Off-Grid Battery System for High-Power AC Loads Using MEAN WELL NTN-5K Inverter/Charger
Introduction
Many engineering applications involve running high-power equipment remotely in the absence of grid power. This creates a design constraint when developing portable, field-deployable power systems.
Specifically, how do we implement steady and continuous power for a mobile system without excessive complexity?
While energy storage is the obvious starting point, getting that stored energy to reliably deliver multi-kilowatt output for extended periods is where the real design tradeoffs emerge. We encountered these challenges in a client project where we needed to support continuous high-power off-grid operation. To meet these demands, we selected the MEAN WELL NTN-5K.
The NTN-5K is a 5kW inverter/charger that converts DC input to AC output, with integrated battery charging and transfer switching functionality.
System Concept Overview
The starting point is a straightforward but demanding requirement. The system called for 3.5kW of continuous AC power (120VAC) for at least two hours without the use of grid power during operation. Additionally, a practical battery charging solution and system portability for field deployment were key considerations.
Based on these requirements, we selected the NTN-5K-148 as the core of the system. The key driver was reducing integration complexity, as the NTN-5K combines inverter, charger, and transfer functionality into a single unit.
With a continuous output rating of 4kW, the NTN-5K-148 provides sufficient margin to support a 3.5kW load under sustained operation.
On top of that, we integrated a 48V lithium battery bank compatible with the NTN-5K for energy storage.
This results in a self-contained off-grid AC power system capable of operating independently from the grid while balancing weight, runtime, and performance.
Role of the MEAN WELL NTN-5K in the System
In this design, the NTN-5K serves as the central power conversion unit. Its integrated charger and transfer functionality significantly reduce external components compared to discrete inverter/charger setups, simplifying system design and improving reliability.
Operating Modes
The system operates in two primary modes depending on grid availability.
Charging/Bypass Mode
When grid power is available, AC input is passed directly to the load through the internal bypass while simultaneously powering the integrated charger to charge the battery bank through a controlled charging profile for lithium batteries.
While the AC bypass feature of the NTN-5K is valuable, especially in UPS-style applications, the primary advantage for this system is the integrated charger, which enables the battery bank to recharge fully within a few hours between discharge cycles.
Battery Powered Operation
When grid power is not available, the system transitions to battery-backed operation, where DC power from the battery bank is supplied to the NTN-5K. The inverter then delivers a regulated AC output to the load.
Depending on the specific model, MEAN WELL’s NTN-5K series supports a broad range of AC output voltages while accommodating a wide spectrum of battery input voltages. For this application, the NTN-5K-148 was utilized. Below is a comparison of the different models.
Figure 1: NTN 5K Series Specifications
Battery Sizing
The system was designed to support a target load of 3500W. Accounting for efficiency losses during power conversion, the continuous power required from the battery bank can be calculated. The NTN-5K-148 has a typical efficiency of 91%, meaning the input power required can be calculated as:
Input Power Required = Load ÷ Efficiency
~3850W = 3500W ÷ .91
To ensure the system can sustain the load for the required runtime, we first calculate the input power at the DC side. We then apply a ~25% margin to account for depth of discharge limits and system inefficiencies.
Minimum Stored Energy = Input Power Required* Run Time * 125%
~9240Wh = 3850W * 2 Hours * 125%
With a DC input voltage of 48Vdc, which falls within the specifications of the NTN-5K-148 input range of 40 ~ 66Vdc, the required ampacity can be calculated as follows.
Minimum Ampacity = Minimum Stored Energy ÷ Battery Voltage
~200Ah = 9240Wh ÷ 48Vdc
This specification ensures the battery bank can deliver the necessary power while providing the desired runtime. Without this calculation, the system could potentially be undersized and fail to maintain output under sustained load.
The required discharge current for a 3850W load at 48Vdc is approximately 80A. By selecting lithium iron phosphate batteries, which support high discharge rates, the system can safely handle this load while minimizing weight. The result is a compact battery solution optimized for efficiency and reliability.
System Architecture
Presented below is a simple block diagram illustrating the system architecture of the application.
During Charging Mode:
Figure 2: Grid Online Charging Mode / AC Bypass
During Operation Mode:
Figure 3: Grid Offline Battery Operation
Key Advantages
This system is designed to meet the load and uptime requirements while keeping integration straightforward. By using the NTN-5K as the core, inverter, charger, and transfer function are consolidated into a single unit, reducing wiring complexity and minimizing potential failure points.
The system operates with the load fully supported by the battery, with grid input used only for charging between cycles. Battery capacity is sized based on the real power demands, accounting for conversion losses to ensure the required runtime is achieved.
From an operational standpoint, recharge time is a key consideration. The integrated charger enables the battery bank to recover within a few hours, making the system practical for repeated field deployments. Overall, this results in a compact, reliable solution for delivering multi-kilowatt off-grid power.
Summary
What was shared here was just one example of the many possibilities in utilizing the MEAN WELL NTN-5K series. This started with a real client requirement, where the challenge was delivering consistent power in a setup that had no room for complexity or failure points. Using the NTN-5K allowed us to simplify the system and still meet the multi kilowatt demand. With the right battery sizing and a clear understanding of the load, this same approach can be applied to many other field applications.