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How to choose the best solar battery for your needs

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Solar battery technology is one of the core pieces of the electrification and solar power revolution that’s happening right now. Reliable and affordable battery technology, after all, not only helps keep the lights on when the power is out, it can help store solar energy for use when the sun isn’t shining. 

Read more about solar batteries for residences in our guide, Solar Batteries for Home: A Comprehensive Guide.

Of course, no battery storage article would be complete without mentioning the Inflation Reduction Act (IRA), which unambiguously makes battery storage eligible for the Investment Tax Credit (ITC) — now at 30% until 2032. 

Even plug-in electric vehicles, which use similar energy storage, are playing a significant role in accelerating the advancement of the solar battery.

All of this is leading to significant improvements in battery technology, and drops in battery storage pricing. Likewise, more homeowners are considering battery storage as part of their solar projects. Let’s look at some of the decision factors to consider when helping a customer determine the right battery for their home.

Are batteries worth the cost?

For solar customers truly looking to make the most of their PV system, a quality home solar battery can be a good choice. There’s no sugarcoating that they’re pricey — usually between $10,000 and $20,000 installed — but if a solar customer can afford it, the benefits of installing a solar battery are substantial. And, as mentioned earlier, solar batteries are eligible for a 30 percent federal solar investment tax credit, and many local utilities offer incentives as well, which brings the cost down considerably. 

Considering that solar batteries play triple-duty as a power generator for emergencies, can help lower energy bills over the long run, and decrease a home’s carbon footprint, they are often well worth the investment if the adopter plans on keeping a home long term. The payback period for solar batteries can be as little as five years, give or take a year or two depending on other factors such as total system capacity and design and available incentives. Of course, some benefits of solar batteries — such as peace of mind and resiliency — are priceless to some solar customers, and should also be a factor in deciding if solar batteries are worth it. 

Of course, knowing ROI and showing ROI to customers are two different things. Aurora’s Energy Storage tool lets you model battery load, system configurations and customization, and more for customers, so they can easily see the benefits. 

Click above to see how you can add battery storage to your offerings.

In addition, the rapid advancements in solar battery technology mean that newer batteries are entering the market while the older ones are still on the shelves. From traditional lead-acid, today’s solar shoppers now have a wealth of battery types, technologies, and sizes to choose from.

There have been numerous advancements in the electrical energy storage industry in the past decade. One of the most notable is the development of modular systems, such as the Tesla PowerWall. These types of batteries have greatly made energy storage more flexible, easy to install and transport, and more affordable to maintain.

If you are looking at battery storage for a solar project, the first thing you need to know is how to choose the right one.

Choosing a battery system

Most people, particularly homeowners, venture into solar power with limited know-how. The market has adapted to their needs by generalizing what a buyer should “expect” when investing in a solar system. This information is not always accurate.

The wealth of solar battery options can make it quite a daunting task. While most people go for a one-size-fits-all approach, this may not always be the best choice.

The problem with one-size-fits-all batteries

There are three good reasons why you should not go for a one-size-fits-all type of solar battery:

  1. The technology the battery uses is rarely ever emphasized. Most one-size-fits-all batteries use lead-acid technology to store energy. This is not the best technology in the market.
  2. These batteries may be bulky in size, but they often lack power storage capacity. The cost rarely ever justifies the power capacity.
  3. These standardized solar batteries are almost always either oversized or undersized. The undersized batteries cannot meet power output demand. The oversized batteries, on the other hand, are not always fully charged, especially in the winter.

The one-size-fits-all battery is touted as the ideal choice for most people because a majority of buyers rarely ever know what to look for. However, it often trades various features and capabilities to meet the minimum requirements of different use cases.

Solar battery specs

All solar batteries are made differently. Some manufacturers use robots, while others assemble batteries manually. The form of quality control can affect the quality of the batteries. Some manufacturers are known to use more lead and heavier grids, which impacts the performance of the cells in the battery. Most importantly, some brands of batteries are tested exhaustively for safety and performance while others are not.

As a result, it is not uncommon for batteries with similar specifications to have different performance and lifespans. Finding the right batteries for your solar setup may mean the difference between good and poor power system performance. It may also mean the difference between negligible or high maintenance.

Here are the most important considerations you should have on your checklist when shopping for a solar battery.

Battery capacity

Batteries are rated in amp-hours, or simply amps. The indicated power rating is typically the fully developed capacity of the battery. This means that it may take tens to hundreds of charging cycles before the battery can reach the indicated full capacity. In other words, it can be misleading to test your battery after only a few cycles of charges.

You do not need to understand the physics behind electricity to estimate your power needs or properly size your batteries. If you already use power from the grid, this guide can help you estimate your power consumption based on your electricity bills.

As a rule of thumb, always estimate your peak power requirements using amp-hours. A battery rated 100 amp-hours, for instance, can theoretically put out 1 ampere of electric energy for 100 hours or 10 amps for 10 hours. When selecting a solar battery, understanding your power needs is the key to choosing the battery with sufficient energy storage.

Note that batteries with long warm-up cycles before reaching full capacity are more likely to outlast batteries that tout a high initial capacity.

Lifespan and charge/discharge cycles

The lifespan of a battery is a crucial factor that manufacturers compete on when designing robust solar batteries. The design process often focuses on making the battery resist heat and cold cycles to deliver peak performance for longer. The type of battery technology also plays a significant role in determining the lifespan of the battery.

Three factors that affect the longevity of a battery that you should check when shopping for one are:

  • Depth of discharge: This is the extent to which the battery is discharged or used, relative to its capacity. Since batteries degrade as they are used, their capacity deteriorates over time.
  • Cyclic life: This is the number of charge and discharge cycles of the battery. During regular use, flooded batteries typically last for between 300 and 700 cycles. Gel batteries can store and deliver peak power for as many as 500 to 5000 cycles. Lithium batteries can last for up to 200 cycles.
  • Temperature: The chemical activity inside batteries increases with temperature. To extend the lifespan of your solar batteries, install them in a temperature-controlled room.

Flooded vs. sealed batteries

Solar batteries can be broadly categorized into two: flooded and sealed.

Flooded batteries are the standard lead-acid batteries used in vehicles and off-grid solar installations. They are affordable, and because they can be easily cleaned and serviced, have longer lifespans. When in use, these batteries generate small amounts of hydrogen gas.

Sealed batteries are also known as VRLA (valve regulated lead acid) batteries. They cannot be serviced or maintained because they are sealed. A charge controller maintains the fluids and plates inside the battery to prolong their lifespan. These batteries do not emit hydrogen gas when in use.

Peak power output

Solar power batteries can be classified by their kilowatt peak or kWp. kWp is the theoretical peak power output of the system in ideal conditions. The peak output is more of a measure of comparison than an absolute unit.

When choosing a solar battery, the kWp rating indicates the highest amount of power it can output at its best performance: the higher the peak power output rating, the better the battery.

Round-trip efficiency

The round-trip efficiency of a battery is the amount of energy that can be computed as a percentage of the energy used to store it. For instance, if 100 kWh of electricity is fed into a battery, and it can only output 90 kWh, the round-trip efficiency of the battery would be 90% (90 kWh / 100 kWh x 100).

Always go for batteries with a higher round-trip efficiency because they are more economical.

Ambient working temperature

Ambient temperature is the average air temperature surrounding the battery, or the temperature of the room in which the battery is installed. The rating indicates the optimum temperature under which the battery will perform normally.

The ambient working temperature of a solar battery is a crucial rating that is often overlooked. This is particularly important for people living in regions with extreme temperatures.

Brand and warranty

Many different manufacturers are competing to develop the ideal solar battery. Their design and manufacturing processes differ, and as such, the final products are also different.

Brand is an important factor when choosing solar batteries. Your priorities and budget should dictate whether to buy a battery developed by a new startup or a major automotive company. Regardless of your choice, be sure to scrutinize the warranty details and go for the product that offers the most extended guarantee.

Cost

The prices of solar batteries range widely. The cost of solar batteries ranges between $200 and $750 per kWh. Lead-acid batteries on average cost around $260 per KWh and lithium-ion batteries average at $271 per KWh. This brings the total cost of the batteries to between $5,000 and $7,000. The actual prices may vary depending on your location and available brands.

Note that the Federal Investment Tax Credit (ITC) provides an incentive for installing a solar power system in the US. Again, the tax credit for installing a residential solar system is 30% until 2032 thanks to the ITC update.

The different types of solar batteries

The type, or technology, is the most crucial consideration when shopping for a solar battery. Your budget and specific needs should determine the type of battery that you choose.

1. Lead-acid

Tried and tested, lead-acid batteries are the standard for electrical energy storage. This type of battery has been around since it was invented in the 17th century, yet it is still the most used in storing power. Until five years ago, these were the only practical batteries that could be used to store electricity for domestic or industrial use.

Pros of lead-acid batteries

The most notable strength of lead-acid batteries is that they are affordable. They are widely installed in rural and remote areas because they are cheaper to buy than to pay for a power mains grid extension.

Lead-acid batteries are deep-cycle batteries, meaning that they can output steadily over a long period. Their discharge rate is constant. These batteries come in both flooded and sealed varieties. They both work on the same principle.

Cons of lead-acid batteries

At first look, lead-acid batteries are dull — they are bulky, ugly, and heavy. Because they take up a lot of space and their ambient working temperature is below room temperature, they must be installed in a climate-controlled shed.

Where lead-acid batteries are used

Lead-acid batteries are the first choice for an off-grid solar system installation. Their price, and stability, make them very dependable and easy to upgrade or replace. Most emergency power backup systems in the country also still use lead-acid batteries.

2. Lithium-ion

Li-ion batteries are becoming popular because they are the go-to power storage for electric vehicle manufacturers. The potential of lithium-ion as an energy storage medium is yet to be fully explored, but they are promising. However, at the rate that they are being improved, it is just a matter of time before they become the most popular battery for solar power storage. Tesla’s Powerwall battery is the most popular power storage solution that uses this technology.

There are two types of Lithium-ion batteries in the market. The first, and most popular among electric vehicle manufacturers, is the NMC (nickel-manganese-cobalt) chemistry type. The other is LiFePO 4 (lithium iron phosphate) type battery.

The NMC-type battery has a high cycle life, making it ideal for use in off-grid installations. LiFePO batteries perform exceptionally well in extreme temperatures, making them suitable for use in regions with extreme temperatures

Pros of lithium-ion batteries

Li-Ion batteries require minimal to no maintenance. They have a higher battery energy density. This means that a Lithium-ion battery can store more energy than a lead-acid battery of the same physical size.

Because they have longer life cycles, they have longer lifespans and higher depth of discharge. The Lithium-ion battery can deliver between 4,000 and 6,000 cycles at an 80% depth of discharge and still last for up to 15 years.

Cons of lithium-ion batteries

The main downside of Lithium-ion batteries is that they are expensive. They cost as much as double the price of lead-acid batteries with similar energy storage capacity. These batteries, unlike lead-acid batteries, are also very fragile and require a stabilizing circuit to ensure safe operation.

Where lithium-ion batteries are used

Lithium-ion batteries have found a home in the automotive industry. The demand for this battery is at an all-time high as electric vehicle manufacturers jostle to get a hold of it.

3. Flow

Also known as redox flow, the flow battery is a new entrant into the solar battery race. These batteries use a water-based zinc and bromine solution and vanadium to store electrical charge. There are only a handful of companies making this battery today, the most notable being Redflow, an Australian company.

Pros of flow batteries

Flow batteries are highly scalable. This means that the capacity and outputs of the battery can be increased or reduced proportionally to the battery size. They differ from the other batteries on this list in that deep discharge has no effects on the performance or lifespan of the battery. They have a long life cycle and very low self-discharge. It is also noteworthy that flow batteries do not heat up during use.

Cons of flow batteries

The fluids used to make the flow battery are prohibitively expensive. While the technology on which they work has been around for decades, these batteries are barely known in the mainstream because few companies produce them commercially.

Because of their chemistry, flow batteries are bulky. The zinc and bromine elements in the battery are also highly corrosive and toxic.

Where flow batteries are used

Flow batteries are ideal for use in situations where the batteries undergo multiple charge/discharge cycles every day. They are ideal for use in large-scale installations. 

4. Sodium-nickel chloride

The sodium nickel chloride battery is a formidable competitor to the lithium-ion battery. This energy storage uses a unique chemistry that makes it fully recyclable. It does not emit toxic chemicals and presents no heating or fire risk. Unlike lithium-ion batteries, sodium-nickel chloride batteries do not require sophisticated cooling systems to work efficiently.

Pros of sodium-nickel chloride batteries

Because of its chemistry, the sodium nickel chloride battery is safe and reliable. It can operate optimally even at extreme temperatures of between -4°F and140°F. The batteries are fully recyclable because they have no hazardous or toxic chemicals in them.

Cons of sodium-nickel chloride batteries

They have a limited lifespan of about 3,000 cycles and only an 80% depth of discharge. This means that as much as 20% of the power it stores cannot be used. These batteries are also quite costly to install, particularly for residential solar systems and large projects.

Where sodium-nickel chloride batteries are used

Sodium Nickel Chloride batteries are best used in large installations in solar off-grid power installations and emergency power backup systems.

Click above to see how Aurora can help you sell more storage.

Battery types comparison

Lead-acid

      • The cheapest in the market
      • Easy to maintain; sealed lead-acid batteries require no maintenance
      • Highly reliable
      • Easily recycled or disposed
      • Bulky, and take up a lot of valuable storage space
      • Short lifespan of between 1000 and 3000 cycles. On average, a lead-acid battery can last for 5 to 8 years
      • Shallow discharge depth of ~60% and an ambient temperature of 70º
      • Good for off-grid solar systems and emergency power backup storage

Lithium-ion

      • Require minimal to no maintenance
      • High battery energy density saves space
      • Longer life cycles and lifespans
      • Highest depth of discharge
      • Relatively expensive
      • Relatively fragile and must be enclosed in metal
      • Use an electronic circuit to provide a stable power output
      • Good for electric vehicles, remote cameras, and drones

Flow battery

      • Can provide over 10,000 cycles with negligible loss of efficiency or storage capacity.
      • Fast recharge rates
      • Little to no heat or fire hazard
      • Relatively expensive
      • Hard to dispose of and non-recyclable
      • Good for large-scale installations

Sodium-nickel chloride

      • Safe and reliable
      • Can operate normally even in extreme temperatures
      • Recyclable
      • Short lifespan
      • Shallow 80% depth of discharge.
      • Relatively expensive
      • Good for large-scale installations, power backup systems

Conclusion

The right battery and size for your customer depends on their specific power needs. Most first-time buyers use a solar battery storage analyzing tool to get faster and more accurate estimates.

The most highly recommended battery for most industrial and residential installations today is the lithium-ion battery. As the battery technology evolves, the batteries are getting more compact, power-dense, and cheaper.

If the budget is tight, or you need to install a basic solar system, then lead-acid batteries can be just as good. However, because environmental factors directly impact the performance and longevity of these batteries, be sure to weigh its features against expected consumption and climate, among other factors.

Schedule a personalized demo to learn more about how Aurora can help you add battery storage to your offerings.

FAQs

Do solar panels have batteries?

Solar panels themselves do not contain batteries. Solar panels produce electricity from the sun, and this energy is either immediately consumed or stored in external batteries for later use

What type of battery backups do solar systems use?

The most commonly used batteries in solar are:

  • Lead-acid
  • Lithium-ion
  • Flow batteries
  • Sodium-nickel chloride 

What is the best way to choose a battery system?

When choosing a battery system, it’s important to balance two key factors: 

  1. How much storage does the customer need? For example, a battery for providing a few hours of electricity during the evenings will look a lot different than a battery meant to power a home through a week-long natural disaster. 
  2. What is the solar customer’s budget? If money is tight, you might still be able to get the power needed with several tradeoffs. 

Which type of batteries last the longest?

Lithium-ion batteries will last the longest and perform the best over the course of their service life. 

Which battery chemistry is safest?

Lithium-ion batteries — and more specifically, lithium iron phosphate (LFP) batteries — are the safest batteries on the market today. 

How many solar batteries are needed for my home?

To determine how many batteries needed for the solar project, calculate your total daily electric requirements (measured in watt hours, or Wh), multiplied by how many days of electricity you need the battery to store. 

For example, for a 30 kWh home to run two days on battery power alone, the house would need six 10 kWh batteries. 

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