0Bills Photovoltaic Batteries Information
The choice of a solar battery backup is one of the most critical decisions that needs to be made when designing a grid-backup or enhanced self-consumption solar PV system.
Deep Cycle Batteries – Solar 101
Today’s solar batteries are better than ever. So are the devices that regulate and protect them. But you still need to do your homework to make sure you get the best solar battery storage solution for your needs. It is all about the right sizing based on your overall power and peak power needs.
There are two main kinds of deep cycle batteries: lead acid and lithium. Lead acid batteries have a lower upfront cost, while lithium batteries are a bit pricier but have the longest lifespan. Flooded lead acid batteries require maintenance, and more expensive sealed lead acid batteries are maintenance free. Due to the depth of discharge of lead acid batteries compared to lithium ion solar batteries basically you would need twice as much installed battery power.
Batteries are the primary storage source for off-grid systems, but they also work as an emergency backup power source for grid-tied systems. Installing a grid-tied system with a solar battery backup also gives you the option to sell excess stored power back to the utility company at a later time.
We use deep cycle batteries to store power generated by solar panels, but they have plenty of other applications. Small electric vehicles and industrial equipment like forklifts, floor cleaners, scissor lifts, and golf carts are also powered by deep cycle batteries.
For more in-depth info on batteries, be sure you visit our Energy Storage product pages.
Future Trends in the Solar Battery Market UK, USA and EU
Companies world-wide are quickly adjusting to the increased global market for solar systems by developing batteries that are better suited for photovoltaic systems. In the future, it is likely that lead-acid batteries will become extinct, as newer technologies in lithium ion and Nickel metal hydride continue to evolve.
Because lead-acid batteries are under the hood of virtually every car, advancements in lead-acid technology are still being made. New developments in lead-acid technology usually originate in the auto industry. Efficiency ratings are constantly going up, as new sensors and improved materials are helping batteries achieve longer lifespan.
The two main types of battery commonly chosen for solar PV systems are Lead Acid and Lithium Ion with various different specific types and products from many different manufacturers available on the market. The table below gives a summary comparison of the key attributes of these two different battery technologies.
0Bills Photovoltaic Battery Information | Types of Solar Battery Technologies
Attribute |
Lead Acid |
Lithium Ion |
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Total Storage Capacity |
An individual lead-acid battery will typically have a gross storage capacity of 100Ah – 200Ah @ 12V or 1.2kWh – 2.4kWh. They may be connected in series for a higher voltage and/or in parallel for greater capacity at the same voltage. A typical lead-acid pack suitable for a residential grid-backup solution will be in the range of 8kWh – 25kWh depending on the length of time required to operate off-grid and the total power of the loads to be supported.
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Lithium Ion battery packs typically are supplied as self-contained units with a built-in battery management system (BMS). Gross capacities vary from about 2kWh up to 8 – 10kWh depending on the model and manufacturer. Some models may be connected in parallel, others may be extended with expansion packs and all need to be fully supported by the software in the battery charger/inverter chosen. |
Daily Usable Capacity |
There is a close relationship between the amount of the total battery capacity that is used each day and the life of the battery as expressed by the number of cycles and typically it is recommended to only discharge a lead-acid battery down to about 50% of the total capacity of a lead-acid battery, this if referred to as a 50% Depth of Discharge (DOD). This makes the storage capacity available for daily use only 50% of the gross storage capacity.
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Most lithium-ion batteries can be used daily down to about 90% of their gross storage capacity with little or no impact on their lifetime in terms of number of cycles. This makes the storage capacity available for daily 90% of the gross storage capacity. |
Full Cycle Efficiency |
Lead-acid batteries tend to get less efficient the nearer to full capacity they reach which either results in a low full cycle efficiency of less than 80% if they are re-charged near to their full capacity or designing the system to only use about 80% of their full capacity in order to maximize their efficiency.
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Most lithium-ion batteries have a full cycle efficiency around 95% even for a cycle from their full depth of discharge up to full capacity making them ideally suitable for daily use applications like solar PV systems which need to use most or all of their retained energy in the evening/night and charge up again fully during the day. |
Lifetime (Cycles) |
The number of cycles that a lead-acid battery can be used for is directly related to the amount of energy charged and discharged in each cycle. With a system configured to utilize 50% of the gross storage capacity of a daily basis a typical lead-acid battery will have a lifetime of 2,000 – 2,500 cycles. Allowing for some degradation over the life of the battery a useful lifespan of about 5 years in a well designed system may be expected. | A good quality lithium-ion battery may have a lifetime of 5,000 – 7,000 cycles which is considerably more than 10 years of normal usage. The built-in battery management system will ensure that the battery condition is always maintained in optimum condition and a full 10 year life may be expected. |
Cost |
The initial investment cost of a lead-acid battery will be relatively cheap when expressed as Rand per kWh of gross capacity but all comparisons should always be done a Rand per kWh of usable capacity which makes a lead-acid battery twice as expensive as it may initially appear. | The initial investment cost of a lithium-ion battery may be 2.5 – 3 times more expensive per kWh of gross capacity compared to a similar sized lead-acid battery but when comparing the Rand per kWh of usable capacity the difference will be typically about 1.5 times as expensive. The lithium-ion battery will however last twice as long as the lead-acid so over a 10 year period the lithium-ion will almost always be a cheaper option with no need to renew the battery after 5 years. |
Weight |
A lead-acid battery may weigh between 70kg and 80kg per kWh of usable capacity so a typically 5kWh – 6kWh domestic battery pack may weight in excess of 350kg which may cause difficulty in locating a large battery pack in a residential property as a strong floor will be required.
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A good quality lithium-ion battery pack will typically weigh between 10kg and 15kg per kWh of usable capacity so considerably less than a equivalent lead-acid pack but a typically residential battery pack will still weigh 75kg – 100kg requiring some consideration as to where to place it. |
Charge / Discharge Power |
Most lead-acid batteries can be charged and discharged relatively rapidly and when connected in parallel the total charge/discharge rate is in effect increased. In a typical solar PV system a lead-acid battery pack may be charged and discharged in 2 – 3 hours with a peak discharge rate much higher for short period of times.
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Most lithium-ion batteries have a relatively restricted charge/discharge rate often needing 3 – 4 hours to charge and a maximum discharge rate of between 1kW and 2kW for a typical residential system. A system utilizing lithium-ion batteries therefore needs to be designed to take care to only connect essential loads to the circuit that will be powered from the battery pack. |
Operating Temperature |
Lead-acid batteries are significantly impacted by the ambient temperature and an increase from 20c to 30c can result in a 25% reduction in the lifetime as defined by the number of cycles and a 50% reduction in the lifetime as defined in years. | Lithium Ion is less impacted by moderate temperature changes and ambient temperatures in the range of 15 – 30 degrees centigrade will not significant impact the lifetime nor performance of the battery. |
The choice of battery type is not a simple decision with many different factors to take into account but we would always recommend that a comparison is made using the above considerations and looking at the total cost over the life of the system and not simply choosing the lowest initial cost option which in many cases may be more expensive over the life of the system.
Equally critical is the size of the battery with one too small providing insufficient benefit and one too large being a significant additional unrequired expense. Detailed below are some of the factors that need to be considered when determining the size of battery required:
Solar Battery Sizing
Attribute |
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Essential Load Energy Usage |
For a grid-backup solution the most important thing to consider is the loads that need to be supported when the grid has failed. It is not generally practical to consider powering all the loads in the property, e.g. an electric oven, geyser and pool pump will all consume considerable amounts of electricity and would require a very large battery to run even for a short time. A good way to consider this is to generate a list of essential energy loads to be backed up and the amount of time they’re needed in a typical day. An essential load is basically something energy must always be available for. This could be something normal like a freezer or burglar alarm, or something site specific like a fish tank. If no power was available, would it lead to loss of (fishy) life or just defrosted ice cream? In the UK, power cuts are relatively rare but for more remote locations or other countries it is definitely worth considering. A lot of loads won’t require their maximum power all the time, so you can add a factor to take that into account. Once that’s done, you’ll have an accurate baseline of energy consumption and be able to consider the appropriate battery capacity.
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Battery Operating Time |
The next critical decision is to decide the number of hours that the system needs to power the essential loads for. Typically a planned grid outage due to load shedding will last for 4 – 6 hours whereas a failure due to a grid fault will typically last for between 1 and 24 hours. The decision on how many hours to allow for is largely driven by the budget available as the cost of the battery pack will be directly related to its size and its size will be directly related to the number of hours chosen. Usually a system will be sized to support the essential loads for between 12 – 24 hours.
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Space Available |
Especially when choosing a lead-acid battery the space available to hold the installed battery and the strength of the floor may be a consideration that imposes a limit on the maximum size of the battery that can be installed. With a Li-Ion battery this is unlikely to be a major concern as a Li-Ion battery will be much smaller and lighter than a similar usable capacity of lead-acid battery.
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Charging Time and Rate |
The battery will be charged from the surplus energy available from the PV system, this is the difference between the energy generated by the solar PV system and that used by the loads during the daylight hours. It is therefore important to ensure that the battery can be fully recharged during a typical day of sunlight, especially in the winter months. A battery pack which is too large relative to the PV system will not get fully recharged and therefore not be fully available to provide power in the event of a grid failure.
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Maximum Depth of Discharge |
Each battery pack will have a recommended maximum depth of discharge, e.g. lead-acid might be 50% and Lithium Ion might be 90%. Having determined the total energy required to be generated from the battery pack with the equation : ‘essential loads energy in 24 hours divided by 24 multiplied by the required battery operating time’ then the gross battery capacity needs to be determined by dividing by the recommended DOD. e.g. 7690W / 24 * 12 hours / 90% DOD = 4272kWh. |
How to Equip a Grid-tied Solar System With Battery Backup?
There are three scenarios how you can add solar batteries to your existing solar panel system:
- AC Coupling: Splice your AC wiring to add a storage-ready inverter and batteries
- DC Coupling: Splice your DC wiring to add a storage-ready inverter and batteries
- Inverter Replacement: Replace existing inverter with a storage-ready inverter
We’ve noticed a surge of calls lately from people looking to add battery backup to their existing grid-tie solar system.
Many of these calls come from our home state of California, where PG&E has announced rolling blackouts to limit the impact of wildfires. With the prospect of scheduled blackouts looming, solar owners have been pushing to add battery backup to their systems to keep the lights on during grid outages.
Unfortunately, this process isn’t as easy as simply hooking up a new battery bank. Grid-tie inverters are designed to convert DC (direct current) from solar panels, but they are not designed to integrate with a battery bank. You’ll typically need to add new components to make your inverter work with your batteries.
It’s also not cheap. Batteries are the most expensive part of a solar system. Between an appropriately-sized battery bank and a battery-based inverter like the Outback Radian, you’re looking at something like 10 grand minimum to add batteries to an average-sized grid-tied system. (We wanted to make this really clear upfront, since people who call us often get sticker shock when we tell them the backup power package can cost more than the system itself!)
If you are concerned about recent blackouts and want the most cost-effective solution, your best bet may be a power generator. It’s going to cost less upfront, and it may be easier to pair it with your existing system because there are less restrictions on system sizing. A gas generator is usually large enough to back up most or all of your household, where an inverter and battery bank is usually sized to power only the essential appliances, because large battery systems can get expensive quickly.
Power generators have their own downsides: they are noisy, less environmentally friendly, require maintenance and a fuel source. But there is no question they are the most cost-effective option upfront.
Batteries have higher upfront cost, but are maintenance-free and much more versatile. The main appeal is storing and managing energy produced by your panels so you can recharge with solar during a long-term power outage. Another benefit of using batteries is that they can turn on and provide power to your home almost instantly, usually under 1 second and without any interruption to your appliances. A gas generator will take a few minutes to start the engine, warm up and begin providing power.
If you do decide that battery backup is the way to go for you, this article covers the 3 approaches you can take to get it done:
- AC Coupling
- DC Coupling
- Replace grid-tie inverter with hybrid inverter (storage-ready inverter)
Scenario #1: AC Coupling
Grid-tied inverters need the power grid to operate—they constantly sense grid voltage and frequency and will shut off if it falls out of range.
In an AC coupled system, the grid-tied inverter is paired to an off-grid inverter and battery bank. The off-grid inverter provides a second power source, which effectively tricks the grid-tied inverter into staying online. This allows you to charge your batteries and run essential appliances during a power outage.
The best option for AC coupling is the Outback Radian. The newest firmware supports frequency shift AC coupling, which will work with any inverter certified to EC or UL 1741 standards.
This feature causes the off-grid inverter to shift its frequency to control the output of the grid-tied inverter. The Radian limits the power coming in from the solar array when needed to prevent overcharging the batteries.
Here are the basic sizing guidelines for picking an inverter:
- The Radian should have at least 25% higher nameplate capacity than the grid-tied inverter.
- The GS8048A can AC couple with grid-tied inverters rated up to 6 kW (5 kW max for Fronius inverters)
- The GS4048A can AC couple with grid-tied inverters rated up to 3 kW (2.5kW max for Fronius inverters)
- Requires MATE3s remote with updated firmware for both the inverter and remote
See the Outback site for more info on using AC coupling to add battery backup to an existing grid-tied system.
Pros of AC Coupling
This is the easiest way to retrofit your system, especially a microinverter system. The battery bank connects to the Radian, which is installed between the grid-tied inverter and your load panels. The existing grid-tied inverter does not need to be removed.
Cons of AC Coupling
Strict guidelines for inverter and battery size make the process of sizing the addition a challenge. The system will perform poorly or not work at all if the inverter or battery bank are undersized. In addition, if the existing grid-tied inverter is large, an AC coupled system can get very expensive.
Compatible With:
- Most grid-tied inverters on the market
Scenario #2: DC Coupling
In a DC-coupled system, the solar array is connected directly to the battery bank using a charge controller.
This is how off-grid systems work, and it could be done to a grid-tied system if they are using a 600-volt string inverter. This works with the SMA Sunny Boys, many Fronius inverters, or any other 600-volt string inverter.
This Morningstar 600-volt charge controller is designed to retrofit grid-tied systems with batteries. It can be combined with any one of our pre-wired power centers that doesn’t have a charge controller.
The 600V charge controller would be installed between the existing PV array and your grid-tied inverter. It includes a manual switch to switch between grid-tie and off-grid modes. The downside of this method is it can’t be programmed—the switch has to be physically turned to start charging the batteries.
The battery-based inverter can still automatically turn on and power your critical appliances, but the PV array won’t charge the batteries until the switch is turned. So, you have to remember and be on site to turn on the solar charging. Otherwise, you might find your batteries are drained and you won’t be able to recharge from solar.
Pros of DC Coupling
In comparison to AC coupling, DC coupling works with a broader range of off-grid inverters and battery bank sizes.
Cons of DC Coupling
The manual transfer switch means you have to be available to initiate the PV charging. If you forget or aren’t there, your system will still provide backup power, but the battery bank won’t recharge from solar until someone manually flips the switch on the controller.
Compatible With:
- Most residential string inverters rated for 600 Volt max input
Scenario #3: Replace Your Grid-Tie Inverter With a Hybrid Inverter
The last option is usually the most expensive: you can remove your existing grid-tie inverter and replace it with a storage-ready inverter instead.
This approach is going to be the most flexible option—it works for all existing grid-tie systems. There are a handful of inverters on the market designed specifically to accommodate energy storage for grid-tie systems:
- The Fronius GEN24 can replace many SMA, classic Fronius and other 600V-1000V string inverters
- The StorEdge or the SolarEdge Hybrid can replace standard SolarEdge inverters
- Microinverters would need to be removed and replaced with any hybrid or storage-ready system
Ideally, you want to replace your existing inverter with one that is about the same size and can use the same array wiring.
In many cases, this solution is preferable to adapting your existing system with AC or DC coupling because these inverters are designed from the ground up with energy storage in mind. They include some cool features, like storing energy and selling it back to the utility during peak time-of-use (TOU) periods to take full advantage of your local net metering policy.
This approach is tough with micro-inverters because it takes more work to rip out the old ones and retrofit every panel with a replacement. The labor is a bit more expensive and time-consuming, so an AC-coupled solution is often a better alternative for microinverter systems.
Pros of Replacing Your Inverter
- Works for any system
- Storage-ready inverters come with additional features
Cons of Replacing Your Inverter
- Most costly option, especially for microinverters
Compatible With:
- All systems
Wrapping Up
Retrofitting solar systems with new equipment can be tricky because of all the different equipment options and various methods for incorporating energy storage. New equipment can change the electrical characteristics for the entire system, and that could introduce faults if the components are not designed to work with each other properly.
If you need help picking the right products to add battery storage to your existing grid-tied system, drop us a line. We have been designing solar panel systems since 2007 and have more than 3,000 solar battery systems in our reference list. Call us +44 333 772 0506 or +1-786-600-1814 or fill out this form to request a free consultation with a member of our design team. We’re happy to help you work out the details.