How Many Batteries Do I Need For a 4kw Solar System

4kw solar system kits are becoming popular among homeowners looking for a backup power source. These systems can also reduce your monthly energy consumption, and it is ideal for off-grid cabins and various applications. These kits do not include batteries however, so we crunched the numbers so you don’t have to.

The battery requirements of a 4kw solar system depends on the load and how long you want to run it. If you need 4kw for 16 hours a day, that would require 16×200ah 24v deep cycle batteries.

How Many Batteries Does a 4KW Need?

The number of batteries you need depends on your power consumption and how you intend to use the system. In most setups, solar panels supply electricity to the batteries which in turn power the household.

The calculation is:

Battery voltage x battery capacity x depth discharge = battery watt capacity

Total watt consumption / battery watt capacity = number of batteries needed

Example 1

You have a 4kw solar system and need to run it for 16 hours straight. Follow these steps.

4kw x 16 = 64kw (64000 watts)

The next step is to figure out how many batteries are needed. To do this, divide the kilowatts by the system or battery voltage. For systems this large batteries like the LiTime 24V 200ah are often used.

Most of these batteries have a depth discharge rate of 85%, so there is 4080 usable watts in a 200ah 24V battery.

24V x 200 x .85 = 4080

64000 / 4080 = 15.6

You need about 16×200ah 24V batteries to run a 4kw solar system for 16 hours.

Example 2

In this scenario we will again use a 4kw solar system. This time you need to run a 3kw load for 24 hours.

The formula is the same. You start by multiplying the battery voltage, its capacity and depth discharge. Assume you are using 200ah 24V batteries with a 75% depth discharge.

24×200 x .75 = 3600

One 24V 200ah battery with a 75% DOD has a capacity of 3600 watts.

To run 3kw for 24 hours, you do the following:

3×24 = 72kw (72000 watts)

72000 / 3600 = 20

You need 20×200ah 24V batteries to run a 3kw load for 24 hours.

In this example we are using batteries with a 75% depth discharge. If you buy high quality batteries its discharge rate is even better.

How Long Will Batteries Last on a 4kw Solar System?

It depends on how much load the batteries carry and for how long. The depth discharge also plays a factor.

There are three types of deep cycle batteries used with solar systems: lead acid, AGM and lithium.

Lead acid batteries are the most affordable, but they require a lot of maintenance. The depth discharge is also the worst at 50%.

If you have a 200ah lead acid battery, you can only use 100ah before recharging. You could use up all 200ah, but that will wear out the battery quickly. For this reason we do not recommend this for large solar systems.

AGM is a type of lead acid battery, but with a better discharge rate of 70%. A good example of this is the Renogy 200ah AGM deep cycle battery.

The best option by far are lithium or lifepo4 batteries. With a typical DOD of 85-90%, you can use more of its capacity. Some of them even allow you to use up to 100%.

Aside from the DOD you also have to consider the duration and load. If you follow the battery calculations given earlier, you can see how the difference in DOD and power consumption affects battery life.

How Many Solar Panels Does a 4KW Require?

4kw solar system kits usually come with 10 solar panels, but the output varies. How many solar panels depends on your power consumption.

The formula is:

Power consumption / sun hours per day / inverter efficiency rate = number of solar panels

Example:

You are going to run a 4kw system at full load for 16 hours. Since the sun is only out for 8 hours a day on average, you should have a battery bank to store the power generated by the panels.

4kw x 16 = 64kw

Assume the sun is out for 7 hours a day:

64kw / 7 hours = 9.14kw

Now divide 9.14 by your inverter efficiency rating. Assume that it is 90%.

9.14 / .90 = 10.15kw

You need a 10.5kw of solar power or rounded off, 10.5kw.

10.5 kilowatts of solar power is equal to:

• 42×250W solar panels
• 35×300W solar panels
• 52×200W solar panels
• 30×350W solar panels

Factors That Affect Solar Panel Output

There are some assumptions we made in the calculations. First, we assume there is 7 hours of sunlight available. Second, the inverter efficiency is 90% and third, you run the 4kw system at full load.

If you change any of those factors, the number of solar panels required might be different. An array of 35×300W solar panels can produce 4kw if there are 7 hours of sunlight available. But if there is only 4 hours of sun per day, the output will go down.

The other factor is inverter efficiency. An inverter converts direct current produced by solar panels into alternating current so the power can be used by home appliances. During the conversion, some of the power is lost, anywhere from 5-15%.

Related: How many solar panels can I connect to my inverter?

The third element is how much power you are going to load and for how long. In this example the system loads 4kw and runs for 16 hours. You can change the power load and the duration but the calculation remains the same.

While you can run appliances using only solar panels (provided you have an inverter), batteries are necessary for two reasons.

One, solar panels only work when the sun is out. If it rains or night time, there is no power available. With a battery bank you can store the power and use it later.

The second is that solar panel output varies throughout the day. By using a battery bank you have a consistent source of power even if the sky turns cloudy.

Related Questions

Can a 4kw solar system run a house?

It depends on how much power you consume, but for a small, energy efficient home, yes. To find out, check your monthly power consumption. You can figure out how many solar panels and batteries are needed using that as a guide.

Do I need a charge controller for my batteries?

Yes. Charge controllers keep solar panels from overloading the batteries with current. How many charge controllers you will need depends on the system setup.

Why is my 4kw solar system producing less power?

Solar panel output depends on several factors, and the rating given is the maximum it can yield. For example, a 4kw solar system means it can generate up to 4kw. This does not mean the system will always produce 4kw. Depending on the weather, environment etc. it might be 3.5 kw or less.

This applies to all solar panels. A 300W solar panel can produce up to 300 watts. But that assumes the conditions are ideal. If the sky turns cloudy, the output is going to drop.

How many batteries can I connect to a solar system?

There is no limit to the number of batteries you can install. For practical purposes, it is better to get the an accurate idea of how many is required.

Can I install a 4kw solar system by myself, including the batteries?

There are DIY installation kits available. If you are handy with these stuff you can install them. If not, you can hire a professional to install the solar system including the batteries. The batteries are a separate purchase and will require an additional cost. The same with the charge controllers.