Solar Energy Storage Methods in 2024: Best Ways to Store Solar Power Efficiently
Solar energy is an abundant, clean, and cost-effective source of electricity, making it an increasingly popular choice for homeowners and businesses alike. However, one common challenge remains: what happens when the grid goes down? Most people assume that once they have installed a photovoltaic (PV) solar system, their power needs are covered, but this isn’t always the case.
When connected to a grid-tied solar system, solar panels produce electricity during the day, converting sunlight into direct current (DC). This DC power is then transformed into alternating current (AC) by an inverter and sent to your home’s main panel to power your appliances. But when the grid goes down, whether at night or during the day, your solar system shuts off, leaving you without electricity.
What happens when the grid goes down?
Solar panels generate electricity from the sunlight during the day. This means that if the grid goes down at night when solar panels do not generate electricity, you you will remain without electricity.
At least, this is what most people think when they install a photovoltaic system. But many people also get surprised when their solar system disconnects when the grid goes off during the day!
First, it is important to know why the grid-tied solar system disconnects.
There are two main reasons:
#1 Protection of your appliances
Since the sun’s radiation is a variable source of energy, there will be occasions when the power produced by your solar panels will not be sufficient to supply the full load.
If your panels are unstably producing electricity during the day and they are connected to the main panel, your electrical appliances could experience flickering, drops in voltages and other issues that could damage them [1].
#2 Security standards of utility companies
The security reason focuses on the safety of the utility technical workers. In most cases, the grid goes off because of a failure in the distribution system. The utility company, therefore, sends technical experts to fix the problem.
If your solar system would continue generating power during the blackout while it is connected to the grid, utility company employees could be seriously injured by a back-feed in the line (distribution line with electrical current from the energy injected by your solar system).
That is why it is forbidden by the utility industry that any household, commercial or industrial grid-tied PV system is connected to the grid when the power goes off [1].
So the question remains:
Can solar panels provide electricity for your house when the grid goes off?
The answer is: Yes! They can.
Best Solar Energy Storage Solutions for Homes in 2024
When you install a grid-tied solar system, the power grid acts as an immense source of energy storage. The other option you have that is a stand alone system with a solar battery storage. In this scenario, a solar battery bank simply acts as a replacement of the grid.
In short: if you add a battery bank to your PV system, you will be able to have electricity even when the grid goes off. Your system will be independent of grid fluctuations.
How Do Solar Batteries Store Energy?
The principle of storing energy in batteries, first pioneered by Alessandro Volta in 1793, forms the foundation of how modern solar batteries store power today. By converting electrical energy into chemical energy, batteries offer a reliable way to store solar energy for use when needed—whether during the night or during a power outage.
In solar batteries, when electricity is generated by your solar panels, it is stored in the form of chemical energy inside the battery. When you need to use this stored energy, the battery converts the chemical energy back into electrical energy. This process of charging and discharging the battery relies on the movement of electrons between two terminals: the positive terminal (anode) and the negative terminal (cathode).
Here is a simplified breakdown of how it works:
Charging the battery (storing solar energy):
When sunlight hits your solar panels, they generate DC (direct current) power. This energy flows through the charge controller, which ensures that the right amount of current and voltage is delivered to the battery. As electrons are stored, the battery’s chemical structure changes, allowing it to hold onto the energy for later use.
Discharging the battery (using stored energy):
When the battery is called upon to power your home, the stored energy is converted back into electrical energy through a process called oxidation-reduction. During this process, electrons move from the anode to the cathode through an external circuit, supplying electricity to your appliances.
This first battery model was known as a voltaic cell and generally represents a value close to 2 volts. It is possible to achieve higher voltages by combining several cells together and summing up their voltage – that is how a battery pack is created. Generally, with 3, 6 or 12 cells it is possible to obtain battery packs of 6 volts, 12 volts and 24 volts respectively [2].
Now that you have an idea of the basic principle of how batteries store electricity, you can better understand how they store solar energy.
Once the radiation from the sunlight hits solar panels, photons release electrons. This makes DC current flow through solar cells. These electrons are then collected by the bus bars in the panels and sent through electrical wires into the charge controller.
Charge controller is a device that is programmed to charge batteries in such a way that the exact current flows in and the voltage limits are within the acceptable boundaries.
Phases of solar battery charging:
The process of charging a solar battery is not instantaneous. It happens in several stages so that the battery is charged safely [3]:
Bulk charge:
The first phase, where the battery is charged at a constant current of 10 to 20% of its nominal rating.
Absorption charge:
As the battery reaches its optimal voltage, the current decreases while the voltage continues to increase.
Finish charge:
In this phase, the current stabilizes, and the battery nears its full capacity.
Floating charge:
Finally, the battery enters a maintenance phase where it holds a steady charge, ready to supply power when needed.
Safe Solar Battery Storage: Best Practices to Protect Your PV Solar System
Solar batteries are an essential component of any solar power system and require careful consideration when it comes to storage. Choosing the right location is crucial for both the safety and longevity of your batteries.
Solar battery storage space cannot be any place. You need to take some important criteria into consideration. Remember that batteries function based on chemical reaction, and therefore, factors like temperature, humidity, dust, and pressure can affect the battery lifetime, efficiency and in some cases even its safety.
Here are the key factors to keep in mind when selecting the perfect solar battery storage location:
Temperature control:
Store batteries in an area where the temperature remains stable, ideally around 25°C (77°F). Higher temperatures can reduce battery efficiency, while extreme cold can impact battery performance.
Ventilation:
The space should be well-ventilated to prevent overheating, which can degrade battery performance and reduce its lifespan.
Dry environment:
Batteries should be stored in a clean, dry, and dust-free space. Exposure to moisture, dust, or oil can lead to leakage currents, self-discharge, and potential short circuits.
Humidity levels:
Keep the humidity below 90% to prevent damage from moisture accumulation.
Avoid heat sources:
Never place batteries near heat sources such as radiators or heaters, as this can cause overheating and decrease battery efficiency.
Presence of any of these elements can lead to leakage currents in the battery, which can lead to self-discharge, short-circuits, moisture and sulfation.
Charge controllers must also be placed in clean and ventilated areas with easy access. For both components, relative humidity values should remain below 90 percent [3].
Maintaining Ideal Temperature for Solar Battery Storage
Temperature is another important consideration. High operating temperatures will shorten battery efficiency. It is recommended that these devices are stored in areas with ambient temperatures close to 25 degrees Celsius (77 degrees Fahrenheit).
Although solar batteries are able to work in a relatively wide range of operating temperatures (depending on the type of solar battery), the average range oscillates between minus 20 to plus 50 degrees Celsius (-4 to 122°F) for VRLA batteries, and between 20 to 45 degrees Celsius (68 to 113°F) for ventilated batteries (more information will be discussed below).
Batteries should not be placed by any means close to heat sources like radiators or heaters, and should not be exposed to cold weather either [3].
Different Types of Solar Batteries Explained
Batteries for any solar system must fulfill one fundamental prerequisite: they must be deep-cycle batteries.
These batteries are specifically designed to handle the frequent deep discharges required for solar power systems. To supply electricity throughout the night implies that solar batteries often get drained to their minimum levels. Batteries that are not conceived as deep-cycle would get damaged if they would be submitted to such operating patterns every day. In some cases, they could even set on fire. That is why the so-called ignition batteries cannot be chosen for solar energy applications.
Batteries contain two types of electrodes which are from different metals. The properties and performance of each battery depend on the materials used for the electrodes. Today, the market offers several types of solar batteries, each with its own unique characteristics.
Popular types of solar batteries:
- Lithium-ion batteries
- Nickel batteries
- Sodium sulfur batteries
- Flow redox batteries
- Lead acid batteries
When it comes to choosing the right battery for your solar power system, several options are available on the market. Each type of battery has its own strengths and weaknesses, depending on your budget, energy needs, and application. Let’s explore the most popular and newest solar battery technologies on the market more in detail.
Lithium-Ion Batteries: The Best Overall Choice
Lithium-ion batteries have become the go-to option for most solar energy systems due to their high efficiency, long lifespan, and low maintenance. They can handle a high number of charge and discharge cycles without significant degradation and have a deep depth of discharge (DoD), often reaching up to 90%.
Pros
- High efficiency up to 90%
- Long lifespan of 15 years
- Compact and lightweight
- No maintenance
- High energy density
Cons
- Higher upfront cost compared to lead-acid batteries
- More sensitive to extreme temperatures
Lithium-ion batteries are ideal for homeowners who want the best performance and are willing to invest upfront for long-term savings. They are widely used in Tesla Powerwall and other modern solar battery systems.
Sodium-Ion Batteries: An Emerging Contender
Sodium-ion batteries are a relatively new technology that is gaining traction due to their potential to offer high efficiency and lower costs compared to lithium-ion batteries. While they are still in the early stages of commercialization, they are expected to become a viable alternative for solar energy storage, especially where lithium supply issues or environmental concerns arise.
Pros
- Lower cost and abundant materials
- Comparable performance to lithium-ion in some cases
- Environmentally-friendlier option
Cons
- Shorter lifespan compared to lithium-ion
- Still in development phase for widespread use
Flow Redox Batteries: Ideal for Large-Scale Solar Storage
Flow redox batteries use a liquid electrolyte stored in tanks to generate electricity. They offer scalability, making them ideal for large-scale solar projects where you need to store massive amounts of energy over long periods. They are known for their long lifespan and low degradation, making them well-suited for commercial or utility-scale solar installations.
Pros
- Extremely long lifespan up to 25 years
- Can handle frequent deep discharge cycles without degradation
- Scalable to meet larger energy storage
Cons
- High upfront cost
- Lower energy density (bulkier compared to lithium-ion)
Lead-Acid Batteries: The Budget-Friendly Option
Lead-acid batteries remain a common choice for solar energy storage due to their affordability and reliability. While they don’t offer the same performance or lifespan as lithium-ion batteries, they are still a viable option for solar systems, especially when budget constraints are a priority.
Lead-acid batteries consist of electrodes made from lead oxide (PbO₂) and pure lead (Pb), with sulfuric acid acting as the electrolyte. They are divided into two main categories:
Vented Lead-Acid Batteries (VLA)
VLA batteries are an older technology that requires regular maintenance, such as topping off water levels and proper ventilation. They are less commonly used in solar power systems today due to their high maintenance needs, but they remain one of the most cost-effective options.
Pros
- Low upfront cost
- Widely available
- Suitable for off-grid systems on a budget
Cons
- Requires regular maintenance by adding distilled water
- Shorter lifespan of 3 to 5 years
- Lower efficiency at around 80%
Valve-Regulated Lead-Acid (VRLA) Batteries: The Best Low-Cost Solar Battery
These are the top-rated battery option for solar PV applications. VRLA batteries are regulated and partially sealed to avoid evaporation of the electrolyte. The battery contains a valve which is sensitive to pressure and automatically controls the emission of gasses. The valve is normally closed, but if it detects a high level of pressure inside the battery, the valve opens to release the pressure.
A great advantage of these batteries is that they recombine oxygen and hydrogen through the electrochemical process that allows them to recover lost water in the energy conversion. This process is 99 percent efficient, which is why it is generally assumed that there are no water losses, and therefore, no maintenance is needed from your side.
The downside of these batteries is that they are more sensitive to temperature changes and they do not allow for reliably checking the State of Charge (SOC).
They can be divided into two main groups:
Gel batteries – Gel batteries add a particular compound of silicon to the electrolyte, which changes its consistency to a gel. This process allows them to have a longer lifetime (more than VLA) and also allows them to endure more cycles of charge and discharge.
Gel batteries also perform deep cycles with high temperature values and even with vibrations. They offer a stable discharge voltage and do not require maintenance at all. They can be placed in any position (because the electrolyte is gel and does not spills) and are also resistant to low temperatures [3,5].
Absorbed Glass Mat (AGM) batteries – these batteries contain electrolyte that is absorbed by a fiber glass base that acts like a sponge and immobilizes the sulfuric acid. These batteries also allow the performance of deep cycles, but they can withstand higher voltage charges than gel batteries. This at the end means higher efficiency [3,5].
Pros
- Low maintenance compared to VLA batteries
- Widely available & affordable
- Suitable for smaller systems on a budget
Cons
- Lower efficiency and shorter lifespan compared to lithium-ion
- Sensitive to temperature changes
- Difficult to monitor State of Charge
Which Battery Is Right for You?
When choosing a battery for your solar power system, the best option will depend on your budget, energy needs, and maintenance capabilities:
As battery technology continues to evolve, newer options like sodium-ion batteries and flow batteries may become more accessible and affordable in the coming years. However, for now, lithium-ion and lead-acid batteries dominate the market, offering a balance between performance and cost. Be sure to consider your specific needs and long-term goals when selecting the best battery for your solar energy system.
The selection of the solar battery type for your solar system also depends on:
- the specifications of the inverter
- the nominal voltage of the system
- charge regulation requirements
- autonomy and size
- maximum depth of discharge
- self-discharge
- efficiency
- operating temperatures
- maintenance availability
- costs
- lifetime expectancy [7]
Besides, you must also keep in mind the capacity and power of the battery. The capacity (measured in Ah) will tell you how much energy the battery can deliver within a specified time.
The other important parameter to take into account is its power rating. Power rating tells you how much power a battery is able to deliver instantly – generally measured in kW.
Another parameter to observe is the useful capacity. This parameter reflects how much capacity is available for the use from the battery, while not exceeding the maximum depth of discharge limit. It is generally measured in Ah and is calculated by multiplying the nominal capacity by the maximum depth of discharge.
As you can see, there is not a rule of thumb to decide which battery is the best for your solar system. The key lies in balancing all these factors and making the decision which battery suits the needs and the requirements of your home better.
Solar Battery Lifespan: How Long Do They Last?
The battery performance and condition over its lifetime determines how long it will last.
To understand this, it is necessary to explain some technical parameters of the battery. One of them is the maximum depth of discharge (DoD).
The DoD refers to the maximum amount of energy that can be extracted from a fully charged battery without damaging the battery.
The other parameter of interest is the maximum number of cycles (charges and discharges) that the battery is designed to endure. This value is intrinsic to the depth of discharge, as the higher the DoD, the less maximum number of cycles [8].
Now that those parameters are clear, the lifetime expectancy of a battery will depend on these two factors, on the environmental and operational conditions, and on the selected type of battery.
Lithium-ion batteries:
Lithium-ion batteries have a longer lifespan of around 10 to 15 years. They are designed to handle between 3,000 and 5,000 cycles at a DoD of up to 90%, making them one of the most efficient options for solar energy systems.
Lead-acid batteries (VLA and VRLA):
VLA batteries typically have a lifespan of 3-5 years and are designed to last through approximately 1,500 cycles. While they are cost-effective, they require regular maintenance and have a shorter lifespan compared to other types of batteries.
VRLA batteries have a lifespan of 5-10 years, depending on the DoD and environmental factors. They can typically handle between 250 to 500 cycles. While they require less maintenance than VLA batteries, their cycle life is relatively short, and they are more sensitive to temperature changes.
Flow batteries:
Flow batteries have one of the longest lifespans among solar battery types, often lasting 25 years. They can handle unlimited cycles, as their capacity does not degrade significantly over time. They are ideal for large-scale solar energy storage systems.
Sodium-ion batteries (emerging technology):
Sodium-ion batteries are a newer technology that is gaining interest due to their potential for lower costs and environmental benefits. While they are still being developed and commercialized, early prototypes suggest a lifespan of 10 years with a cycle life of 2,000 to 3,000 cycles. They could become a viable alternative to lithium-ion batteries as technology matures.
General rule: When the battery’s capacity is reduced to 80 percent of its original capacity, its lifetime is over [8]. At this point, the battery may still function, but its efficiency and ability to store energy will be significantly reduced. For most solar energy systems, this means replacing the batteries at least once during the lifetime of the solar panels.
How Many Batteries Do You Need for Solar Power Storage?
Whether you are setting up a grid-tied system with battery backup or going completely off-grid, understanding your energy needs and system configuration will help you figure out the right number of batteries for your solar setup.
This is a question that actually depends on several aspects:
#1 Type of your PV solar system
There are two main types of PV solar systems with a battery backup: a grid-tied system with battery storage and an off-grid configuration.
Grid-tied systems with battery backup:
If you choose the first option, your solar battery system does not need to be so big because blackouts in most western countries usually do not last over prolonged periods. Your solar system should be fine even with a small battery bank.
Off-grid solar systems:
For an off-grid system, the situation is different. Your battery bank needs to store enough energy to cover all your household’s energy needs for multiple days, especially during cloudy weather or low solar production periods. An off-grid solar battery system must be large enough to supply power 24/7.
#2 Calculating your energy demand (Watt-Hours or Wh)
Before deciding on the number of batteries, you need to know your household’s energy demand. Here’s how to estimate it:
- Step 1: Check your electricity bills to find the average monthly energy consumption.
- Step 2: Identify the most essential devices (lighting, refrigerator, TV, electronic outlets, etc.) that will run off your solar battery system.
- Step 3: Exclude non-essential appliances like microwaves, dryers, air conditioners, and heaters in an off-grid scenario to reduce the load.
- Step 4: Calculate the power consumption of each essential device in watts and multiply that by the number of hours you plan to use them.
Once you have this information, sum up the watt-hours (Wh) for all devices to determine the total energy demand your batteries need to cover.
#3 Battery autonomy (days of backup)
Battery autonomy refers to the number of days you want your solar power system to function without recharging from the grid or solar panels. In off-grid systems the minimum value is 3 days, while in the grid-tied systems with battery backup the autonomy is between 12 to 24 hours.
#4 Battery capacity (Ampere-Hours or Ah)
The capacity of the battery determines the amount of energy it is able to provide within a specified time. Capacity is usually measured in Ampere-hours (Ah) and is usually accompanied by the Cx acronym, where the “x” establishes the number of hours when the battery can provide a constant electrical current.
As an example, if the battery is 200 Ah C10, then it can discharge a constant of 20 A for 10 hours.
If you would like to know the amount of energy it can deliver in kWh, you need to multiply this number by the voltage of the battery (typically 12 V) [9].
#5 Voltage of the system (V)
Voltage of the system depends on the amount of power you want to back up. In general, most home appliances consume between 800 to 1,600 Watts. In this case, the voltage you need is 24 Volts.
When power ranges between 1,600 to 3,000 Watts, then a 48 Volt bank is the right choice.
For higher power demand, increase the voltage accordingly, but keep in mind that current levels should remain below 100 A in the total circuitry [11].
Once all of these parameters are clear, then you can estimate the capacity of the power bank by using this formula.
The next step is to choose a battery model and check its capacity (CCB) as well as the voltage (VB). With these two parameters you will be able to determine how many batteries you need to connect in parallel and how many in series.
Connecting Batteries in Series vs. Parallel
Connecting batteries in series increases the voltage but keeps the same capacity. For example, if you need a 48V system, you can connect four 12V batteries in series to achieve the required voltage.
Connecting batteries in parallel increases the overall capacity while keeping the voltage the same. For example, if you need more capacity (Ah), you can connect two 12V 200Ah batteries in parallel to double the capacity to 400Ah.
By balancing these configurations, you can build the battery bank that suits your energy demands and system requirements.
Alternative Solar Energy Storage Solutions Without Batteries
Batteries are the most used form of solar energy storage, but there are even other options to store electricity of your PV system.
One of them is directing the electricity from your PV to water electrolysers, which generate hydrogen gas. Hydrogen is then stored and used as feedstock for fuel cells to generate electricity when needed. This is called R&D solution and is more suited for industrial applications.
Another option is to store electricity in super capacitors, which can be later discharged to generate electricity when needed. This method is very expensive.
A brilliant option is to store solar electricity in the form of potential energy of water pumped to higher elevations. When needed, this stored water potential can be converted into kinetic energy and spins turbines, which generate electricity (a combination of hydroelectricity and PV) [12].
There is also an option to store solar energy in the form of heat, which is the main form of storage in concentrated solar power plants, where the heat transfer fluid passes through the receiver (where all the heat is concentrated), absorbs thermal energy and then stores it in hot thermal tanks that are available for usage when the electricity is needed.
Finally, one of the most interesting solutions to store PV electricity comes from E.ON – the German utility company. E.ON customers can freely feed the excess electricity to the “E.ON Solar Cloud,” which is a virtual electricity account that can be accessed at any time. You can even access the data from your mobile application to check how much stored energy is available in the cloud.
The utility makes sure that the grid is balanced all the time by taking the feed-in and the consumed electricity from this cloud. This highly innovative solution is perfect for solar energy storage, as no capital nor maintenance costs are involved. The proposal was recently announced in January of 2018, so it is just a matter of time until it will be applied in other places as well [13].
While batteries are the most common solution for storing solar energy in residential setups, there are several alternative options available that can also be effective for homeowners. These solutions, though less conventional, offer unique advantages for storing the energy generated by your solar photovoltaic (PV) system. Let’s explore the most promising residential solar energy storage options that don’t rely on batteries.
Can Solar Panels Store Energy for Later Use? (Answered)
No, solar panels only generate electricity. They are not able to store energy in any way.
The concept of solar panels is to transform the radiation of the sunlight into DC electricity and send it to the main panel of your house.
[2] https://www.scientificamerican.com/article/how-do-batteries-store-an/
[3] http://www.trojanbattery.com/pdf/TrojanBattery_UsersGuide.pdf
[4] http://repository.udistrital.edu.co/bitstream/11349/3663/1/ANA%CC%81LISIS%20TE%CC%81CNICO%20DE%20LOS%20DIFERENTES%20TIPOS%20DE%20BATERI%CC%81AS%20COMERCIALMENTE%20DISPONIBLES%20PARA%20SU%20INTEGRACIO%CC%81N%20EN%20EL%20PROYECTO%20DE%20UNA%20MICRORRED%20AISLADA.pdf
[5] https://www.tecnocio.com/blog/diferencias-entre-baterias-de-gel-plomo-agm-y-litio-caracteristicas/
[6] https://upcommons.upc.edu/bitstream/handle/2099.1/9360/Anexos_Sebasti%20n_Bardo.pdf?sequence=2
[7] http://api.eoi.es/api_v1_dev.php/fedora/asset/eoi:45302/componente45301.pdf
[8] http://academica-e.unavarra.es/bitstream/handle/2454/21830/TFG_GuembeZabaleta.pdf?sequence=1&isAllowed=y
[9] http://www.olajedatos.com/documentos/baterias_plomo.pdf
[10] https://www.letsgosolar.com/faq/what-is-a-solar-battery/
[11] M.Casa and M. Barrio. Instalaciones solares fotovoltaicas. Marcombo Formación
[12] https://www.researchgate.net/post/ways_of_storing_solar_pannel_energy_other_than_storing_it_in_a_battery
[13] https://www.kallanishenergy.com/2018/01/31/e-ons-technology-to-store-solar-power-without-battery/
[14] http://www.idae.es/en/publications/instalaciones-de-energia-solar-fotovoltaica-pliego-de-condiciones-tecnicas-de-instalaciones-aisladas