As the solar power industry continues to grow, more and more homeowners are experimenting with solar power systems to power different electrical systems, such as powering their properties, which not only saves them a significant amount of money on their electricity bills, but also increases the value of their properties and the transaction price of their secondary sales. Off-grid solar panel systems are popular with many people, and within an off-grid solar panel system, the battery bank is an important component in increasing its power supply. In this article, we will introduce you to how to arrive at the size of the battery required for an off-grid solar panel system from a number of aspects.

How to calculate the battery size needed for an off-grid solar panel system?

Calculating the size of the battery capacity in an off-grid solar system needs to take into account the influence of various factors, so accurate and complete calculations need to be derived through the following steps.

Determine the daily energy demand of the property

Before you start calculating the size of the batteries you need for your off-grid solar panel system, you need to know the daily energy usage (usually measured in KWh) of your property, as this largely determines the size of the batteries you need to prepare. Typically, you can find out the average daily energy consumption directly by checking your monthly electricity bill payment, but this is best done by collecting electricity bills over a 12-month period to avoid large errors in the statistics due to seasonal differences.

Examine the peak hours of sunshine at the location of your property

The next step is to check the abundance of sunlight resources where your property is located, that is, the average daily peak sunlight hours. This greatly affects the amount of power generated by the solar panel system and thus has an impact on the size of battery capacity required. In order to get an accurate figure, you need to make an estimate of the peak sunshine hours in your local area: this value varies greatly depending on the location. You can check a map to estimate the average peak sunlight hours for your location or use a peak sunlight hours calculator to get an accurate value for your location. However, it is important to note that when using the Peak Sunshine Calculator, you need to place the device at the azimuth of the surface of the roof where you are going to install the solar panels.

Generally speaking, peak sunshine hours above 5h-6h already belong to high sunshine resource areas.

Determine the type of battery required

After checking the objective data above, you need to start thinking about the attributes of the chosen battery, which will also have an impact on the size of its capacity.

First, there is the type of battery you choose. The main common solar battery packs on the market are lithium iron phosphate and lead acid batteries. These two types of batteries have different chemical and physical properties. Lithium iron phosphate batteries have a higher charging and discharging efficiency, capable of reaching about 80%, which means that they lose less power throughout the charging and discharging process, in addition to which they have a greater depth of discharge, allowing you to more fully utilize the capacity of the battery. The service life of lead-acid batteries is much shorter than lithium iron phosphate batteries, and the efficiency of charging and discharging will be lower, the depth of discharge can only reach about 50%.

To summarize the advantages and disadvantages can be seen, lithium iron phosphate battery is a better choice for solar battery. Although in the same capacity, lithium iron phosphate battery price is higher, but its service life is longer, the later maintenance costs are lower, and in its gradual price reduction under the premise of the choice of lithium iron phosphate battery is clearly a better choice.

Determine the required battery voltage

Generally speaking, the common voltage of the batteries used in solar energy systems are 12V, 24V, 48V. there are many factors to consider when choosing the battery voltage, the system voltage of the battery not only affects its compatibility with charge controllers, solar panels and circuits, but also affects the power supply effect of its household appliances.

Determine the number of days of battery backup required to have

Battery back-up days are the number of days that the battery will be able to power the property independently of the solar power system in bad weather. This greatly affects the size of the battery you need. If you are in an area where the energy supply is unstable, and there is often bad weather and grid outages, you can increase the number of battery backup days to 5-7 days. If you are in an area where there is plenty of sunshine and the energy supply is warm, the number of battery backup days can be lowered to 3-5 days. Battery backup days can be reduced to 3-5 days if your area is sunny and the energy supply is warm.

With all of these influences combined, you can begin to make calculations about the size of the batteries you will need for your off-grid solar panel system.

First, calculate how much energy needs to be consumed during the time when the solar panels are not able to generate electricity, the exact value of which is equal to the product of your household’s daily energy consumption and the number of days of battery backup, ie:

Total Energy Consumption (Wh) = Daily Energy Consumption (Wh/day) x Battery Backup Days (days)

Next, is the total energy consumption divided by the depth of discharge of the battery, this step is to take into account that the battery does not completely exhaust its capacity power when discharged, then the formula for calculating the labelled capacity of the battery is as follows:

Battery capacity (Wh) = total power consumption (Wh) ÷ battery discharge depth

Finally, the unit of battery capacity will be converted from Wh to Ah, only need to divide the Wh value by the voltage of the battery, the specific calculation formula is as follows:

Battery capacity (Ah) = Battery capacity (Wh) ÷ Battery voltage (V)

In order to facilitate your better understanding and mastery of the above series of formulas, the following will be used as an example of a set of off-grid solar power generation system using 12V lithium iron phosphate batteries, with a daily power consumption of 2000Wh/day, and battery backup days of 5 days:

Its total power consumption is 2000Wh/day × 5 days = 1,0000Wh, and the battery capacity is:

1 0000Wh ÷ 80% = 1 2500Wh, i.e. 1 2500Wh ÷ 12V = 1041Ah.