What An Inverter Has To Do To Make Grid-Shaped Power

Panels and batteries produce direct current at a voltage that drifts with conditions. Everything in a house expects alternating current at a fixed voltage and frequency, and the inverter's job is that conversion.

Switching fast is how a smooth wave is built

An inverter cannot generate a curve directly. It switches its output between fixed levels thousands of times per second, varying how long it spends at each level.

Averaged over time, the sequence of pulses traces the shape of a sine wave, and a filter of inductors and capacitors smooths the steps into a continuous curve.

The switching frequency has to be far above the output frequency for this to work, which is why inverters contain fast semiconductor switches and substantial heat sinking.

Cheap designs approximate the shape

A modified sine wave inverter switches between a few levels and produces a stepped waveform that is close enough for resistive loads such as heaters.

Motors, transformers and some electronics respond badly to those steps, running hotter or producing audible noise because the current they draw follows the shape they are given.

Anything intended to connect to the grid must produce a genuine sine wave, since a stepped output would inject harmonics into everyone else's supply.

Grid connection means following, not leading

An inverter feeding the grid cannot set the voltage or frequency. It measures the utility waveform continuously and pushes current in step with it.

Being slightly out of phase means delivering reactive power rather than useful energy, so the control loop tracks the grid's zero crossings and adjusts constantly.

To export at all, the inverter must present a voltage slightly above the grid's at the point of connection. Current flows toward the lower potential, which is the whole mechanism behind sending power back.

The panels have their own moving target

A solar array's most productive operating voltage shifts with sunlight and panel temperature, and drawing at the wrong voltage costs real output.

Inverters therefore run a tracking routine that continually adjusts the load they place on the array, testing slightly higher and lower and moving toward whichever produces more power.

Shade complicates this, because a partly shaded array can have several local peaks and a simple tracker may settle on the wrong one.

Efficiency is a curve, not a number

Conversion losses come from switching, from resistance in the semiconductors, and from the magnetic components, and these scale differently with load.

The result is a curve that peaks somewhere below full output and falls off at very low power, where fixed overheads dominate whatever is being converted.

Sizing an inverter far larger than the array it serves therefore costs output, because the system spends most of the year operating in the inefficient bottom of that curve.