When makers and off-grid builders ask how to make an inverter, they are rarely asking how to solder MOSFETs onto a breadboard to generate a square wave. They are asking how to build a complete, reliable inverter power system that takes stored DC energy and turns it into usable AC power without tripping breakers, melting wires, or destroying battery cells. Building this system requires matching your DC source to your AC loads through precise sizing math, correct topology, and hardware that can handle real-world surge currents.
This guide walks through the exact engineering steps to size, select, and wire a 48V inverter system capable of sustaining a 2000W continuous load. We will terminate this guide with a concrete, off-the-shelf hardware recommendation so you can order parts and start building.
The Anatomy of a DIY Inverter System (Source to Load)
A functional inverter system is not just a single box; it is a chain of components where the weakest link dictates your maximum safe power. The energy flow follows a strict source-to-load block path:
- DC Source (Battery Bank): Stores chemical energy and provides raw DC voltage (nominally 48V, operating between 42V and 56V).
- DC Disconnect and Overcurrent Protection: A manually operable switch and a Class-T or ANL fuse located within 18 inches of the battery positive terminal to protect the main feeder cables.
- Inverter/Charger: The core power electronics. It draws DC to synthesize a pure sine wave AC output, and reverses the process to charge the batteries when grid or generator AC is available.
- AC Load Panel / Transfer Switch: Distributes the inverted AC power to your circuits, protected by standard thermal-magnetic breakers.
- Control Signal Path: A low-voltage communication cable (usually RS485 or CAN bus) linking the Battery Management System (BMS) to the inverter to command shutdowns if cells drift out of safe voltage limits.
Battery Bank Sizing: Series vs. Parallel and C-Rate Limits
Your battery bank topology determines your system voltage and capacity. The choice between series and parallel wiring fundamentally changes how the bank behaves under load.
Series vs. Parallel Consequences
- Series Wiring: Voltages add, Amp-hours (Ah) remain the same. Wiring four 12V 100Ah batteries in series yields a 48V 100Ah bank. This is the preferred method for high-power systems because higher voltage drastically reduces current draw (Amps), allowing for smaller, cheaper wire and reducing I²R heat losses.
- Parallel Wiring: Amp-hours add, Voltage remains the same. Wiring four 12V 100Ah batteries in parallel yields a 12V 400Ah bank. While this provides high capacity, pulling 2000W from a 12V bank requires over 170 Amps of continuous current, demanding massive 4/0 AWG cables and heavy-duty busbars.
Charge/Discharge Limits: C-Rate and Depth of Discharge
You cannot use 100% of a battery's rated capacity, nor can you pull energy out infinitely fast. These limits are defined by Depth of Discharge (DoD) and C-rate.
- Depth of Discharge (DoD): For LiFePO4 (Lithium Iron Phosphate), a safe daily DoD is 80% to 90%. For Lead-Acid/AGM, DoD should be limited to 50% to prevent rapid sulfation and capacity loss.
- C-Rate Limits: The C-rate defines charge and discharge speed relative to capacity. A 100Ah battery discharged at 1C is delivering 100A. Most high-quality LiFePO4 server-rack batteries are limited to 0.5C continuous discharge (50A for a 100Ah battery) to preserve cycle life and prevent BMS overheating.
Inverter and Charger Sizing Math
To size the inverter and battery bank, we must work backward from the AC load, applying efficiency factors and Peukert's law to find the true DC demand.
Applying Efficiency and Peukert's Law
Assume a target continuous AC load of 2000W. Inverters are not 100% efficient; high-frequency pure sine wave inverters typically operate at 88% to 93% efficiency under heavy load. We will use a conservative 90% (0.90) efficiency factor.
DC Power Required = AC Load / Inverter Efficiency
DC Power = 2000W / 0.90 = 2222W
Next, we account for battery voltage sag. A 48V nominal LiFePO4 bank sits around 51.2V at rest but sags to roughly 48V under heavy load.
Continuous DC Current = 2222W / 48V = 46.3 Amps
Where Peukert's Law Applies: Peukert's law dictates that a battery's effective capacity shrinks as discharge current increases. For lead-acid batteries, the Peukert exponent is typically 1.3, meaning high current draws severely reduce usable Ah. However, as Battery University notes, LiFePO4 chemistry has a Peukert exponent very close to 1.05. For lithium, you can largely ignore Peukert capacity derating, but you must respect the BMS thermal limits and voltage sag at high currents.
Inverter and Charger Sizing for the Stated Load
Your continuous draw is 46.3A. Motors, compressors, and power supplies have startup surges that can be 2x to 3x their running wattage. You need an inverter with a continuous rating comfortably above 2222W, and a surge rating of at least 4500W. A 3000VA (approx. 2400W continuous / 5500W surge) inverter is the correct physical size.
For the charger: to maximize lithium lifespan, charge current should ideally be between 0.2C and 0.5C. For a 100Ah bank, a charger output of 35A to 50A is optimal, requiring roughly 1800W of AC generator/grid input to feed the charger.
Decision Tree: Picking Your Exact Inverter and Battery Hardware
Stop guessing. Use this decision matrix to select your hardware based on your 48V, 2000W continuous load requirement. This path terminates in a single, highly reliable hardware stack.
| System Requirement | Hardware Path / Constraint | Concrete Pick (Part Number) |
|---|---|---|
| System Voltage | Must be 48V to keep DC current under 50A for 2000W loads. | 48V Nominal Architecture |
| Battery Chemistry | Must be LiFePO4 for 6000+ cycle life and 1.05 Peukert exponent. | 48V 100Ah Server Rack LiFePO4 (e.g., SOK 48V or EG4 48V100) |
| Inverter Topology | Low-frequency toroidal transformer preferred for heavy motor surges and grid isolation. | Victron MultiPlus-II (Transformer-based) |
| Continuous Power | Must handle 2400W+ continuous and 5500W surge. | MultiPlus-II 48/3000 VA |
| Charger Size | Must provide ~35A DC to charge 100Ah bank at 0.35C. | MultiPlus-II 48/3000/35-16 (35A charger model) |
| Final System Pick | Order this exact combination for a bulletproof 2000W off-grid build. | Victron PIN: PMP482305016 (MultiPlus-II 48/3000/35-16 120V) |
Wiring, Fusing, and Commissioning the Build
With the Victron Wiring Unlimited guidelines as our standard, here is how you physically assemble the DC side of this system.
1. Wire Sizing and Routing
The maximum continuous DC current is ~46A, but the inverter can pull up to 65A continuously and over 110A during surges. According to NEC-style ampacity tables for 75°C rated wire in free air, 2/0 AWG copper THHN (rated for ~195A) is the correct choice. This provides a massive safety margin, prevents voltage drop, and keeps the wire cool during sustained surges. Keep DC cable runs under 5 feet to minimize voltage drop.
2. Fusing and Disconnects
Install a 150A Class-T fuse on the positive battery cable, within 18 inches of the battery terminal. Class-T fuses are mandatory for lithium banks because they have a high interrupting capacity (AIC) of 20,000 Amps, meaning they will safely extinguish the arc during a dead short. Standard ANL fuses (often rated for only 2,700A AIC) can fail catastrophically on large lithium banks. Follow the fuse with a 200A rated DC disconnect switch or marine battery isolator.
3. Torque and BMS Communication
Use a digital torque wrench. Loose DC connections create high-resistance hot spots that melt lugs and cause fires. Torque the MultiPlus-II DC terminals to 15 Nm (11 lb-ft) and the battery busbar bolts to the manufacturer's spec (usually 5-6 Nm for M8 bolts). Finally, connect the RJ45 BMS CAN-bus cable from the battery's COM port to the inverter's BMS-CAN port, and terminate the line with the included 120-ohm resistor.
4. Verification Step
Before applying AC power, use a multimeter to verify DC polarity at the inverter terminals. Read the voltage: it should be between 48V and 54V. Close the DC disconnect. The inverter should boot, read the BMS state-of-charge, and display a green LED. Only then should you apply AC grid/generator power to the AC-In terminal to test the 35A charging relay.
