider whether AC or DC coupling is more appropriate. Coupling refers to how your solar panel is wired to your battery.

AC Coupling vs DC Coupling

In an AC-coupled battery system, the direct current that comes from the solar panels is inverted to alternating current. A switchboard is used to distribute the electricity, prioritising where the electricity depending on consumer requirements. When the demand on electricity is low the switchboard sends AC electricity to another inverter which converted to DC for the battery to receive. This electricity can be discharged back to the inverter, converted to AC and sent via the switchboard to the consumer.

One of the big advantages of AC-coupled storage is it can store energy from both solar panels and the grid. This means even if your solar panels aren’t generating enough electricity to fully charge your battery, you can still fill the battery with electricity from the grid to provide you with backup power. Another advantage is that AC-coupling is easier to retrofit. Therefore anyone wanting to add a battery component after historic solar panel installation will find AC-coupling is the best option.

In a DC-coupled battery system, the DC current that comes from the solar panels can flow directly into the battery. Only one inverter is required in order to convert the current into alternating for the consumer.

Each time the current is inverted from DC to AC (or vice versa), power is lost as heat. As a result, DC-coupled energy systems are more efficient due to fewer inversions. Some existing solar panels have built in inverters which does make storing energy in a battery difficult.

Which Batteries are Used?

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Most batteries used in solar batteries use lithium-ion technology. Lithium ions are free to move between the cathode and anode within the battery depending on whether charging or discharging is required. The most common lithium-ion batteries used in solar batteries are NMC and LFP batteries.

The acronyms stand for nickel manganese cobalt oxide and lithium ferro phosphate (or lithium iron phosphate), which are the key materials within the battery and provide the lithium ions. They are the cathode material which releases the lithium ion to the anode during the charging process.

Batteries manufactured using these materials are typically efficient with long lifespans. LFP batteries offer more life cycles than NMC batteries without the need for precious metals such as nickel and cobalt. This means they can be charged and discharged more times before battery degradation. However, NMC batteries do offer higher energy density than LFP batteries, so they do store more energy per volume. They also perform better in extreme temperatures and can deliver higher power outputs.

Battery Considerations

The choice of battery within the solar battery set up is dependent on the specific system requirements. The energy generated by solar panels can be easily predicted based on weather forecasts and previous data. Therefore, the key variable is the usage on the other end and the power/energy that is required. Why is energy storage needed for the local system? Is it to protect a home from power outages or does a solar panel owner want to contribute to a virtual power plant?

The key considerations solar battery owners must take into account are:

• How much power do I need? - how much power (kW) is needed at once to run your home or workplace?
• How much energy do I need? - how long will you need to power your house? Energy is measured in kWh so how long would you need to power your home from your battery alone?

Solar Battery Usage

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Solar batteries are typically used as a backup, for everyday consumption or a mixture of the two. This means that the battery is either used when the grid goes down (backup) or as soon as power is needed (self-consumption). The main difference is that the solar battery can isolate itself from the grid in critical backup but not in self-consumption.

Backup

One of the most popular reasons for owning a solar battery is the ability to provide backup power during a power outage. This set up also allows the system to still work independently of the grid so solar energy can still be harnessed and used as well as the battery being used.

The amount of time the battery can support the home during an outage depends on the capacity of the battery and the desired usage. Therefore, the backup battery system typically only supplies power to "essential" powered items such as lights, refrigerators and medical devices.

Everyday Consumption

Self-consumption mode for a solar battery allows owners to reduce their usage of the grid. This is a cost saving application of solar batteries where the storing and using of solar energy is cheaper than selling excess electricity to the wider grid.

The solar battery in this case is directly linked to your main electrical panel and will discharge power as soon as solar production is not able to keep up with energy demands. This means that you will only have full charge as long as the solar energy that is being produced meets the needs our your power demands and the excess is making it to the battery. Therefore on days where you are already consuming power from your battery even before the sun has gone down means you will probably need to dip into energy from the grid before the sun rises the next day depending on your energy demands.  

Combination of Backup and Everyday Consumption

It is also possible to have the best of both worlds. A household can be protected from power outages and also be less reliant on the grid when it is working properly. However, if this is the case, you will need to know when a power outage might be possible to make sure you are starting the outage with as much battery as possible. This might mean relying on solar and grid energy only (not the battery) in the lead up to needing the backup mode. This will also mean you are having to balance energy density and power density with batteries usually being better at one than the other.

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