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How to effectively choose batteries to Store Solar Energy for Maximum Benefit?




The popularity of solar energy is surging tremendously. With new technologies emerging every day and a push from government authorities to make it a viable renewable source, there is no denying that solar is the energy of the future.

Everyone knows the benefits of solar energy and how the energy is consumed; the question which arises more often is their reliability when sunshine is not performing its best.

There are many technologies invented to consume solar energy, among all these inventions, a grid-tied photovoltaic system has gained popularity.

To understand how to effectively store solar energy, you need to first understand how a typical grid-tied photovoltaic system works?

How does a grid-tied photovoltaic system work?

The system consists of a set of solar panels, an inverter and a box to house all the wiring of the system. The solar panels collect the energy from the sun and turn it into electricity, which is passed through the inverter and converted into a form that can be utilized by a household.

Most of the solar PV systems are connected to a power grid, so when your solar panels are producing extra energy than your home needs the surplus is sent to the power grid and when your solar panels generated less energy the power grid supplies the additional energy required by your home. This process is called net-metering.

So what happens when the sun is absent that is at night or when the grid system is shut-off?

How do you continue to consume solar power in such cases?

The solution to this problem is to add or replace the power grid with a battery, so you will be able to have electricity even when the grid is off.

So the next question arises…

How do batteries store solar energy?

To understand how batteries store solar energy, you need to first know how batteries work.

Basically, electrochemical cells consist of two electrodes separated by a distance in a container. The space between the electrodes is filled with an electrolyte – an ionic liquid that conducts electricity. One electrode – the anode possess the ability to donate an electron, another electrode – cathode possesses the ability to receive an electron. The energy is stored in the compound formed as a result of this electrolysis, which is a combination of the compounds that make the anode, cathode and the electrolyte.

Simply put, the chemical energy at the anode makes it choose an electron and the reaction at cathode makes it receive an electron the path taken by an electron to travel from anode to cathode is captured in the form of electricity by an external circuit and the liquid electrolyte. This process continues until the anode loses all its electrons.

So now you understand how batteries store energy, it will be easier for you to understand how it stores solar energy.

Once the sunlight hits the solar panels, it loses electrons; due to the interaction between photons and electrons the electron travels in the form of direct current through solar cells. These electrons are then captured by the bars in the panels and sent through electrical wires to the charge controller.

A charge controller is a device used to charge the batteries in such a way that the exact current flows in and the voltage is maintained within acceptable boundaries.

How Many Batteries do you need for your PV solar system?

This depends on the various factors such as;

  • Type of your PV System – If you have a grid-connected system, you don’t need a big battery backup. The grid handles major electricity requirements. If your system is off-grid you require a big battery backup, as these systems are independent to store their own electricity.
  • Energy Demand – You need to calculate the average energy your household demands. Calculate the power demands each device requires, calculate the number of hours you want to use the devices without grid power in Watt-hours. Once you get the Watt-hours of individual devices, add them up and that sum total is your average energy demand.
  • Capacity – Capacity is usually measured in Ampere-hours (Ah). If you would like to know the power it can deliver in Watt-hours, just multiply its capacity with the voltage it handles. You need to choose battery capacity which matches your energy demand.
  • Autonomy – This represents the number of days you want to spend without the power grid
  • Voltage – Voltage depends on the power required by your household. If you know the power of the battery and your current requirements, then you can use the formula P= I*V to find the voltage. Where P = Power, I = Current and V = Voltage

When you calculate all these details, you can use the below two formulas to calculate the number of batteries.

                                                         LD  = ED/V

Cb =  LD * A /PD*ɳrb*ɳinv

LD      = Load Demand

ED      = Your Calculated energy Demand

V      = Voltage of the System

Cb      = Required Capacity of the battery

A      = Autonomy Days

Pd   =Depth of discharge ( The amount a battery can deliver in a single cycle without getting damaged)

ɳrb    = Efficiency of the charge controller and battery (around 85%)

ɳinv  = efficiency of the inverter (around 90%)

The next step is to choose a battery and note down its capacity and the voltage. This will help you calculate the number of batteries you need.

                                                      NB,P = CB/CCB


Number of batteries  =  NB,P * NB,P

NB,P  =   Number of batteries connected in parallel.

CB      =  Required capacity of the battery

CCB   =   Capacity of chosen battery

NB,S  =   Number of batteries connected in serial

V        =  Voltage of the system

VB    =    Voltage of the selected battery

The Best Batteries to store solar energy

In general, lead-acid batteries are considered to be the best for solar energy storage.

But when you are selecting a particular battery, you need to keep this criterion in mind.

  • The specification of your inverter
  • The voltage produced by your solar panels
  • Autonomy
  • Operating temperature
  • Maintenance cost
  • Battery lifetime
  • Cost
  • Self-discharge
  • Size of your solar panels
  • Maximum depth of discharge
  • The efficiency of your battery and solar panel

I hope this blog post helped you understand how batteries work and how you can select the one best suitable for you.

This is Amit working as a Solar Energy Adviser & content writer. I love to write on topics solar industry to help people improve their solar energy knowledge and stay update continues.