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How many solar batteries do I need?

Updated: 16 October 2020

The number of solar batteries you need will depend on the size of your system, your power usage and what you want to achieve with your battery.

The solar battery you will most likely need is a 14kWh which comes as a single unit. It is by far the most popular size and suits solar systems 5 – 6.6kW in size. This will be sufficient power to run your home while the sun is not shining.

If you are looking to go totally off-grid, you will need a battery 4 – 5 time bigger to account for cloudy and rainy days where power harvested from panels is reduced.

Sizing your solar battery system

Sizing your solar battery system is the process through which you calculate the size of batteries you require for the storage of power on your system. Through this, you are able to compare your power requirements, with your solar panels, days of power storage, and come up with the right batteries capacity required.

The process of sizing your solar battery system is affected by the amount of consumption at your home, the number of days that you would like to store your energy, and the type of batteries you get.

You have to make sure you have enough batteries for your energy needs and the type of batteries that can be charged by your solar panels.

 

STEP 1

The first step is calculating the total energy consumption at your home. If you already have a system installed you can skip to step 2.

You will need to have a look at your power bill and work out the average power used by your home each day. If it is a monthly bill, take the kiloWatt hours and divide by 30 to get a single day usage.

 

STEP 2

The second step is to calculate the best battery size for that amount of energy.

Let’s say for example you have a daily usage of 20kWh on average. This is great for a 5kWh solar system which produces that amount of power each day.

Because we use 60-70% of our power at night, we need to store 70% of the power the system produces for later use.

20kW X .07 = 14kW

For this example, a standard size 14.4kWh battery like the Tesla Powerwall 2 or the SonnanBattery will do the job.

Which type of battery should I go for?

Currently, you have 2 x realistic options a Lead acid battery similar to the ones you find in your car. Or a Lithium-ion battery.

Lead Acid Battery

When calculating the size of the lead-acid battery, we have to consider that there is a 50% discharge depth. Hence you multiply by 2. Then you have to account for the inefficiency factor, which is 1.2.

That is, 10 kWh x 2 x 1.2 = 24 kWh.

Basically, you will need double to capacity. It’s worth noting that lead-acid battery technology is over 100 years old and has its limitations. It is quite cheap to get set up though.

That being said they only last about half as long as Lithium-ion batteries so it’s not really worth it.

Lithium Battery

Lithium-ion batteries have a 10-year warranty and will last longer than that. They have a discharge depth of 80% + and require very little to no maintenance, unlike their lead-acid counterparts.

Lithium-ion is the way to go as they are smaller, last longer and are more efficient. You will pay double, but they last more than twice as long at greater efficiency.

Solar battery calculator

The solar battery calculator is designed to help you understand how many batteries you need for your solar system. This is because each solar system has a specified number of batteries it should be able to charge.

By using a solar battery calculator, you are able to precisely know how many batteries to buy for what system to avoid wastage of money.

https://www.gwstore.co.za/solar-battery-calculator/

Number of batteries by solar system size

Getting the exact number of batteries you need for a solar system is determined by the type of batteries you are using and how many days you want the energy stored. It also depends on the amount of energy you would like to store. The depletion depth is also a major factor in this case.

3kw – Energy output, in this case, is 12 kWh. A battery with 8.4 kWh will be required.

4kw – Energy production here is 16 kWh per day. This means that you need a battery that is able to 11.2 kWh.

5kw –   Makes 20 kWh per day. Since a home will require around 70% solar power at night and 30% during the day. A battery of 14 kWh, in this case, is ideal.

6kw –   This system makes 24 kWh per day. Meaning you need a battery with a capacity of 16.6 kWh.

10kw – It is a system making 40 kWh of power per day. As a result, one will need to have a battery with a capacity of at least 28 kWh.

30kw – The total output here is 120 kWh per day. This means that you will need to get batteries with a capacity of 84 kWh. However, in case you do not find a single battery that can generate this amount of power, it is okay to have several batteries. In the case of Tesla Powerwall 2 storing 14 kWh, you need 6 batteries.

100kw – For this system, the total energy output is 400 kWh per day. This means that you will need a batter that can store up to 280 kWh. When using Tesla Powerwall 2 with a capacity of 114 kWh that is 20 batteries.

Note that most suppliers allow for expandable steps.

The following are some key things to note for such a system. When you are calculating the number of batteries, you must consider the voltage required or selection since it affects efficiency as well as cost in terms of cable sizes used. As a result, for any power system with a power of 3kWh, you need a voltage of 12v, between 3 and 5 kWh is 24v and above 5 kWh is 48v.

Days of storage can also affect the number of batteries you need in your system.

https://www.gwstore.co.za/solar-battery-calculator/

Which is the best solar battery?

The best solar battery is a Lithium-ion battery.

BatteryCost14 kWCost per kWCapacityWarrantyType
Alpha Ess$7,302$9,346$66810kWh10 yearsLithium Ion
Sungrow$3,500$10,220$7304.8 kWh10 yearsLithium Ion
Ampetus Energy$2,300$10,580$7563kWh15 yearsLithium-iron-phosphate
LG$8,195$11,718$8379.8kWh10 yearsLithium Ion
Soltaro 2$1,700$11,900$8502kWh10 yearsLithium Ion
Aquion Energy$2,200$12,320$8802.5 kWh5 yearsSalt Water
Sungrow-Samsung sbp4k8$4,610$13,461$9624.8kWh10 yearsLithium ion
Tesla Powerwall 2$14,000$14,000$1,00014kWh10 yearsLithium ion
ZEN Energy$10,000 to $15,000$14,000$1,00010kWh to 20kWh10 yearsLithium Ion
sonnenBatterie Eco$15,000$15,000$1,07114kWh10 yearsLithium ion
Fronius Solar Battery$5,000 to $20,000$15,555$1,1114.5kWh – 12.0kWh7 yearsLithium-ion phosphate
Redflow Zcell$12,600$17,640$1,26010kWh10 yearsZinc-bromine
Leclanche Apollion Cube$9,200$19,320$1,3806.7kWh10 yearsLithium ion
Simpliphi Batteries$4,750$19,570$1,3983.4kWh10 yearsLithium Ferrous Phosphate
Solax$5,390$21,560$1,5403.5kWh5 yearsLithium-ion
Magellan Power$20,600$22,536$1,61012.8 kWh5 yearsLithium ion
Enphase Energy$2,057$23,998$1,7141.2kWh10 yearsLithium Ion
AKASOL’s neeoQube$12,000$30,000$2,1435.5kWh10 yearsLithium Ion
Sunverge SIS$26,000$31,200$2,22911.6kWh10 yearsLithium Ion
Redback Technologies$10,745$31,375$2,2414.8kWh5 yearsLithium Ion
Hansol Technics$13,500$37,800$2,7004.98kWh5 yearsLithium ion

When comparing batteries to find out the best, there are several factors that are worth comparing to make sure that you know which is better.

That includes;

 

Depth of discharge

Depth of discharge is also referred to as DOD. Discharge depth is the amount of energy you should use on the battery when using it. This is because it is not advisable to use 100% of the charge on a battery.

As a result, a different brand of batteries has various DOD. This means that a battery with a high DOD will allow you to use more energy on your battery than one with a low DOD.

 

Power and Capacity

The capacity basically refers to the amount of energy a battery is able to store. It is measured in kWh. As a result, different batteries have different capacities. A good battery is one that is able to store more energy.

However, in this case, power also comes to play. When the capacity shows how big a battery is, its power rating shows how much power a battery can deliver at a given time.

This means that a battery can have low capacity but a high rating on power. The result is that it can power a home comfortably but for a few hours.

A battery with a large capacity but the low rating on power, on the other hand, is able to power a small area but for several hours.

 

Battery life

Battery life and warranty refers to how long a battery can last. This can be in terms of years or cycles. A good battery is one with many cycles or a long lifespan. When looking at lifespan, lithium-ion batteries have the longest lifespan followed by Saltwater batteries then Lead Acid.

 

Round-trip efficiency

This is a question of how much you can derive from your battery compared to how much is feed in. Batteries use energy to store energy. As a result, you cannot get 100% energy you stored. You have to consider this efficiency percentage when selecting a battery to get one with the most efficiency.

Based on the above factors, Lithium-ion batteries are the best compared to Lead-acid and Saltwater batteries. This is because they have a high DOD, and their lifespan is the longest.

Other advantages of Lithium-ion batteries include;

  • No ventilation/off-gassing
  • No maintenance required
  • Power usage is efficient
  • Their storage capacity is better and usable.

What is the cheapest solar battery?

Finding the cheapest solar battery is not easy since it depends on several factors. If we are talking battery type, then it’s anything lead-acid. But in the Lithium-ion space, the best bang for your buck is going to be the……

WINNER: Alpha Ess Smile5 – INV

Alpha ess smile 5 inverter
BatterySizeCostType
Alpha Ess Smile5-INV10kW$9346Lithium-ion

How long do solar batteries last?

Between 5 and 15 years.

The lifespan of solar batteries is affected by some factors. These factors include the type of battery, care, and maintenance.

A solar battery well maintained will last longer. Maintenance is mostly on Lead-acid batteries that have you regularly clean terminals of any corrosion, avoid overcharging, among other maintenance procedures. Proper maintenance will extend its lifespan.

The type of battery is the next factor that affects the lifespan of a battery. In this case, you will find Lithium Batteries to have the longest lifespan followed by Saltwater batteries then Lead Acid batteries.

Lithium batteries- 10 to 15 years

Saltwater batteries- 8 to 10 years

Lead Acid batteries – 5 to 7 years.

Ben McInerney head shotAuthor: Ben McInerney is a renewable energy enthusiast with the goal of helping more Australians understand solar systems to make the best choice before they purchase. Having an accredited solar installer in the family helps give Ben access to the correct information, which allows him to break it down and make it easily understandable to the average homeowner.