How to size battery bank for off-grid solar system

Introduction: Why Battery Capacity is Crucial

In off-grid systems, the battery bank forms the foundation of the system's reliability. Unlike grid-connected systems, off-grid solar power systems are capable of providing independent power supply in areas without a grid or where grid output is low. Therefore, the selection of battery capacity is of paramount importance. An inappropriate choice can lead to insufficient power supply at night or on overcast days, excessive battery discharge resulting in a shortened lifespan, unstable system operation, or even system shutdown, as well as increased initial investment or future expansion costs. For EPC contractors, system integrators and energy project developers, accurately determining the battery bank capacity for off-grid solar systems is crucial to the system's long-term performance. So, how should one select the appropriate capacity? In this article, GSl ENERGY, a storage manufacturer, will explore this topic with you.

Step 1: Calculate daily electricity consumption (kWh)

To select the appropriate battery capacity, you must first determine the total daily load. For example, for a household or project, list all the loads individually and calculate the total capacity.

You may refer to the following formula:

Total electricity consumption (kWh) = Total power of all appliances × Duration of use

Example:

Lighting: 5 kWh

Appliances: 25 kWh

Cooling system: 20 kWh

Total: 50 kWh per day

This serves as the baseline for calculating battery capacity.

Step 2: Determine the backup duration (Autonomy Days)

The number of backup days determines the system's survivability in the absence of sunlight:

·1 day: Minimum configuration (low cost, high risk)

·2–3 days: Standard off-grid system (recommended)

·≥3 days: High-reliability scenarios (medical, communications, mining, etc.)

Step 3: Consider the depth of discharge (DoD)

For modern lithium iron phosphate battery systems:

Typical depth of discharge (DoD) range: 80%–90%

A higher depth of discharge (DoD) implies greater available capacity, but must remain within safe limits.

Step 4: Account for System Losses

Reasons for efficiency losses:

Inverter losses

Cabling losses

Temperature effects

Typical loss factor: 10%–15%

Step 5: Apply the Battery Sizing Formula

Use this engineering formula:

$Battery\ Capacity\ (kWh)=\frac{Daily\ Load\ (kWh)\times Autonomy\ Days}{Depth\ of\ Discharge}$

Given conditions:

Daily electricity consumption: 40 kWh

Backup duration: 2 days

DoD: 85%

Calculation:

Base capacity:

40 × 2 ÷ 0.85 ≈ 94 kWh

Add 10% system redundancy:

Final recommended capacity ≈ 100 kWh

In actual B2B projects, modular design is the mainstream solution.

Example configuration:

Single battery module: 14.34 kWh

Target system capacity: 100 kWh battery

Configuration: 7–8 modules in parallel

Advantages of the modular solution

Flexible scalability (supports future expansion)

Easier installation and maintenance

Reduced transport and deployment costs

Improved system redundancy and reliability

Call to Action

Need help designing your off grid solar battery system?

GSL Energy provides:

Custom battery sizing and system design
LiFePO4 battery manufacturing
OEM / ODM / OBM solutions
Global off-grid project experience

Contact us today to get a tailored solution for your project.