How does an inverter work? - DIY Solar Power Forum

The power P drawn from a battery of voltage V by a resistor of resistance R (say, a light bulb) is not constant---it is given by P = V^2/R. As time evolves, V decreases and so does P. Does an inverter operate the same way? --- Suppose there's a household appliance connected to the output of an inverter, requiring a constant power from the inverter. Will the inverter try to take that power from the battery no matter what? If so, then we should observe an increase in current from the battery to the inverter with time, as the battery's voltage decreases. Unfortunately I didn't find any videos on YouTube showing this. I would really appreciate if anyone can give an input!

Does an inverter operate the same way?

NO, An inverter will draw as much current as it needs to drive the loads.
As the battery voltage goes down, the battery current will go up if the inverter output power remains constant.

For an inverter, Power-in = Power-out / Efficiency.

In terms of Voltage and Current:
(Vbat) * (Ibat) = (Vac * Iac) / (Inverter Efficiency)

Note: The inverter specs should tell you efficiency, but for a first approximation you can use 90%. (Cheaper or smaller inverters are often closer to 85%. A decent large inverter should be 95% or better.)

If I want to figure out the maximum sustained current an inverter will draw on a 48V LiFePO4 system I use this formula

(Inverter power rating)/(Inverter Efficiency) = 48V * (Max Battery Current)

48V is at the bottom end of the battery discharge curve and gives a higher current than if I used the voltage of a full battery (58V)

NO, An inverter will draw as much current as it needs to drive the loads.
As the battery voltage goes down, the battery current will go up if the inverter output power remains constant.
For an inverter, Power-in = Power-out / Efficiency.
In terms of Voltage and Current:
(Vbat) * (Ibat) = (Vac * Iac) / (Inverter Efficiency)
Note: The inverter specs should tell you efficiency, but for a first approximation you can use 90%. (Cheaper or smaller inverters are often closer to 85%. A decent large inverter should be 95% or better.)
If I want to figure out the maximum sustained current an inverter will draw on a 48V LiFePO4 system I use this formula
(Inverter power rating)/(Inverter Efficiency) = 48V * (Max Battery Current)
48V is at the bottom end of the battery discharge curve and gives a higher current than if I used the voltage of a full battery (58V)

Thank you 1. Input Stage: DC Power from Battery

• Input: The inverter receives 48V DC from a battery bank (typically lead-acid, lithium-ion, or similar).
• Protection: The DC input passes through protective components like fuses, circuit breakers, or reverse polarity protection diodes to safeguard the inverter from overcurrent or incorrect wiring.
• Filtering: Capacitors smooth out any voltage ripple or noise from the battery to provide a stable DC input.

2. DC-DC Boost Conversion (High-Voltage DC Generation)

• Purpose: To generate split-phase 120/240V AC, the inverter first needs a higher DC voltage (typically 300-400V DC) because the peak voltage of a 240V AC sine wave is around 340V (240V × √2).
• Process:
  • A boost converter or transformer-based H-bridge steps up the 48V DC to this high-voltage DC level.
  • This is achieved using high-frequency switching (via MOSFETs or IGBTs) controlled by a pulse-width modulation (PWM) circuit.
  • Inductors and capacitors store and release energy to facilitate the voltage increase.
• Topology: Some inverters use a high-frequency transformer here, while others rely on a transformerless design with a boost stage.

3. Inverter Stage: DC to AC Conversion

• H-Bridge Configuration:
  • The high-voltage DC is fed into an H-bridge circuit made up of four high-power transistors (MOSFETs or IGBTs).
  • The H-bridge switches rapidly (typically 20-50 kHz) to create a square wave or modified square wave approximation of AC.
• PWM Modulation:
  • Pulse-width modulation refines this square wave into a smooth sine wave.
  • A microcontroller or DSP (digital signal processor) generates PWM signals to control the transistors, adjusting the duty cycle to mimic a 60Hz sine wave (standard in North America).
• Split-Phase Output:
  • The H-bridge is configured to produce two 120V AC outputs that are 180° out of phase with each other.
  • These two "hot" legs (L1 and L2) are referenced to a neutral (N), providing 120V from L1-N or L2-N, and 240V from L1-L2.

4. Output Stage: Filtering and Stabilization

• LC Filter:
  • The raw PWM AC signal contains high-frequency harmonics, so an inductor-capacitor (LC) low-pass filter smooths it into a clean sine wave.
  • This removes switching noise and ensures the output meets power quality standards.
• Transformer (Optional):
  • In some designs (especially older or lower-cost units), a low-frequency output transformer steps up or stabilizes the voltage and provides galvanic isolation between the DC input and AC output.
  • Modern high-efficiency inverters often skip this, relying on transformerless designs for lighter weight and better efficiency.

5. Control and Monitoring

• Microcontroller/DSP:
  • A central processor monitors input voltage, output voltage, current, temperature, and load conditions.
  • It adjusts PWM signals in real-time to maintain stable output (e.g., 120/240V ±5%, 60Hz ±0.5Hz).
• Feedback Loop:
  • Sensors measure the output waveform and feed data back to the controller to correct distortions or voltage drops under load.
• Cooling: Fans or heat sinks dissipate heat generated by switching losses and resistive elements.

6. Split-Phase Specifics

• Wiring:
  • The inverter outputs three lines: L1 (hot), L2 (hot), and N (neutral).
  • L1 and L2 are 180° out of phase, providing 240V across them, while either leg to neutral provides 120V.
• Load Balancing:
  • The inverter can handle unbalanced loads (e.g., 120V appliances on L1-N and nothing on L2-N), though efficiency may drop slightly if heavily unbalanced.
• Power Capacity: At W continuous output, the inverter can deliver up to 25A per 120V leg (W ÷ 240V = 25A across L1-L2), assuming perfect efficiency.

7. Efficiency and Losses

• Efficiency: Typically 90-95% for modern inverters. Losses occur due to:
  • Switching losses in transistors.
  • Resistive losses in inductors, capacitors, and wiring.
  • Transformer losses (if present).
• Peak Power: Many inverters can handle short surges (e.g., 12,000W for motor startups) by drawing extra current from the battery (up to 250A at 48V for 12kW).

Simplified Internal Workflow

1. 48V DC enters → Boosted to ~340V DC → H-bridge converts to raw AC → PWM refines to sine wave → LC filter smooths output → Split-phase 120/240V AC delivered.

Example Components

• MOSFETs/IGBTs: Handle high-current switching (e.g., 100A+ at 48V input).
• Capacitors: High-voltage electrolytic or film caps (e.g., 400V rated).
• Inductors: Toroidal or ferrite-core for filtering and boosting.
• Microcontroller: STM32, Texas Instruments DSP, or similar for PWM control.