Sizing battery bank sizing and calculations

We will do our best to explain the sizing of the battery bank storage for your solar configuration.

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.

Sizing Battery Bank Storage

Considerations for sizing a battery bank

Sizing a battery bank can be challenging but it is actually not too difficult. The first step is to calculate the amount of power you’ll be using for a period of time or per day. If you’re using more kWh per day than that your batteries can provide, you will be left without power and also, batteries can discharge below 50% which can harm the batteries in the process. However, if you have too many batteries in your battery storage, you will need to buy more solar panels to accommodate the batteries, otherwise, the amount of power from the solar panels won’t be sufficient to charge all of the batteries. Basically, every aspect of solar energy component should have a balance. Also, expanding your battery bank, especially when using lead-acid batteries, with new lead-acid batteries to old batteries, can lead to a quick degeneration of your newly added batteries. However, lithium-ion can be expanded because lithium-ion batteries uses a battery management system of BMS which regulates the different batteries in the bank.

Factors to solar battery sizing

The amount of batteries you would require will depend on some limitations or factors:

Your battery bank sizing budget.

Batteries can be expensive, usually the most expensive component in a standard solar setup. Always ensure that you have enough money for battery storage before you purchase a solar panel bulk package with a 10kWh inverter.

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%.

Voltage Multiply Amps Equals Watts
Battery Sizing Calculating Formulas

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.

How to 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?

To correctly size a battery bank, you would need the continues hours of use 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 of 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 with 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.

Steps For Solar Battery Sizing For Power To Home
Solar Battery Bank Steps And Calculations
How Long Will Solar Batteries Last

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.

Solar Battery Bank Configuration

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

Higher voltage rating

Series configurations can be achieved with batteries of the same or different types of batteries. However, batteries of the same voltage such as 12V batteries and batteries with Ah capacity similarities, can be connected to increase the overall voltage of the battery storage bank. Connecting the positive terminal from the first battery to the negative terminal of the secondary battery and then to the 3rd and so on. This is to reach the desired voltage levels and will be classified as a series battery connection. The voltage would be the total voltage calculated together. The Ah, cranking performance and reserve capacity remains unaltered.

Solar Batteries In Series
Solar Batteries Wired In Parallel

Parallel battery configuration

Higher amperage rating

Batteries connected in parallel should be of the same type, voltage and Ah capacities. Therefore, increasing the overall capacity of the battery storage bank. To setup a parallel battery configuration, firstly, connect all the positive terminals of the batteries to a conductor. Secondly, connect all the negative battery terminals in a likewise manner. The voltage of the battery bank will be unaltered, while the capacity of the battery storage bank is the total sum of each battery in this configuration. Ah, Cracking Performance and reserve capacity increases although, voltage remains the same.

Batteries In Series And Parallel

Solar batteries in series and parallel

Higher volts and amperage rating

This mix combination of series and parallel connections, offer 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 are as short as possible to minimize energy loss due to resistance. Therefore, more electrical current can flow and more energy can be produced.

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