Solar Battery Sizing – As more and more people turn to solar power to meet their energy needs, battery storage is becoming increasingly important. In a solar system, batteries serve as a way to store the energy generated during the day for use at night or during times of low sunlight. Solar battery sizing is an important step in designing a solar power system. A properly sized battery can ensure that your system runs smoothly and efficiently, while an undersized battery can cause issues such as system failure and reduced battery life. In this blog post, we will explore some of the key factors to consider when sizing batteries for a solar system.
You may check out my blog post on Sizing Solar Panels for Your Home
Step 1: Determine Your Energy Needs
The first step in sizing batteries for a solar system is to determine your energy needs. This will involve calculating your daily energy usage in kilowatt-hours (kWh). This information can be obtained by examining your past energy bills or by using an online calculator. You can also estimate the daily energy consumption by using a Wattmeter or from the nameplate rating of appliances.
To calculate your daily energy usage, you can use the following formula:
Daily Energy Usage ( kWh ) = ( Appliance Wattage x Hours of Use )/1000
For example, if you have a 10-watt LED light bulb that you use for 5 hours per day, your daily energy usage for that light bulb would be:
(10 watts x 5 hours)/1000 = 0.05 kWh
Once you have calculated your daily energy usage, you can add up the energy usage for all of your appliances, lights, and devices to determine your total daily energy needs.
You can get the power rating from the Name Plate of the appliance or you can measure the actual power consumption by using a wattmeter.
Manual Calculation:
If you’re running 2 Nos of 6W LED bulbs for 5 hours a day, 1 No of Fan (80W) for 4 hours, 1 No of Laptop (65W) for 3 hrs, and a WiFi Router (6W) for 24 hours.
1. LED Bulb: 2 x 6W x 5 hr = 60WH
2. LED TV: 1 x 65W x 3 hr = 195WH
3. Ceiling Fan: 1 x 80W x 4 hr = 320WH
4. WiFi Router: 1 x 6W x 24 hr = 144WH
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Total = 719WH = 0.719kWh
Once you have this information, you can use it to determine the size of the battery bank you will need.
Step 2: Determine Your Battery Bank Capacity
Once you have determined your energy needs and solar panel output, you can calculate the size of your battery bank. The capacity of a battery is measured in amp-hours (Ah) and will determine how long the battery can provide power at a certain rate. To determine the capacity of your battery bank, you will need to divide your daily energy usage by the voltage of your battery bank (typically 12, 24, or 48 volts) and then multiply this by the number of days of autonomy you require. Autonomy refers to the number of days you want your battery bank to be able to provide power without being recharged.
Step 3: Choose the Right Battery Type
There are several types of batteries to choose from when sizing batteries for a solar system. Two of the most common battery chemistry types are lithium-ion and lead-acid. Apart from these NiCd is also used for renewable applications, but here we will discuss only the first two as they are the most popular in the market.
Lead-acid batteries are made with lead, while Lithium batteries are made with the metal lithium. Lithium and lead-acid batteries can both store energy effectively, but each has unique advantages and drawbacks.
1. Lead-acid Battery:
These batteries are the most affordable option for storing solar energy. They have been used for decades and are well-known for their reliability. Lead-acid batteries are available in two types: flooded and sealed.
Flooded Lead-Acid (FLA) :
These types of batteries are submerged in water. These must be checked regularly and refilled every 1-3 months to keep them working properly. It also needs to be installed in a ventilated place to allow battery gases to escape.
Sealed Lead-Acid (SLA):
SLA batteries come in two types, AGM (Absorbent Glass Mat) and Gel, which have many similar properties. They require little to no maintenance and are spill-proof. The key difference in AGM vs. gel batteries is that gel batteries tend to have lower charge rates and output. Gel batteries generally can’t handle as much charge current, which means they take longer to recharge and output less power.
2. Lithium Battery:
Lithium is a premium battery technology with a longer lifespan and higher efficiency, but you’ll pay more money for the boost in performance.
The Lithium batteries that are employed in solar systems are Lithium Iron Phosphate (LiFePO4) which has great thermal stability, high current ratings, and a long life cycle. This new technology lasts longer and can be put through deeper cycles. They also require no maintenance or venting, unlike lead-acid batteries. The main downside for lithium batteries is their higher price compared to lead-acid batteries at the moment.
Step 3: Consider Battery Depth Of Discharge ( DoD )
The battery’s Depth of Discharge ( DOD ) is the percentage of the battery capacity that can be safely drained without damaging the battery, usually expressed as a percentage of the total capacity of the battery. The depth of discharge of a solar battery can have a significant impact on its overall performance and lifespan. Generally, it is recommended to avoid discharging a solar battery below 50% DoD on a regular basis to avoid premature aging of the battery. However, some batteries ( Li-Ion and Li-FePo4 ) are designed to handle deeper discharge cycles, and their specifications will indicate the maximum recommended DoD for optimal performance.
As you can see in the above figure, the more a battery is allowed to discharge, the shorter its lifespan. Deep cycle batteries are designed to discharge 80% of their capacity but are recommended to choose a value of around 50% as a good trade-off between longevity, and cost.
For a deep cycle battery, 50% and for a lithium battery 80% DOD is considered as good practice.
Example: Let’s consider a 12V lead-acid battery with a total capacity of 100 Ah. If we discharge the battery to 50% DoD, we can use 50 Ah of energy before recharging the battery. This means that the usable capacity of the battery is 50 Ah. If we discharge the same battery to 80% DoD, we can use 80 Ah of energy before recharging the battery.
Step 4: Estimate the Days of Autonomy
Days of autonomy refers to the number of days a solar power system can supply power to the load without receiving any charge from the solar panels. This is an important factor to consider when sizing a battery bank for a solar power system because it determines the amount of energy storage required to keep the system running during periods of low or no solar energy input.
The days of autonomy required for a solar power system depend on factors such as weather conditions, location, and load requirements. In general, a solar power system should have enough battery capacity to provide power for at least two days of autonomy. This ensures that the system can continue to provide power even if there is no solar energy input for an extended period.
However, the days of autonomy required can be higher in areas with long periods of cloudy weather or low solar radiation. It is also important to note that increasing the days of autonomy requires larger battery banks, which can increase the overall cost of the system.
Step 5: Calculate Battery Bank Capacity
Battery Capacity (in Amp-Hours) = ( Daily Energy Consumption ( kWh) x 1000 x Days of Autonomy )/ (Battery Voltage x DoD)
Example:
Daily energy consumption =719WH ( Calculated in the earlier step )
Battery Voltage = 12V
DOD = 50% for Flooded Lead Acid Battery
Days of Autonomy =2
Battery Capacity = ( 0.719 x 1000 x 2) / ( 12V x 0.5 ) = 239.6 AH
You have to select a battery with a capacity of more than 239.6 AH. You can purchase 2 x120 Ah Battery.
Consider Other Factors
Finally, there are several other factors to consider when sizing batteries for a solar system. These include the temperature range the batteries will be exposed to, the depth of discharge (DOD) you require, and the rate at which the batteries will be charged and discharged. These factors will affect the lifespan and performance of your battery bank and should be taken into account when choosing the right batteries for your solar system.
Conclusion
In conclusion, sizing batteries for a solar system is a complex process that requires careful consideration of several factors. By determining your energy needs, considering your solar panel output, determining your battery bank capacity, choosing the right battery type, and considering other factors, you can ensure that your solar system is equipped with the right battery bank to meet your energy needs.
