How Many Batteries for 2 kW Solar System?
Knowing how many batteries you will need for your solar system can be daunting, and it is. Because there are a number of factors that must be taken into account.
Including batteries type and capacity, the voltage of the system, and the amount of backup energy you want to store.
Do you have a 2 kW solar system? And looking for the correct battery capacity? Well, the following article explains an easy way to estimate how many batteries for 2 kW solar panels.
Understanding Battery Capacity And C-Rate
When considering purchasing batteries, there is an important criteria that distinguishes each battery, which is the recommended charging current or the recommended C-rate (in this case, charge).
The C-rate is an important standard used to determine the battery's ability to withstand charging and discharging currents.
The C-rate of charge represents the charging current that can charge the battery relative to its capacity, in other words how fast we can charge the battery.
For example, if you have a 100 Ah battery and the C-rate is 1, it can charge at a rate of 100 Amps (= 1*C(Ah)). If the C-rate is 0.25C, it can charge at a rate of 25 Amps (= 0.25*C(Ah)), and so on.
The C-rate depends on the type of battery (chemistry), whether lead-acid or lithium. The C-rate for lead-acid batteries is usually less than one, for example, 0.2C. This value is relatively low due to the nature and efficiency of lead-acid batteries, and the recommended value is ≤0.2C for charging mode and 0.1C-0.05C for discharging mode.
For lithium batteries, the C-rate is usually higher, for example, up to 1C. However, manufacturers recommend a rate ≤0.5C.
Generally, lithium batteries are better able to withstand high currents compared to lead-acid batteries.
When sizing the battery capacity for a solar system, it is important to consider the recommended C-rate for charging and discharging the battery.
For example, if a battery has a capacity of 100 Ah and is connected to a 100 A charge controller or a 12V-1000W inverter, which is a 1C rate, it may be charged or discharged in less than an hour.
However, If the battery is charged or discharged at a higher current than the recommended C-rate, the battery may be stressed, leading to capacity loss due to the strong discharge current.
How Many Batteries are needed for 2 kW Solar System?
The battery is connected to the solar panel array through a charge controller and inverter, Therefore, it is important to consider both the charge controller controller size (or the charging current) and the inverter size (or the current that the inverter will draw from the battery) when you do battery sizing.
Therefore the smallest size of battery needed to keep its operating safely and reliably with a 2kW system will depend on the largest current.
For a start, we’ll look at how to determine the size of your battery bank. Here are the steps to do it:
Step #1 Calculate the maximum charging current
To calculate the maximum charging current you need to decide what system (battery) voltage you will use.
- 12V
- 24V
- 48V
When you design your battery bank, you’ll have less power loss (through wires) if the voltage is higher. For 2kW Solar System, If you are able to design a 24V system, you can conserve more power than when you use a 12V system.
Max. charging current (A) = Parray (W) ÷ nominal system (Battery) voltage (V)
Parray = Maximum power output of the array.
For a 2kW system the max-power output of the array is 2000 watt.
- Imax (A) = 2000 W ÷ 12V = 167 Amps (for 12V system)
- Imax (A) = 2000 W ÷ 24V = 83 Amps (for 24V system)
- Imax (A) = 2000 W ÷ 48V = 42 Amps (for 48V system)
Step #2 Find the recommended charge current for the battery to be used, and extract the C-rate.
When charging or discharging a battery, it is important to respect the recommended charging and discharging current (C-rate or hour rate). Therefore, before using any battery, it is essential to refer to its datasheet.
C-rate of charge = Recommended charging current (A) ÷ Nominal battery capacity (Ah)
If you have not yet decided what type of battery to use, you can use the following values:
- Lead Acid C-rate: 0.2C
- Lithium (LiFePO4) C-rate: 0.5C
Step #3 Calculate the battery capacity required to respect the recommended C-rating of the battery
The battery capacity can be calculated using following formula:
Total Battery Capacity (Ah) = Max. charging current (A) ÷ C-rate of charge
Lead-acid Batteries
- Ctotal (Ah) = 167 A ÷ 0.2C = 835Ah (for 12V system)
- Ctotal (Ah) = 83 A ÷ 0.2C = 415Ah (for 24V system)
- Ctotal (Ah) = 42 A ÷ 0.2C = 210Ah (for 48V system)
We can see that for 12V lead-acid battery the smallest battery bank capacity recommended for the 2kW solar system is 835Ah, for 24V lead-acid battery is 415Ah, and so on.
Lithium (LiFePO4) Batteries
- Ctotal (Ah) = 167 A / 0.5C = 334Ah (for 12V system)
- Ctotal (Ah) = 83A / 0.5C = 166Ah (for 24V system)
- Ctotal (Ah) = 42 A / 0.5C = 84Ah (for 48V system)
We can see that for 12V lithium (LiFePO4) battery the smallest battery bank capacity recommended for the 2kW solar system is 334Ah, for 24V lead-acid battery is 166Ah, and so on.
Having a low battery capacity will still work, but remember that if you keep on doing this (charging your battery with higher current) you will shorten the battery’s lifespan.
Step #4 Calculating the number of required batteries
Number of required batteries = Total Battery Capacity (Ah) / Single Battery Capacity (Ah)
|
Battery types and capacity
|
Lead-acid |
Lithium
|
||||
|---|---|---|---|---|---|---|
| 12V-100Ah | 12V-200Ah | 24V-100Ah | 12V-100Ah | 12V-200Ah | 24V-100Ah | |
|
number of required batteries for 2kW system |
8 | 4 | 4 | 4 | 2 | 2 |
We can see that we need fewer lithium batteries than lead-acid batteries. This is because the lithium battery chemistry allows for higher charging currents.
Note:
If your inverter capacity is greater than 2kW and the continuous wattage your devices use is more than 2000 watts (inverter draw more than 2000W from your battery), it would be better to calculate the number of required batteries using the inverter capacity instead of the capacity of your solar panel array, because the discharge current will be greater than the charging current.
Battery Sizing Calculator:
I put in your hands a battery sizing calculator that can perform all of these calculations (just enter the solar array capacity (2000 watts) in the inverter capacity input field if your inverter capacity is less than 2000 watts).
Mohamed Bellamine
I'm the blogger behind Goo SolarPower. As a solar engineer, and a strong supporter of renewable energy. I've always been interested in Solar devices. I loves to write articles, guides, and tutorials about solar PV technology. Through goosolarpower.com I want to help people build and understand how solar power systems work.