For a 5kW continuous AC load on a 48V nominal battery bank, you need an IGBT-based inverter rated for at least 6.25kW peak to handle surge currents and efficiency losses. At this power level, you should utilize 1200V/50A minimum half-bridge IGBT modules, specifically the Infineon FF50R12RT4. While MOSFETs dominate in 12V and 24V systems under 2kW due to their high switching speeds, Insulated-Gate Bipolar Transistors (IGBTs) win in 48V and higher architectures because they offer vastly lower conduction losses at the high continuous currents required by 5kW+ loads.

System Block Description: From 48V LiFePO4 to AC Load

Before sizing the silicon, you must understand the power path. A high-power inverter is not just a switch; it is a carefully managed energy pipeline. Here is the exact block sequence from source to load:

  1. Source (Battery Bank): 16-series (16S) LiFePO4 cells providing 48V nominal (51.2V resting).
  2. DC Protection: Class-T fuse or DC breaker sized 1.25x the max continuous current, followed by a DC disconnect switch.
  3. DC Bus & Precharge: Bulk electrolytic capacitors smooth the DC bus. A precharge circuit (resistor and relay) prevents inrush current from welding the main contactors or blowing the DC fuse when the capacitors are initially empty.
  4. IGBT H-Bridge: Four IGBT switches (or two half-bridge modules) chop the DC bus into a high-frequency PWM waveform.
  5. LC Filter: An inductor-capacitor low-pass filter smooths the PWM chops into a clean 50/60Hz sine wave, removing high-frequency switching harmonics.
  6. AC Output & Load: The filtered sine wave passes through an AC breaker and into your subpanel or critical loads panel.
Series vs. Parallel Consequences: Wiring cells in series increases voltage while Ah capacity remains the same (e.g., 16S 100Ah = 48V, 100Ah). Wiring in parallel increases Ah capacity while voltage remains the same (e.g., 2P = 200Ah). Never parallel mismatched cells, different brands, or strings of different ages. Always parallel identical, pre-top-balanced strings to prevent circulating currents that will degrade the weaker string and trigger BMS faults.

Sizing Math: Inverter, IGBT Modules, and Battery Bank

Let us run the exact math for a 5,000W continuous load. We must account for inverter efficiency, battery voltage sag, and the specific discharge characteristics of lithium iron phosphate.

1. Inverter DC Input Power:
Assuming a conservative 90% inverter efficiency ($\eta = 0.90$) at full load:
$P_{dc} = P_{ac} / \eta = 5000W / 0.90 = 5,555W$

2. Continuous Battery Current:
Using the low-voltage cutoff threshold (44.8V for a 16S LFP bank) for worst-case current sizing rather than the 51.2V nominal:
$I_{cont} = 5555W / 44.8V = 124A$

3. Peukert's Law and C-Rate Limits:
Peukert's Law describes how battery capacity drops at higher discharge rates. While lead-acid batteries suffer heavily (Peukert exponent $k \approx 1.3$), LiFePO4 chemistry is highly linear ($k \approx 1.02$ to $1.05$). This means a 100Ah LFP battery will still deliver roughly 98Ah at a 1C discharge rate. However, the real limit is voltage sag. Pulling 124A from a 100Ah bank (a 1.24C rate) will cause severe voltage sag, tripping the BMS low-voltage disconnect.

To keep the continuous discharge at a safe 0.5C to 1C rate and maintain an 80% Depth-of-Discharge (DoD) for a 6,000+ cycle lifespan, we size the bank up:
Required Bank Size: 200Ah at 48V (10.24 kWh total capacity, 8.19 kWh usable at 80% DoD). At 200Ah, our 124A draw is a comfortable 0.62C rate.

4. Surge and IGBT Sizing:
Motors and compressors require 2x to 3x surge current for a few seconds. A 5kW inverter must handle 10kW (approx. 250A DC) for 5 seconds. The IGBT modules must have a pulsed collector current ($I_{cm}$) rating well above this. A 50A continuous IGBT module typically handles 200A+ pulsed, which is why we use two 50A half-bridge modules in parallel per switch leg for high-surge commercial units, or simply oversize to a 100A module for custom builds.

IGBT vs. MOSFET: The Decision Tree for Inverter Switches

Choosing the right switching topology depends on your DC bus voltage and total power. Use this decision matrix to select your semiconductor.

System Parameter MOSFET Topology IGBT Topology SiC MOSFET Topology
Power Level < 2,000W 3,000W to 15,000W > 10,000W
Battery Voltage 12V or 24V 48V (High DC Bus) 48V to 800V DC
Switching Frequency 20kHz - 50kHz+ 10kHz - 20kHz 20kHz - 100kHz
Primary Loss Type Switching losses dominate Conduction losses dominate Lowest overall losses
Cost per Watt Low Medium High

The Decision Path:
If your build is a 12V van conversion under 2kW, choose MOSFETs (e.g., IRFP4468). If you are designing a high-frequency grid-tie or premium off-grid 48V inverter above 5kW and budget allows, choose Silicon Carbide (SiC). However, for the standard 48V, 5kW off-grid solar or UPS build, IGBT is the mandatory default. The lower conduction voltage drop ($V_{ce(sat)}$) of an IGBT at 100A+ saves massive amounts of heat compared to the $R_{ds(on)}$ resistive losses of standard silicon MOSFETs at those current levels.

Charge/Discharge Limits and Lithium Safety Protocols

LiFePO4 Fire and Thermal Runaway Warning: While LiFePO4 is significantly more thermally stable than NMC lithium-ion, a short circuit or severe overcharge can still cause venting and fire. Never bypass the Battery Management System (BMS). The BMS must be hardwired to interrupt both charge and discharge paths via contactors if cell voltage exceeds 3.65V or drops below 2.50V. Always install a secondary Class-T fuse on the positive terminal as a catastrophic fail-safe.

When configuring your inverter/charger parameters, you must respect the electrochemical limits of the cells. According to baseline lithium research from Argonne National Laboratory, violating charge limits causes irreversible structural damage to the anode.

  • Max Charge Rate: 0.5C (100A for a 200Ah bank). Standard charge rate should be 0.2C to 0.3C for longevity.
  • Max Discharge Rate: 1C continuous (200A), 2C surge for < 15 seconds.
  • Low-Temperature Charge Cutoff: You must configure the BMS and inverter charger to stop all charging at 0°C (32°F). Charging LFP below freezing causes lithium metal plating on the anode, which permanently reduces capacity and creates internal dendrites that can pierce the separator and cause a short circuit.
  • Absorption/Float Voltages: Set absorption to 55.2V (3.45V/cell) and float to 53.6V (3.35V/cell). LFP does not require equalization; never enable an equalization cycle on your inverter/charger.

Final Build Recommendation: The 5kW 48V Reference Design

We do not leave system design to guesswork. Based on the 5,000W continuous load requirement, the 48V architecture, and the thermal constraints of a standard off-grid equipment room, here is your concrete bill of materials and part selection.

Component Specification / Part Number Notes
IGBT Module (Custom/Repair) Infineon FF50R12RT4 (1200V, 50A Half-Bridge) Use two modules to form a full H-bridge. Mount to a thermal-resistance heatsink < 0.2°C/W with high-grade thermal compound. See Infineon IGBT Module Specs.
Commercial Inverter (Buy) Victron MultiPlus-II 48/5000/70 If you are buying rather than building, this unit utilizes a robust IGBT-based toroidal topology and handles 5000VA continuous with a massive surge capacity.
Battery Bank 48V (16S) 200Ah LiFePO4 with 200A BMS Provides 10.24 kWh. Keep DoD to 80% (8.19 kWh usable) to guarantee 6000+ cycles.
DC Protection 250A Class-T Fuse & 200A DC Breaker Sized for 1.25x the 124A continuous worst-case draw, while accommodating the 200A BMS limit.
Wire Size (Battery to Inverter) 2/0 AWG Welding Cable (or 2x 2 AWG THHN in parallel) Keep the run under 5 feet to minimize voltage drop below 1% at 125A.

By selecting the Infineon FF50R12RT4 for the switching stage (or the Victron MultiPlus-II 48/5000 as a complete commercial equivalent), pairing it with a properly sized 200Ah 16S LiFePO4 bank, and strictly enforcing the 0°C charge cutoff, you will build a 5kW power system that runs cool, survives motor surges, and lasts for a decade of daily cycling.