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How Many Batteries Do I Need for a 3 kW Solar System?

By Benjamin Strusnik October 14, 2022

As much as a 3KW solar system’s output is in its name, the number of batteries needed in the system, or the size of those batteries is not.

Knowing how many batteries are needed in a solar system depends on variables that can be inputted into an online solar calculator.

Join us in finding out what these variables are and how they can be obtained before placing them into the calculator. 

But before we can dive into how many batteries are needed to power a 3kw solar system, we need to look at the purpose of the battery in a solar system.

The Battery’s Purpose

Adding a battery to existing solar systems expands its use and power in many ways. 

how many batteries do you need for a 3 kw solar system

For starters, a battery will be used to store energy that’s generated during the day. it acts as a repository for energy that can be used when the sun goes down.

Since the sun doesn’t shine during the night, your solar panels are practically useless.

However, if you have a battery that’s been building up its energy capacity during the day, you can make use of it at night.

The best part about incorporating a battery into your solar system is that you aren’t limited to one battery. Batteries can be tied together to create a battery bank that will store the energy coming into your solar system.

Finally, if you’re living off-grid and don’t need to be sending power back or paying your power company, a battery saves you and your wallet.

However, batteries are broken up into 3 different types, each having its pros and cons.

Lithium-ion

Lithium-ion stands out as being the most expensive battery out of the lot, but its quality is well worth the price.  

These are arguably the best batteries for solar systems that make use of an energy repository.

They require little to no maintenance and you don’t need to wait for them to fully charge before using them.

Due to their lengthy charge cycles, it's going to be a long time before you need to replace the battery.

how many batteries are needed for a 3kw solar system

Lithium-ion batteries offer plenty of safety features, but the one that stands out from the rest is their being able to shut down as soon as it overheats to save energy.

So if you’re wondering if you should get a battery with your solar system and you can’t decide which battery to choose, a lithium-ion battery will get the job done.

Flooded Lead-acid

These batteries are more commonly known as wet cell batteries and are the cheapest out of the 3 batteries.

As much as the wet cell is cheap, the quality severely drops when compared to lithium-ion.

They require plenty of maintenance such as checking water levels and equalization charges, and if not done properly, can render the battery useless and dangerous.

We say dangerous because failure in doing these checks causes hydrogen gas to leak out of the battery. 

This means you’re going to need proper ventilation if you don’t want to breathe in the toxic fumes.

These batteries are decent if you have very low solar needs but since we’re dealing with a 3kw solar system, wet cell batteries won’t be your best bet.

AGM Lead Acid

AGM batteries still fall under the lead acid battery category, but they are more durable, and premium compared to wet cells.

how many batteries for a 3 kw solar system

They have fiberglass mats wedged between the cells allowing the electrolytes to be absorbed.

Since there are no water levels, there’s less maintenance and need for repairs. This expands the lifespan of the battery but compared to lithium-ion still falls short.

If an AGM or wet cell needs to be repaired, chances are that you’re better off buying a new battery.

And if you’re constantly buying lead-acid batteries after every hiccup, you might as well buy a lithium-ion battery.

Again, this isn’t feasible in a 3KW solar system. 

Both types of lead acid batteries are 10 times cheaper than lithium-ion batteries, but due to their lacking of safety and overall quality, they are best suited for small or temporary solar systems.

How Many Batteries Are Needed?

To make the calculation simpler, we’re going to convert the kilowatt hours into watt-hours. So, our 3KW system becomes a 3,000W solar system.

We recommend using an online solar calculator as they all have the same approach when it comes to calculations.

You can manually figure out how many batteries you need in your solar system but it’s more of a rough estimate than an accurate statistic.

how many batteries are needed for 3kw solar system

The solar calculator also takes discharge and efficiency into account, something that isn’t simple to do manually.

Solar Needs

The first step in knowing how to calculate battery capacity for solar systems is to figure out your solar needs.

Usually, if we weren’t dealing with a system that already has a total wattage and we want to calculate the solar panel capacity too, we’d find out how much power we use per day.

This can be achieved by gathering your appliances and multiplying the wattage by the hours you spend using them per day.

However, we’re going to assume that we are needing 3,000 watt-hours per day.

Days Without Sun

The next step is to estimate how long we want this 3KW solar system running without the sun.

In other words, how many days of backup power would be needed?

Typically, you would need around 3 to 5 days with the size of this system, but for this calculation, we’re going to stick with 3.

The Lowest Temperature the Solar System Is Exposed to

The final variable in this calculation has to do with the temperature your system is exposed to.

Depending on whether you stay, your lowest temperature might not be that low. For our calculation, we’re going to say around 30 degrees Fahrenheit.

The reason the solar calculator wants this information is to get a good idea of the conditions the battery should function.

Type of Battery

Finally, we need to select the type of battery we are wanting to use in our solar system and the voltage of the battery.

Since this is a bigger solar system, we’re going to make use of a 24V lifepo4 battery which falls under the lithium-ion category.

how many batteries do i need for 3kw solar system

Final Calculation

The calculator does its magic and we’re left with a battery bank capacity of 16,579 watt-hours.

We made life simpler by converting kilowatts into watts, but now we need to convert back to kilowatts to continue that flow of simplicity.

When you purchase a battery, you are greeted with a few readings. The nominal voltage which in this case is 24V, nominal amp hours, and total capacity.

The battery we’re using has a nominal amp hour reading of 100 A/h with a total capacity of 2.56kWh.

If we convert our needed watt hours for our battery bank capacity into kilowatt hours, we can use the total capacity of our battery to figure out how many batteries are needed.

The 1657 watt-hours equate to around 16.5kWh, and since our battery has a capacity of around 2.56kWh, it’s a simple division from here on out.

Our 16.5kWh battery bank will consist of seven 24V batteries, each with a total capacity of 2.56kWh.

How Much Will This Cost?

Knowing how much it costs to add battery storage to a solar system is a good way to determine if you truly need a battery bank.

The battery we’re using in our example can retail anywhere between $800 and $1000, and if you need seven of them, that’s around $6000.

Since this is a larger system compared to the smaller portable solar systems, you would’ve already spent a good amount of money on solar panels and inverters.

In other words, if you have or are wanting a 3KW solar system, be prepared to spend.

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