Solar batteries and how do they work

What kind of batteries are used for solar panels? How long do they last? How do you pick the right one? Get answers to your solar battery questions and information to be a smart solar energy storage system shopper.

Take a look at the various benefits of solar technology, as well as the best practices on how to select the right solar battery for a solar installation. 

Interest in energy storage is growing

Based on a survey of more than 150 of our customers, over 63% expressed interest in energy storage. From initially starting as a solution for individuals living off of the grid, it has expanded beyond that to be able to provide resiliency for homeowners with solar. Solar battery storage systems help solve a variety of issues with solar energy.

By adding a solar battery to a grid-tied solar energy system allows the system to keep providing power to critical loads even when the grid is down instead of having to disconnect and refrain from generating electricity. This feature is commonly referred to as “islanding” and is a critical feature that was lacking previously. Solar batteries can also help commercial consumers reduce peak demand charges, and gain resiliency from power outages.

Some local and state governments have restrictions on the use of carbon-fuel based generators and the duration for which they can be utilized. As the demand for solar storage systems grows, and as states continue to encourage the adoption of solar, we are excited to see how many individuals are thoughtfully thinking about how to make their community a better place. 

Battery storage used for solar applications helps alleviate the demands on our electrical grid by replacing unstable grid energy with clean-green electricity, providing heavy cycling (charging and discharging), and irregular full capacity recharging. There is a variety of battery types fitted for these unique requirements. Considerations for choosing a battery include the cost of a solar battery, installation, cycle life, and maintenance.

Why Solar Batteries Make Sense More Than Ever

Electric utilities all over the country are proposing changes to their utility rate designs. The cost of electricity keeps rising! A preliminary settlement agreement was struck between Southern California Edison (SCE) and the California Solar and Solar Association (CalSSA) on December 6th, 2019, which would boost the winter “super off-peak” rate starting in October 2020. This increase will push the default TOU rate for NEM customers about six (6) cents higher than the current super off-peak rate for that rate plan. With the possibility of additional rate changes occurring between now and October 2020, the final rate increase from SCE may be even higher. 

Time-of-Use Matters! Solar batteries maximize your solar ROI

In California, all of the major utility companies took action to adjust their rate designs to make electricity most expensive when you typically use it the most, between the hours of 4 PM - 9 PM. This change also impacts the benefits of solar panels since your solar energy system generates electricity when the sun is out.

Your consumption is typically when the sun is down. This impacts the overall benefit solar provides for homeowners. The good news is that this threat to solar energy is an opportunity for energy storage. As storage can help limit the erosion of savings from future changes. Developers are leveraging energy storage’s ability to future-proof solar energy savings as a key selling point to close deals.

Flattening the Curve, “Not that Curve!” But the California Duck Curve

Solar batteries are lovely, not just for a homeowner but also for the utility company as well. With solar batteries, you can store the excess electricity you generate during the day when demand is the lowest and use the stored energy when demand is the highest. Helping to flatten the electrical curve. 

When it comes to the electrical load from our utility company, our demand can produce curves that look like a Duck Curve. Based on studies conducted by the California Independent System Operator, an independent agency who helps monitor California's electric power system we can see that our electrical needs and usage fluctuate greatly throughout the day. Based on renewable energy like solar and overgenerating during the daytime hours, it produces a “belly” appearance in the mid-afternoon that quickly ramps up to produce an “arch” that resembles the neck of a duck—hence the moniker of “The Duck Chart”.

What are Solar Batteries? Solar Battery Types & Costs

Solar Battery Battery Types

•Lead Acid

Lead-acid batteries are a tested technology, which has seen quite a bit of adoption from off-grid solar energy systems. Lead-acid batteries possess a relatively short life and are also one of the least expensive options. They typically have these batteries are typically found in your car, boat or golf cart. 

Typical Lifespan of 3-5 Years

Costs: $$

•Lithium-Ion – Iron Phosphate (LiFePO4) 

Iron Phosphate Lithium-ion battery has a deeper DoD than NMC. Lithium-Ion - Iron Phosophate remains cool at room temperature while Lithium-ion heats up faster and may suffer thermal runaway under similar charging conditions. A thermal runaway typically occurs when the solar battery is exposed to high levels of heat, often leading to destructive results. In addition, if a car were to hit a battery with a conventional Lithium-Ion Nickle Manganese Cobalt Oxide, thermal runaway could occur. 

Typical Lifespan of 10+ Years

Costs: $$$$

•Lithium-Ion - Nickel Manganese Cobalt Oxide (NMC) 

NCA battery is the most commonly used storage technology used for energy storage. It typically has a slightly shorter cycle life and a higher energy density (less stability) lighter weight. (Cell phones and electric vehicle market)

Typical Lifespan of 10+ Years

Costs: $$$$

The most significant advantage of Lithium-Ion batteries has to do with their cycling capabilities. Lithium-ion batteries deliver more cycles in their lifetime than lead-acid based batteries. This makes lithium-ion batteries an excellent choice for applications like solar panel installation. 

Providing ancillary services to the grid by helping flatten the energy demand curve. By storing the power you generate during the day and consuming them at night when there is a much higher demand, you will be able to help towards flattening the curve! Providing power and additional ancillary functions hen your solar panels aren’t performing and 

•Flow Batteries

If you use 100% of a battery’s charge, its useful life will be significantly shortened. However, flow batteries, a new entrant on the solar battery market, offers 100% DoD. If you expect to be using your batteries heavily, this is one way to increase their lifespan. However, most flow batteries require vanadium, a relatively rare and expensive metal. 

Typical Lifespan of 5-10+ Years

Costs: $$$$

What are the best batteries for solar?

Solar batteries used for home energy storage typically are made with one of three chemical compositions: lead-acid, lithium-ion, and flow batteries. In most cases, lithium-ion batteries are the best option for a solar panel system, though other battery types can be more affordable. 

How to compare your solar storage options

As you consider solar energy storage system options, you’ll come across a lot of complicated product specifications and questions around what the best equipment is to best match your solar battery. The best place to start evaluating your solar battery options is to best understand the solar battery’s capacity to store energy; it’s power ratings, depth of discharge (DoD), round-trip efficiency, and warranty. 

Sizing Your Solar Battery System

Properly sizing your batter is essential but often overlooked by users and solar installers. Batteries in PV systems are routinely undersized due to cost or because the system loads were underestimated. It’s essential to understand the customer’s energy requirements and correctly design a system that best matches their goals. Online battery sizing calculators provided by solar battery manufacturers can simplify the process of determining battery capacity for load requirements. For most homes, a 20kw system is more than enough. 

Factors to Consider When Buying Solar Batteries

There are several factors that should be considered when evaluating the total cost of adding a solar battery to your solar energy system. 

• Price: A solar battery with a low upfront price tag may come at the expense of quality and battery life, you may be sacrificing a smaller upfront cost for a substantially higher cost in the future. Since the need for frequent battery replacements could boost the cost over time. That is why it’s essential to take a holistic view beyond price when making the decision.

• Capacity: Battery capacity is significant because it’s a measure of the amount of energy that can be stored by the battery

• Voltage: The solar battery bank’s voltage should be taken into account to ensure it matches the solar systems requirements. Your solar energy system will come with a solar inverter, and depending on the solar battery bank’s voltage; you may need to get a different inverter. The solar battery’s voltage is dependent upon the specifications of the inverter and whether it is being installed on a system that is converting DC-to-AC energy as well as the voltage of the priority loads in a DC energy system. 

• Cycle Life: The most critical consideration of a solar battery is its cycle life. The cycle life provides the number of discharge/charge cycles the battery can provide before capacity drops to a specified rated capacity percentage. Solar batteries may have the same capacity size, energy content, and similar weight. However, the design, material, and quality will influence how many times the battery will properly cycle throughout its lifetime. 

Which solar storage option to choose

This can depend on whether you are adding your solar battery to a new system, or retrofitting the solar battery to an existing solar energy system. This can lead to substantial differences in the type of batteries you can choose.

DC Coupled Energy Systems

DC Coupled Solar Energy System Advantages

•Very high efficiency - up to 95% to 97% battery charging efficiency (using MPPT)

•Great low-cost setup for smaller off-grid systems up to 3kW

•Ideal for small vehicles or marine systems that require only 1 or 2 solar panels.

•Scalable - Additional solar panels and controllers can be easily added if needed.

•Very efficient for powering DC appliances and loads.

DC Coupled Solar Energy System Disadvantages

• Becomes much more complex to set up systems above 3kW as usually multiple strings are required in parallel, plus string fusing.

•Can be a much higher cost for systems above 5kW as multiple higher voltage solar charge controllers are required.

Lower efficiency (approx. 90%) when powering large AC loads during the day due to the conversion from DC-DC-AC

AC Coupled Energy Systems

AC Coupled Solar Energy System Advantages

•Higher efficiency when used to power AC appliances during the day, such as air-conditioning or pool pumps, approx 95%.

•Can use multiple solar inverters in various locations allowing for multiple homes to potentially establish AC coupled microgrids.

•Most string solar inverters above 3kW have dual MPPT inputs, so solar panels can be installed at different orientations and tilt angles.

AC Coupled Solar Energy System Disadvantages

•Slightly lower efficiency when charging a battery system. You may see a 7% decrease in total efficiency. 

•Suitable solar inverters can be expensive for smaller systems

•Lower efficiency when powering direct DC loads during the day.

Bi Directional AC-Coupled Energy System with Islanding

Bi Directional AC-Coupled ESS with Islanding Advantages

•Very economical battery system

•Generally easier to install an AC coupled system

•Retrofit ready - can be added to homes with an existing solar installation

•Scalable - The solar battery system can be expanded in a modular way. 

Bi Directional AC-Coupled ESS with Islanding Disadvantages

•Lower efficiency due to conversion (DC - AC - DC) - approx. 90%

•Some AC batteries cannot function as a backup supply (Enphase)

•Not designed to function in off-grid installations.

Key Concepts: 

•Backup power

It provides support loads in the event of a grid failure. Usually done by relocating a subset of the home’s branch circuits to a new subpanel. In the event of a grid failure, storage can support these specific loads. 

•Time of use 

One of the roles for energy storage is time-of-use bill management is to store energy for use later. Storage system logic employed to define high-value versus low-value energy.

•Tax credits and SGIP Rebates

The 30% ITC applies to ESS as long as 75% of the charging occurs from a renewable source. When claimed with a solar energy storage system, SGIP averages around $0.28/Wh in SCE with step-downs built into the program.

•PV self-consumption 

The role of energy storage in a self-consumption or zero-export scenario is to store excess PV production and discharge this stored energy later. 

•Protecting Solar investment

Adding storage will protect solar financial value from TOU and NEM rate change from the utility provider.

Commercial Storage Battery Solutions for All Businesses

•Demand reduction

Demand-reduction applications use stored energy to reduce instantaneous power demand. Demand reduction in attractive business cases for behind-the-meter energy storage in commercial applications.

•Make renewable self-generation feasible

Most companies don’t have a great roof for solar or enough roof space to offset their electrical usage. By combining a solar energy system with solar energy storage, commercial buildings will require fewer solar panels. Making solar energy worthwhile enough in terms of the cost of adoption. 

•Reduce need for backup generators 

A solar energy storage system serves as a backup supply, providing power to critical systems when they need it the most. With no restrictions on the duration, they can run, ease of how it automatically just switches on, and how they can continuously recharge as long as there is sunlight; it is rapidly becoming one of the most common forms of commercial energy backup. Imagine if your business lost power for a day or a week, would your office be as productive? Would you be able to function correctly? With a solar battery, concerns around when the next major power outage will occur would be alleviated. 

•Facilitate the adoption of EV charging stations 

Multi-family housing, commercial properties, businesses, and government agencies worry about the impact that unpredictable electric vehicle (EV) chargers can have on their electricity load profiles. As the adoption of EVs continues to grow, we will start to have more and more EV charging stations. These EV charging stations can be a fantastic way to draw the right type of traffic for your business. Concerns about the cost of electricity shouldn’t outweigh the potential benefits that EV chargers could have for your business. To help balance the potential electricity draw, energy storage systems can automatically detect charging events and discharge power to avoid creating a demand spike.

Secure low-cost electricity for decades with solar panels

Battery Storage Glossary

Capacity - 

Battery storage capacity is the total amount of electricity that a solar battery can store. The amount of capacity a battery has is measured in kilowatt-hours (kWh). Most home solar batteries are designed to be “modular,” which means that you can add multiple batteries with your solar-plus-storage system to scale up your capacity. While a battery’s capacity tells you how big your battery is, it doesn’t tell you how much power a solar battery can provide at a given moment. To evaluate the best battery solution for your needs, not only should you consider the battery’s capacity but also its power rating. 

Power Rating - 

The power rating is the amount of electricity that a battery can provide. The power rating is measured in kilowatts (kW). A battery with a low power rating but a high capacity would deliver a small amount of electricity. Only enough to run a few crucial appliances but for a long time. A battery with a high power-rating and low capacity could run your entire home, but only for a few hours. Therefore it is crucial to understand how the power-rating and the battery capacity relate to one another. 

Depth of Discharge (DoD) - 

A battery’s depth of discharge (DoD) refers to the amount of a battery’s capacity that has been used. Most solar batteries need to retain a minimum level of charge at all times due to their chemical makeup. If you use 100% of a solar battery’s charge, its useful life will be significantly shortened. Most battery manufacturers will specify a maximum DoD for optimal performance. For example, if a 10 kWh battery has a DoD of 90 percent, you shouldn’t allow it to fall below 1 kWh in capacity before recharging it. Typically, a higher DoD results in higher utilization of your battery’s capacity. 

Round-trip Efficiency - 

A solar battery’s round-trip efficiency represents the amount of energy that can be used as a percentage of the amount of energy that it took to store it. For example, if you feed 5 kWh of electricity into your battery and can only get 4 kWh of useful electricity back, the battery has 80 percent round-trip efficiency (4 kWh / 5 kWh = 80%). A higher round-trip efficiency means you will get more economic value out of your battery because it shows that you are able to utilize more electricity.

Solar Battery Frequently Asked Questions (FAQs)

How long will the batteries last?

The life of a solar battery depends on the battery technology. Typically, the solar battery’s useful lifespan ranges between 5 to 15 years. If you install a solar battery today, there is a good chance you will need to replace it at least once if you want to match the 25-year lifespan of your solar panel system. However, just as the lifespan of solar panels has increased significantly in the past decade, solar battery technology is starting to see dramatic technological advancements that are improving its life, performance, and value. 

How can I extend the life of my solar battery?

The primary factor that will impact how long your batteries will last is proper maintenance. The battery’s temperature profoundly affects the lifespan of a solar battery, so it is crucial to ensure that the enclosure is maintained correctly, there is proper cooling to reduce heat, and protection from freezing temperatures.

If a solar battery’s temperature drops below 30 degrees Fahrenheit, it will require more voltage to reach maximum charge; when that same battery rises above the 90 degrees Fahrenheit threshold, it will overheat and require a reduction in charge. To help with this problem, many leading battery manufacturers, like Tesla and LG, provide temperature moderation as a feature. Choosing a quality battery manufacturer and maintenance efforts to ensure proper temperature controls will significantly extend the life of your batteries. 

How Long Can Batteries Power My Home? 

It depends on a variety of variables, including how much energy your household consumes in a given day, the capacity and power rating of your solar battery, what loads your battery will help support, and whether or not you are connected to the electric grid. A battery should be sized to run your home for multiple days if it is fully charged, even if your solar panels are not producing energy. 

According to the US Energy Information Administration (EIA), the average US household uses roughly 30 kilowatt-hours (kWh) of energy per day, and a typical solar battery is typically 10 kWh of capacity. Therefore, if you wanted to run your whole home off of your solar battery, you will need at least three (3) solar batteries to help offset your daily consumption.

But in real-life application, the answer is a bit more complicated. You will also be generating power for about 6-7 hours during the day during peak sunlight hours to help offset the need to utilize your battery. On the other end, most batteries cannot run at maximum capacity and generally peak at a 90% DoD (as explained above). As a result, your 10 kWh battery likely has a useful capacity of 9 kWh.

Ultimately, if you are pairing your battery with a solar PV array, one or two batteries can provide sufficient power during nighttime when your panels are not producing. However, without a renewable energy solution, you may need three batteries or more to power your entire home for 24 hours. Additionally, if you are installing home energy storage in order to disconnect from the electric grid, you should install at least a day or two of backup power to account for days where you might have cloudy weather. 

Read more solar battery FAQs.

Forme Solar Electric: Local Solar Battery Installer

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