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How Many Batteries Do I Need for a 3.5 kVA Inverter?
04 December 2025
If you’ve been wondering, “How many batteries do I need for my 3.5 kVA inverter?”, you’re not alone. This is a common question from homeowners and small business owners who want reliable power backup without overinvesting in batteries. Choosing the right number of batteries is crucial for safety, efficiency, and battery longevity.
As a solar products manufacturer, we design and supply tubular batteries, solar inverters, online UPS systems, EV chargers, lithium-ion batteries, and more. Based on my experience as an online UPS manufacturer, online UPS supplier, and online UPS trader, I can guide you through sizing your battery bank to match your 3.5 kVA inverter.
Understanding Your Power Requirements
Before we talk numbers, let’s understand what a 3.5 kVA inverter actually means.
1. Converting kVA to kW
Inverters are rated in kVA (kilovolt-amperes), which represents apparent power. For most residential inverters, the power factor is around 0.8.
Calculation:
3.5 kVA × 0.8 = 2.8 kW
This means your inverter can handle a maximum load of 2.8 kW. Understanding your load is critical to selecting the right battery capacity.
2. Backup Duration
How long do you want your backup to last? Common durations for a 3.5 kVA inverter are:
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Short: 1–2 hours
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Medium: 3–4 hours
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Long: 5–6 hours
The longer the backup, the more battery capacity you need.
Battery Specifications and Sizing
Next, let’s look at battery specifications. Since we manufacture tubular batteries, lithium-ion batteries, and more, I’ll focus on tubular batteries for this guide.
1. Battery Voltage
For small inverters like 3.5 kVA, 12V or 24V battery banks are common. Using a 24V battery system is efficient and reduces the number of batteries needed.
2. Battery Capacity
Battery capacity is measured in Ah (ampere-hour). For example, a 150Ah tubular battery at 12V stores:
Energy (Wh) = Voltage × Ah = 12 × 150 = 1800 Wh ≈ 1.8 kWh
This is the usable energy per battery before considering efficiency and depth of discharge.
Calculating the Number of Batteries
Now let’s calculate how many batteries you actually need for a 3.5 kVA inverter.
Step 1: Determine Total Energy Requirement
To estimate total energy, multiply your load by the desired backup time.
Example:
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Load = 2.8 kW
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Backup duration = 4 hours
Total energy required = 2.8 kW × 4 h = 11.2 kWh
Step 2: Adjust for Battery Efficiency
Batteries cannot be discharged completely. For tubular batteries, the Depth of Discharge (DoD) is ~80%. Inverter efficiency is ~90%.
Adjusted energy required = 11.2 ÷ 0.8 ÷ 0.9 ≈ 15.5 kWh
Step 3: Determine Number of Batteries
Each 12V 150Ah battery stores ~1.8 kWh.
Number of batteries = 15.5 ÷ 1.8 ≈ 8.6 → 9 batteries
So, for a 3.5 kVA inverter with a 4-hour backup at 2.8 kW, you would need 9 batteries of 12V 150Ah.
Battery Configuration
The total number of batteries must be arranged in series and parallel:
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Series Connection: Increases voltage to match the inverter. For a 24V system, connect two 12V batteries in series.
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Parallel Connection: Increases capacity (Ah) to meet energy needs.
For our example:
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Series strings: 2 batteries in series = 24V
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Parallel strings: 9 ÷ 2 = 4.5 → 5 parallel strings (slightly oversized to ensure backup and longevity)
This ensures balanced charging, safe operation, and longer battery life.
Battery Type Considerations
Selecting the right battery type is essential:
1. Tubular Batteries
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Ideal for consistent loads and long backup hours
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Durable, reliable, and cost-effective
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Good choice for 3.5 kVA inverters
2. Lithium-Ion Batteries
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Higher initial cost but longer lifespan
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Can handle higher currents and deeper discharges
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Require minimal maintenance
As a tubular battery manufacturer, supplier, and exporter, I often recommend tubular batteries for residential or small commercial 3.5 kVA systems. For compact or high-performance setups, lithium-ion is a great alternative.
Charging Considerations
Battery performance depends not just on quantity, but also on proper charging:
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Use a Smart Charger or Online UPS – Protects batteries from overcharging and extends lifespan.
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Avoid Constant High Currents – High currents reduce battery life. For a 150Ah tubular battery, 0.2C (30A) is ideal for daily charging.
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Regular Maintenance – Maintain electrolyte levels for tubular batteries and check connections.
As an online UPS manufacturer, online UPS supplier, and online UPS trader, I always stress the importance of smart charging technology.
Integrating Batteries with a 3.5 kVA Inverter
Proper integration is key:
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Match inverter voltage with battery bank (12V or 24V).
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Ensure charging current does not exceed battery manufacturer’s recommendation.
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Use series-parallel configurations to balance voltage and capacity.
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Monitor battery temperature and maintain ventilation.
This ensures a long-lasting and reliable system.
Practical Example
Suppose a small office requires 2.8 kW backup for 4 hours using 12V 150Ah tubular batteries:
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Energy required: 11.2 kWh
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Adjusted for efficiency: 15.5 kWh
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Number of batteries: 9
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Configuration: 2 batteries in series per string (24V), 5 parallel strings
This setup provides safe, efficient, and reliable backup.
Conclusion
Calculating the number of batteries for a 3.5 kVA inverter requires understanding:
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Your actual load (kW)
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Desired backup duration
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Battery voltage, capacity, and type
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Efficiency losses and Depth of Discharge
For a 4-hour backup at 2.8 kW, 9 batteries of 12V 150Ah are typically required.
As a tubular battery manufacturer, solar inverter manufacturer, online UPS manufacturer, EV charger manufacturer, and lithium-ion battery manufacturer, we provide complete solutions for homes and businesses. We are also trusted suppliers and traders for these products, delivering high-quality, reliable, and efficient systems.