Sizing a battery bank

he sizing of solar battery bank storage is a crucial aspect of your solar electrical configuration.

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What is meant by sizing solar batteries?

The sizing of solar battery bank storage is a crucial aspect of your solar electrical configuration. Most importantly, it is to install solar batteries that would be capable of handling the load received from the solar panels. Also, the solar battery bank should produce sufficient and enough electrical power to meet your needs without discharging too much. If batteries discharge too much, usually around 50% and lower, it can drastically reduce the time to live a period of the batteries. A battery bank is usually a multi-battery setup. If multiple batteries are connected with wires in specific configurations, it can provide more power for longer periods to your home. Also, the batteries in a battery bank, will not discharge as much as only one battery would.

Factors to consider for sizing solar batteries

Sizing solar batteries involves determining the capacity and number of batteries you need to effectively store and utilize the solar energy generated by your solar panels. The sizing process ensures that your battery storage system meets your energy requirements, allowing you to maximize your energy independence and optimize the use of solar energy. Here’s how to size solar batteries for your energy storage system:

  1. Determine Your Energy Needs: Calculate your daily energy consumption in kilowatt-hours (kWh). This information can usually be found on your utility bills. Keep in mind that energy usage may vary throughout the year.
  2. Assess Solar Energy Generation: Estimate the daily solar energy production of your solar panels in kWh. This estimation depends on factors such as panel wattage, efficiency, location, and orientation. Your solar installer or the manufacturer’s specifications can provide this information.
  3. Calculate Daily Energy Surplus or Deficit: Subtract your daily energy consumption from your daily solar energy production to determine if you have an energy surplus (excess solar energy) or deficit (more consumption than generation).
  4. Determine Battery Capacity: To size your battery storage, you generally want to store enough energy to cover your deficit or excess solar energy. This means selecting a battery capacity that can store the energy you would use during periods of low solar energy production, such as nighttime or cloudy days.
  5. Consider Battery Depth of Discharge (DOD): Determine the depth of discharge you’re comfortable with. For example, if you want to use only 50% of your battery’s capacity to extend battery life, you’ll need to size the battery to store twice the amount of energy you plan to use.
  6. Calculate Battery Capacity Needed: Battery Capacity Needed = (Daily Energy Deficit or Excess) / Depth of Discharge
  7. Consider Days of Autonomy: You might want to size your battery system to provide backup power for multiple days without sunlight. Multiply the daily energy consumption by the number of days of autonomy you desire to calculate a larger battery capacity.
  8. Choose Battery Chemistry: Different battery chemistries, such as lithium-ion and lead-acid, have varying energy densities, cycle lives, and costs. Choose a battery chemistry that aligns with your requirements and budget.
  9. Consider Inverter Efficiency: Keep in mind that there will be energy losses during the conversion process from DC to AC using the inverter. Account for this efficiency loss when sizing your battery system.
  10. Consult Professionals: Solar installers, energy consultants, or battery manufacturers can help you accurately size your battery system based on your specific needs, local climate, and energy consumption patterns.

Remember that proper battery sizing ensures that you have enough energy storage capacity to meet your needs while considering factors such as energy usage patterns, backup requirements, and battery longevity. Consulting with experts can help you make informed decisions about the capacity and number of batteries needed for your solar energy storage system.

Days or hours before recharging

This is the most important factor for people looking into solar products and batteries. A question being asked by many people is: “how many hours will my batteries be able to produce power before it will need to be recharged?” The more electrical appliances you need to power for longer periods, the more batteries you will require to keep up with the demand. Too few batteries could lead to power interruptions and faster discharging times which again, could lead to battery health reductions. This is determined by the number of batteries you use and how their wiring configurations are done which will influence your battery bank’s total amp-hours.

High wattage appliances.

High wattage appliances could quickly discharge your batteries. Therefore, it is a best practice to carefully size your battery storage bank. If a device requires many watts per hour, you will need batteries that could produce or store that electricity required to power the high wattage appliances.

Solar array volts.

Volts can be compared to “pressure” in the flow of electricity. Your solar panel array produces power in a specific voltage. If solar panels generate electricity at 48 volts, your battery bank should be able to meet the needs of 48volts. Therefore, more batteries would usually be required. Many people are using 36-volt solar panel arrays which could work perfectly fine with a 24-volt battery bank. If there is a drop in voltage in the solar system, you can be on the safe side if the voltage is a little lower such as the 24-volt system above. A best practice for solar and battery sizing is, ensure that your solar panels can produce a little more power than the size of your battery bank, this protects against sudden voltage drops, electricity fluctuations, and power loss. A larger solar panel array than your battery storage bank is a good practice.

Charging the batteries.

The battery energy source supplying power to the batteries should produce a higher voltage which exists inside the battery. Many popular solar panels come with a 16 to 18 peak powerpoint. A 5% drop in voltage produced will reduce the overall necessary difference in voltage, thus also reducing the current of charge to the battery by a higher percentage. JC Solar Panels would recommend a voltage drop size of about 3%. If you’re using a battery bank that consists of 12volts, solar panels of about 16 to 18 volts would be ideal to use. This will allow unexpected voltage drops that can occur in the system.

Battery storage capacity.

When you’re in the process of sizing a battery bank, it is important to install the correct storage capacity batteries to power your appliances. South Africa is a sunny country, but on overcast days, you would want optimal battery storage or more batteries, to provide you with more amperes in an hour. More amps and an hour of electricity allows you to have more power ready for your appliances. Therefore, the more amp hours, the longer the duration of battery discharge will be.

Battery discharge rate.

To properly implement solar battery bank sizing, you should consider the duration or rate of the battery discharge. The longer duration of discharge, the more power you will have over some time. Battery discharge rates can usually be found in the manual or on the battery. A C-10 marking would allow the battery to discharge in about 10 hours. C5 would indicate that the battery would discharge in about five hours.

DoD or solar battery Depth of discharge

The depth of discharge explains how deep the solar battery is discharged. A battery that is charged to it’s max, its DoD will be 0%. If a battery is charged to 70%, the DoD will be 30%. For lead-acid batteries, the DoD maximum would be 50% or below. Deep discharging a battery or discharging a battery beyond its recommended DoD, can cause damages to the battery which is irreversible.

More solar batteries, more power

Having multiple batteries and adding more to the battery storage, the more power you will have available. Also, the higher capacity batteries or bigger batteries you install, will allow you to power more electrical appliances. Therefore, having a decent size battery bank, your batteries will discharge in smaller amounts or cycles, allowing your batteries to last much longer.  JC Solar Panels  will recommend to always have more batteries for a proper sized battery bank. Also,  we  don’t recommend to ever discharge your batteries beyond 50% for Gel or 90% for Lithium-ion.

Sizing a battery using Watt hours

The most common battery sold by JC Solar Panels, is the popular 12V 105Ah battery. Ah (amp hours) are the indication for storage capacity while V is the push or “pressure” of electricity flow. To determine the amount of power the battery will store, you can calculate the watt hours. It’s simple math and easy to understand. The formula is this: Volts x Ah / 100. You take the volt rating of the battery, multiply it with the amp hours and then divide it with 100. Volts x Amp Hours / 100 = Watt Hours 12V x 105AH = 1260 / 100 = 12.6-Watt Hours Therefore, a fully charged 105Ah 12V solar battery can provide electricity for a 100watt device for about 12.6 hours. Consider doing your own research about solar battery specifications before you purchase and install a solar battery bank. If you know exactly what you require and what you can afford, you can be more confident in your battery solution installation. Solar battery sizing mistakes can be a high price to pay.

Calculate Battery Capacity in more detail

If you determined the amount of power you’ll need in your home, you can use these provided methods for calculating your battery capacities:

Let’s say you need 5kW of power usage per day, you can add the inefficiency of the battery which is 80% for Lead-Acid batteries and 95% for Lithium-ion batteries.

Lead-Acid: 5kW x 1.2 inefficiency

Lithium-ion: 5kW x 1.05 inefficiency

Step two would be to take the depth of discharge into consideration, lead-acid batteries are about 50% while lithium-ion batteries are 80%.

Lead-Acid: 5kWh x 1.2 x 2 for 50% depth of discharge

Lithium-ion: 5kWh x 1.05 x 1.25 for 80% depth of discharge

Step 3 would be to add the charge controller and inverter as an inefficacy to the calculation

Lead-Acid: 5kWh x 1.2 x 2 x 1.05 inefficiency

Lithium-ion: 5kWh x 1.05 x 1.25 x 1.05 inefficiency

Finally, we should take the temperatures of battery operations into consideration.

Lead-Acid: 5kWh x 1.2 x 2 x 1.05 x 1.11 temperature multiplier.

Lithium-ion: 5kWh x 1.05 x 1.25 x 1.05 x 1.05 temperature multiplier.

By using these formulas and calculating the inefficiencies for the batteries, we can determine an estimated idea of how many Kwh in battery capacity we will need. Therefore, the results will be:

Lead-Acid: 5kWh x 1.2 x 2 x 1.05 x 1.11 = 14 kWh Lead-Acid battery capacity.

Lithium-ion: 5kWh x 1.05 x 1.25 x 1.05 x 1.05 = 7 kWh Lithium-ion battery capacity.

The conclusion is, these formulas provide the calculations for the minimum power capacity your battery bank will require to power your home which requires 5kWh of electricity. You should have a large enough solar array to charge these batteries to full if you’re using the system for off-grid purposes. On the other hand, the lithium-ion battery will be more efficient because it has a deeper DOD or depth of discharge rate.

Convert batteries from KwH to Ah

Lead-Acid: Let’s say we use a 28-kWh battery, 28kWh / 12 = 2,333Ah Lithium-ion: If we are using a 14.47 kWh battery bank, 4.8kWh / 12 Volts = 1,205.83Ah Most of the time, the more voltage the system offers, such as 24 to 48 volts, the better the chances that you will find these systems in larger solar configurations. This is because, the higher the system’s voltage is, the more efficient the system will be, because your wire will be much thinner and more solar power can be implemented unto each charge controller. Systems with higher voltages also usually have more inverters installed.

How many batteries do I require for my inverter?

The number of batteries you require for your inverter depends on various factors, including the capacity of each battery, the voltage of your battery bank, the inverter’s input voltage requirements, and your energy storage needs. Here’s how you can calculate the number of batteries needed for your specific inverter setup:

  1. Determine Battery Voltage: Check the voltage rating of your inverter’s DC input. This is typically stated in volts (V) or a voltage range. For example, if your inverter requires a 48V input, you’ll need to set up your batteries in a configuration that provides the required voltage.
  2. Calculate Battery Bank Voltage: Determine the total voltage of your battery bank by arranging batteries in series. If each battery has a nominal voltage of 12V, for a 48V inverter, you would need to connect four batteries in series (4 batteries × 12V/battery = 48V total).
  3. Calculate Battery Bank Capacity: Estimate the energy capacity you need in kilowatt-hours (kWh) based on your daily energy consumption and desired days of autonomy. Use the formula: Battery Capacity (kWh) = Daily Energy Consumption (kWh/day) × Days of Autonomy.
  4. Battery Capacity per Battery: Divide the total battery capacity (in kWh) by the number of batteries in the battery bank to determine the capacity each individual battery needs to have.
  5. Battery Ah Rating: Convert the capacity per battery from kWh to ampere-hours (Ah) by using the formula: Battery Capacity (Ah) = Battery Capacity (kWh) / Battery Bank Voltage (V).
  6. Choose Battery Size: Select batteries with a capacity that matches or slightly exceeds the calculated Ah rating per battery. Keep in mind that battery capacities might be available in specific standard sizes.
  7. Calculate Number of Batteries: Divide the total battery capacity (in kWh) by the capacity of each battery (in kWh) to calculate the number of batteries required. Remember that in practice, you might need to round up to the nearest whole number of batteries.
  8. Check Inverter Compatibility: Ensure that the number of batteries you plan to connect matches the inverter’s recommended input voltage and capacity range. Also, verify the inverter’s maximum charging current to ensure your battery bank can be charged effectively.
  9. Include Safety Margin: Consider adding a safety margin to your calculations to accommodate potential future energy needs or changes in efficiency.
  10. Consult Professionals: It’s recommended to consult with solar professionals or inverter manufacturers for precise battery bank sizing based on your specific inverter model, battery type, and energy requirements.

Keep in mind that battery sizing is a crucial step to ensure optimal performance and longevity of your energy storage system. Accurate sizing will help you avoid overloading the batteries and underutilizing the inverter’s capacity.

To correctly size a battery bank, you would need the continuous hours of use to multiply the amount of Wattages consumed. Hours x Watts = Total Watts. Now you can take the Total Watts amount and divide it by the DC Voltage and it will give you the number of Amps required. Total Watts / DC Volts = Amps.

Example 1

Let’s say we require 4 hours of power for our appliances from our battery bank and we will consume 900 Watts. The formula will be 4hours x 900 Watts = 3600 Total Watts. Now we take, 3600 Total Watts / 12 DC Volt = 300Amps. Therefore, 300Amps are required to be stored in your batteries to power the 900Watt appliances. However, JC Solar Panels won’t recommend discharging batteries below 50%.

Example 2

Now, let’s say you buy a 1000Watt inverter at 12 volts. If you max out the 1kW inverter at 1000Watts, you will be pulling 1000Watts / 12 Volts = 83Ah. If you install a 12 Volt, 200Ah battery to this system, you can do the following: 200Ah battery / 83 amps = 2.4 hours of runtime (144 minutes). Therefore, in 2.4 hours, the battery will be fully depleted and discharged. This can be damaging to your battery storage. However, we recommend a 50% DoD and therefore, we can divide the 2.4hours by 50%. It will look like this: 2.4h / 50% = 1.2 hours (72 minutes). If you only want to discharge 30% of your battery storage and be on the safe side, the formula would look like this:  2.4h / 30% = 0.7 hours (42 minutes).

Example 3

If we work with the same concept but use a 24 Volt battery storage and system, it would look like this: 3000Watt inverter and 2x 12-volt batteries in series which produce a 24Volt 200Ah battery setup. The formula would be: 3000W / 24Volts = 125Ah. Therefore, 200Ah battery / 125Ah = 1.6 run-time hours (96 minutes) while fully discharging the battery. So, to discharging to 50% would be 1.6 hours / 50% = 0.8 hours (48 minutes) and 30% would be 0.5 hours (30 minutes) of runtime.

List of steps to start sizing batteries

Create a list of all appliances which would require power form the battery storage bank and determine their watt ratings and also how long they would need to run on battery power.

Use simple math by multiplying the total watts required for all electrical devices. For example, if you watch TV for two hours per day, and let’s say the TV uses 150W, you would need to multiply 150 by 2 which gives you a total of 300 Watts per day. You can do this for all appliances which will need to be powered by your battery storage. Adding all the wattage of all devices, will provide you with a total sum of wattage you will require on a daily basis.

If you’re using solar panels to generate electricity, you should determine the number of hours they receive sunlight per day.

If you have all these numbers worked out, you can divide the total watts used by your electrical appliances per day with by the hours of sunlight per day. For example, 4Kw or 4000 Watts of electrical appliances with 5 hours of sunlight would look like this:

4000 / 5 = 800 Watts.

Therefore, your solar array should generate 800W of electricity per day to power all 4000-Watt appliances in your home.

How long will solar batteries last?

One important question most of JC Solar Panels customers have is: “how long will my solar batteries last?” AGM or sealed lead-acid batteries are usually rated by the amount of cycles of discharging and recharging they allow before they stop working. If your battery has a 2000 cycle rate, it means you can charge and discharge the battery for approximately 2000 times before it should stop working. However, this is if the battery is not discharged beyond 50% of its capacity. On the other hand, lithium-ion batteries can be discharged deeper and usually comes with a 10-year warranty. Lithium-ion batteries are the highest recommended battery type, but, it doesn’t always suit most people’s budget.

Can multiple batteries be installed?

Yes, multiple batteries can be installed in a series or parallel configuration for recharging purposes. The rate of the battery charger is dependent to the maximum charging current. Therefore, this can limit the amount of batteries that can be recharged at a time. If you’re using parallel or series battery connection configurations, the batteries won’t be harmed in any way. This is if no errors in connecting the batteries were made.

Series battery configuration

Battery series connection is a wiring configuration in which multiple batteries are connected in a chain, end to end, to increase the overall voltage of the battery bank. This configuration is commonly used in solar energy storage systems to achieve the desired input voltage for inverters, charge controllers, or other devices that require a specific voltage level. Here’s how battery series connection works and its benefits:

How Battery Series Connection Works:

  1. Positive to Negative: In a series connection, the positive terminal of one battery is connected to the negative terminal of the next battery, creating a continuous chain.
  2. Increased Voltage: The total voltage of the connected batteries is the sum of their individual voltages. For example, if you connect four 12V batteries in series, the total voltage would be 48V (4 batteries × 12V/battery).
  3. Maintaining Capacity: The overall capacity of the battery bank remains the same as that of an individual battery, but the voltage increases.

Benefits of Battery Series Connection:

  1. Voltage Matching: Series connection allows you to achieve the desired voltage level required by your inverter, charge controller, or other devices. Some devices have specific input voltage requirements for optimal performance.
  2. Efficiency: Higher voltage battery banks can be more efficient when used with certain types of inverters, especially those designed for higher voltage input.
  3. Reduced Current: Higher voltage battery banks can reduce the current flowing through the wiring, which can help reduce energy losses due to resistance.
  4. Compatibility: Series-connected battery banks are often compatible with a wide range of equipment, as they can be adjusted to match various voltage requirements.

Considerations and Limitations:

  1. Balancing: Battery cells within a series-connected battery bank need to be balanced to ensure that all batteries have similar charge and discharge characteristics. An imbalance can lead to reduced capacity and performance.
  2. Safety: Series-connected batteries require careful monitoring to prevent overcharging or over-discharging of individual cells, which could lead to imbalances or damage.
  3. Maintenance: Maintenance and monitoring of battery health become more critical in series-connected configurations to ensure that all batteries are functioning properly.
  4. Voltage Limit: Series-connected batteries can only be used up to the total voltage specified by the equipment. Exceeding this voltage can damage equipment and batteries.
  5. Uniform Batteries: To ensure consistent performance, use batteries of the same type, age, and capacity in a series-connected bank.

Battery series connection is a useful technique when designing solar energy storage systems to achieve the required voltage for your equipment. However, it’s essential to consider the technical requirements, safety precautions, and maintenance practices associated with series-connected battery banks. Consulting with professionals can help ensure a well-designed and safe energy storage system.

Parallel battery configuration

Battery parallel connection is a wiring configuration in which multiple batteries are connected side by side, with all positive terminals connected together and all negative terminals connected together. This configuration increases the overall capacity of the battery bank while keeping the voltage constant. Parallel connection is commonly used in solar energy storage systems to increase the available energy capacity. Here’s how battery parallel connection works and its benefits:

How Battery Parallel Connection Works:

  1. Positive to Positive, Negative to Negative: In a parallel connection, the positive terminals of all batteries are connected together, and the negative terminals of all batteries are connected together.
  2. Increased Capacity: The total capacity of the connected batteries is the sum of their individual capacities. For example, if you connect four 100Ah batteries in parallel, the total capacity would be 400Ah (4 batteries × 100Ah/battery).
  3. Constant Voltage: The voltage of the battery bank remains the same as that of an individual battery, but the overall capacity increases.

Benefits of Battery Parallel Connection:

  1. Increased Energy Storage: Parallel connection allows you to increase the overall energy capacity of the battery bank, allowing you to store more energy from your solar panels.
  2. Longer Discharge Times: A higher capacity battery bank can provide energy for longer periods, especially during times of low solar energy production.
  3. Reduced Current Stress: Parallel-connected batteries share the load, which can reduce the current flowing through each battery, potentially prolonging battery life and reducing heating.
  4. Scalability: You can easily add more batteries in parallel to expand the energy storage capacity as needed.
  5. Compatibility: Parallel-connected battery banks are often compatible with a wide range of equipment, as long as the voltage matches the requirements.

Considerations and Limitations:

  1. Battery Compatibility: When connecting batteries in parallel, use batteries of the same type, capacity, and age to ensure consistent performance and prevent imbalances.
  2. Balancing: Batteries within a parallel-connected bank need to have similar characteristics to avoid imbalances in charging and discharging. A battery management system (BMS) or regular monitoring can help maintain balance.
  3. Charging: Charging multiple batteries in parallel requires adequate charging capacity to ensure all batteries receive the proper charge. It’s essential to use compatible charge controllers and properly set charging parameters.
  4. Wiring and Current: Ensure that the wiring between batteries and connections is appropriately sized to handle the increased current in parallel configurations.
  5. Space Consideration: Parallel connection requires more physical space compared to a single battery, so ensure you have enough room for the additional batteries.

Battery parallel connection is a valuable technique for increasing energy storage capacity in solar energy systems. However, it’s crucial to consider the technical requirements, safety precautions, and maintenance practices associated with parallel-connected battery banks. Consult with professionals to ensure that your energy storage system is properly designed and safely implemented.

Solar batteries in series and parallel

This mix combination of series and parallel connections offers a double voltage and capacity in Amps for your battery storage. If we have 4 x 200Ah batteries, 2x batteries are connected in series to create 24 volts. The other two batteries are similarly connected. Thus, we have 2 blocks of batteries at 24 volt each. We then connect the two blocks of batteries in parallel, thus, creating 400Ah. Now you have a system running at 24 Volts and 400Ah. However, it is recommended to install the correct cabling for this setup and also ensure that the wiring for connections is as short as possible to minimize energy loss due to resistance. Therefore, more electrical current can flow and more energy can be produced.