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Calculating the battery capacity and backup time of solar systems

Determine the Battery Backup Time of Your Solar System

As Nigerians embrace the shift towards renewables, getting the best out of your solar systems requires a good understanding of their solar batteries’ backup capabilities and limits. It’s becoming increasingly valuable for existing and prospective solar system users to know their solar systems' battery capacity and backup time to help them make better energy decisions. 

This analysis explains how to utilize basic solar system specifications to calculate your solar systems' battery capacity and backup time.

Why is the battery backup time important?

  • Optimized energy consumption: It helps you develop effective consumption patterns to deliver the much-desired consistent electricity.
  • Customized power solutions: Help determine the most suitable solar power solution to meet your energy needs and reduce power expenses.
  • Energy independence: Provide the foundation to accelerate your energy sufficiency drive and reduce reliance on traditional energy sources.

What determines your backup time?

  1. Battery Technology: The solar battery technology you use (lithium or lead acid), its amperage, voltage rating, and depth of discharge determine the usable battery capacity and the battery cycle life.

  2. Total Power of Load: The summation of the power rating of all appliances (load) that the solar system will power is critical in determining the battery backup time.

Off-Grid Solar Systems Storage

For off-grid solar power systems, the energy generated from solar PVs is stored in the energy storage system (battery) and used to power the appliances (load). In this case, determining the battery capacity and backup time requires key information about the solar system’s batteries.

Calculating the Battery Capacity

Let's calculate an off-grid solar system's battery capacity and backup time using four (4) units of 100Ah/51.2V lithium (LiFePO4) batteries with 90% depth of discharge.

Key Battery Information

  • Battery Amperage = 100Ah
  • Number of Batteries = 4 Units
  • Nominal Battery Voltage = 51.2V
  • Depth of Discharge (DOD) = 90%

Backup Battery Capacity = Battery Amperage X Number of Batteries X Nominal Battery Voltage X Depth of Discharge

Backup Battery Capacity  = 100Ah  X  4  X  51.2V  X  4  X  90% (0.9) 

Backup Battery Capacity  = 18,432Wh = 18.4kWh
The 18.4kWh capacity means that the off-grid solar system can provide stable backup power to 18.4kW load (appliances) for one (1) hour

Calculating the Backup Time 

You can use the backup battery capacity calculated above to determine the solar system’s backup time. To achieve this, sum up all your appliances' power consumption (watt rating) to get your total power consumption.

Now, let's assume your total power consumption is 3.2kW (equivalent to simultaneously running two units of 1HP inverter AC, ten units of laptops, two units of TV, and five lighting bulbs.)

Key Battery Information

  • Battery Backup Capacity = 18.4kWh
  • Total Power Consumption (Assumed) = 3.2kW

Total Backup Time = Battery Backup Capacity ÷ Total Power Consumption

Total Backup Time = 18.4 ÷ 3.2

Total Backup Time = 5.76 Hours

The above calculations show that the off-grid solar system with a battery backup capacity of 18.4kW can power the (assumed) 3.2kW load (appliances) for approximately six (6) hours.

In reality, navigating our expanding solar solutions market requires a considerable understanding of solar system components and the battery capacities of the respective solar systems. This knowledge is vital and valuable in optimizing solar energy consumption, acquiring a new solar power system, and upgrading an existing one. 

Next article Solar Power Storage:Tips for Installers in Nigeria

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