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# Step 3: How Many Batteries Do I Need? – sizing your battery bank

This tutorial will show you how to calculate the number of batteries you need in your battery bank. It is the third step in six-step tutorials on how to design a solar PV system

Step by step guide on how to design a solar PV System

In this tutorial, I will show you how to calculate the number of batteries you need in your battery bank. This is the third step in six-step tutorials on how to design a solar PV system.

If you jumped into this article from a google search or somewhere like that, I strongly advise you to visit the first step, Determining your Power Consumption Requirements, and the second step, How Many Panels Do I need. The information in those articles is very critical to understanding this third step. More so, if you’re a complete solar energy beginner, I recommend Fundamentals of Solar Energy and Everything You Need to Know About Solar Electricity those articles are written for you in mind.

Before you bring out your calculator, I want to begin the tutorial by explaining what we mean by “Battery Autonomy” in solar PV world. After that, I will make you understand some features of battery that cause inefficiency in PV system and how to cater for them. Then, I will show you how to calculate the number of batteries you need in your battery bank.

Let’s get going!

Battery Autonomy

Battery Autonomy is the number of days that you need the PV system to operate your appliances without recharging your battery system. This implies that the battery bank should be large enough to store sufficient energy to operate the appliances at night and cloudy days. This design is common in coastal states like Lagos and Rivers. Note, however, the larger the battery bank, the larger the budget.

Causes of Inefficiency in Battery and How to Cater for Them

1. Internal resistance during charging and discharging. We lose quite a number of energies from solar panel just by trying to put the energy into the batteries. We include other losses such as interconnection, fuses, protection circuits and wiring. To cater for those losses when designing our battery bank size, we divide the total Watt-hours per day by 0.85 (0.85 is the standard international best practice)

2. Depth of Discharge (DoD) – Batteries cannot fully discharge all the energy it has stored. DoD is the percentage of battery’s energy it can discharge in a cycle.

Different battery-types have varying Depth of Discharge (DoD). The depth of discharge for a typical Lead-Acid battery is 50 – 60% while Lithium-ion battery is 80-90%. The DoD of every battery in the market is indicated on the pack or user manual. Be aware that some bad manufacturers may over-size the rating to meet their sales demands. Always go for an established brand for your battery purchases. In this tutorial, we will adopt 60% as our DoD. This is because, the Lead-Acid battery I use in my projects has 60% DoD, guarantee!

How You Can Calculate Number of Batteries You Need

It’s time to bring out your calculator.

If you have done that, let’s go!

To get the number of batteries you need in your battery bank, follow these 5 easy steps:

A. Calculate Total Watt-hours per day used by appliances.
B. Cater for battery loss by dividing the total Watt-hours per day by 0.85
C. Cater for Depth of Discharge by dividing the answer obtained in “B” above by 0.6.
D. Divide the answer obtained in “C” by the nominal battery voltage.
E. Multiply the answer obtained in “D” with days of autonomy (number of days without recharging) to get the required Ampere-hour capacity of deep-cycle battery.

Practical Example:

Recall that the Total Watt-hour per day used by our appliances in the step 2: Sizing Solar panel is 6,656Wh (after catering for losses in panel). You may like to go back to the step 2 to find out how we arrived at the figure.

In this example, the battery available to us has the following specification:

1. Nominal Voltage of 12V

2. Rating of 200Ah

3. Depth of Discharge of 60%

Because we are in Abuja where there’s enough sunlight every day, we need 1-day autonomy.

To get the number of batteries we need in our battery bank, in the example:

A. Calculate Total Watt-hours per day used by appliances.

Total Watt-hour per day used by our appliances is 6,656Wh

B. Cater for battery loss by dividing the total Watt-hours per day by 0.85

Therefore, 6,656Wh Divided by 0.85 = 7,830.59Wh

C. Cater for Depth of Discharge by dividing the answer obtained in “B” above by 0.6.
Therefore: 7,723.53Wh Divided 0.6 = 13,050.98Wh

D. Divide the answer obtained in “C” by the nominal battery voltage.
Therefore: 13,050.98Wh Divided by 12V = 1,087.58Wh

E. Multiply the answer obtained in “D” with days of autonomy

Therefore: 1,087.58Wh Multiply by 1day = 1,087.58Wh

Total Ampere-hours required = 1,087.58 Ah

So, the battery should be rated 12 V by 1,087.58Ah

Since a single battery rated 12V by 1,087.58Ah is not available to us. What is available is 12V by 200Ah. So, we have to divide 1,087.58Ah by 200Ah = 5.44

So, we need six (6) 12V by 200Ah batteries for the project and have extra room.