How Does a DC to AC Inverter Work? Topology, Sizing Math, and System Design

The Core Working Principle: From DC Source to AC Load

A DC to AC inverter works by using high-frequency electronic switches (typically MOSFETs or IGBTs) to rapidly alternate the polarity of a DC voltage source, creating a high-frequency square wave that is then filtered into a pure sine wave matching standard grid power (120V/240V at 60Hz in North America, or 230V at 50Hz in Europe). Modern pure sine wave inverters use Pulse Width Modulation (PWM) to vary the width of these DC pulses, effectively synthesizing the smooth, curved voltage profile required by sensitive electronics and inductive motors.

To understand the energy flow, trace this system block description from source to load:

1. DC Source: Battery bank provides stable DC voltage (e.g., 48V nominal).
2. DC Disconnect & Overcurrent Protection: A Class T fuse and rotary disconnect isolate the bank.
3. Inverter DC Terminals: Heavy-gauge copper lugs transfer high current into the inverter.
4. H-Bridge Switching Stage: Internal MOSFETs chop the DC into high-frequency AC.
5. LC Low-Pass Filter: Inductors and capacitors smooth the chopped wave into a pure sine wave.
6. AC Output Terminals: Clean 120/240V AC exits the inverter.
7. AC Subpanel & Load: Power is distributed via standard breakers to appliances.

For off-grid and backup systems, low-frequency (LF) inverter-chargers with heavy toroidal transformers are generally preferred over high-frequency (HF) models. LF units handle heavy inductive surge loads—like well pumps and compressor startups—far better because the transformer provides physical magnetic mass that absorbs transient spikes without tripping the inverter's solid-state protection.

Series vs. Parallel: Configuring the DC Source

Before sizing the inverter, you must configure the battery bank. The choice between series and parallel wiring fundamentally alters your voltage and amp-hour (Ah) capacity, which dictates your wire sizing and busbar requirements.

As the table illustrates, wiring in series increases voltage while keeping Ah constant. Wiring in parallel increases Ah while keeping voltage constant. Total energy (Watt-hours) remains identical in both scenarios (4,800Wh), but the 48V series configuration is vastly superior for any inverter over 1500W. By quadrupling the voltage, you quarter the current. Lower current means less voltage drop, less heat generation, and significantly cheaper copper wire.

Sizing Math: Peukert, Efficiency, and Surge Limits

Sizing an inverter and battery bank requires calculating both continuous draw and surge capacity, while factoring in inverter efficiency and battery chemistry limits.

The Scenario: You need to run a 1500W microwave and a 500W refrigerator simultaneously. The fridge compressor has a Locked Rotor Amps (LRA) surge of 2000W for a few milliseconds.

Step 1: Inverter Sizing
Total continuous load = 2000W. Total surge = 3500W (1500W + 2000W).
Inverters are not 100% efficient; a good LF inverter operates at roughly 90% efficiency under heavy load.
DC Input Power required = 2000W / 0.90 = 2222W.
Add a 20% safety margin for continuous thermal headroom: 2222W * 1.20 = 2666W.
Result: You need a minimum 3000W inverter with a surge rating of at least 5000W to handle the compressor startup.

Step 2: Battery Sizing and Peukert's Law
At 2222W DC input on a 12V system, the current draw is 185A. On a 48V system, it is 46.3A. Let us look at how battery chemistry handles this 46.3A draw on a 100Ah bank.

If you use Lead-Acid (AGM or Flooded), you must account for Peukert's Law. Peukert's Law states that as the rate of discharge increases, the available capacity of a lead-acid battery decreases non-linearly. A 100Ah AGM battery rated at a 20-hour discharge rate (5A) will only yield about 60Ah of usable capacity if you pull 46A from it. Furthermore, lead-acid batteries suffer severe sulfation if discharged below 50% Depth of Discharge (DoD). Therefore, a single 100Ah lead-acid battery is entirely inadequate for a 3000W inverter; you would need a massive 400Ah+ bank just to keep the C-rate low and respect the 50% DoD limit.

Lithium Iron Phosphate (LiFePO4) does not suffer significantly from the Peukert effect. A 48V 100Ah LiFePO4 battery will deliver nearly its full 100Ah capacity even at high discharge rates. However, you must respect the Battery Management System (BMS) continuous discharge limit, typically 100A (1C) for a 100Ah cell.

Charge, Discharge, and Safety Limits

Every battery chemistry has strict charge and discharge limits that dictate your inverter's low-voltage disconnect (LVD) settings and your solar charge controller parameters.

Decision Matrix: Picking Your Inverter and Battery Bank

Use this decision path to select the correct voltage architecture and hardware for your specific load profile. Refer to the Victron Energy Wiring Unlimited guide for exact torque specs and busbar layouts when assembling your chosen system.