A solar inverter turns the direct current (DC) from panels into the alternating current (AC) used by homes and the grid by switching it on and off at very high speed through semiconductor switches, then smoothing the result into a sine wave. It works in stages: a tracking stage draws the most power from the panels, a DC bus holds that energy briefly, a switching bridge reverses it back and forth, and a filter shapes it into clean 230 volt, 50 hertz power in step with the grid. Around those stages sits a layer of measurement and protection that decides whether the inverter is allowed to run at all.

The overview in how solar power works introduces pulse-width modulation and maximum power point tracking. This article follows the energy through the inverter in more detail.

Stage by stage through a string inverter

1. DC input and tracking. Each string connects to an MPPT input. A DC-to-DC converter, often a boost converter, adjusts the voltage the panels see so they operate at their best point, while delivering a steadier, higher voltage to the next stage.
2. The DC bus. A bank of capacitors holds energy at a stable voltage. It acts as a small reservoir between the variable solar input and the output, and it absorbs the ripple that single-phase output creates 100 times a second.
3. The switching bridge. In a single-phase inverter, four transistor switches, typically IGBTs or MOSFETs, are arranged in an H-bridge. Closing one diagonal pair sends current one way through the output; closing the other pair reverses it. Three-phase inverters use six or more switches to create three outputs spaced a third of a cycle apart.
4. Pulse-width modulation. The switches do not simply flip 50 times a second. They switch thousands of times a second, varying the width of each pulse so the average voltage traces a sine wave: narrow pulses near the zero crossing, wide pulses near the peaks.
5. Output filter. Inductors and capacitors smooth the pulse train, removing the high-frequency switching component and leaving a clean sine wave with low harmonic distortion.
6. Relays and connection. Only after the controller confirms that grid conditions and internal checks are acceptable do the output relays close and connect the inverter to the switchboard.

Staying in step with the grid

A grid-connected solar inverter normally behaves as a controlled current source that follows the grid, rather than setting its own voltage. A control loop called a phase-locked loop continuously measures the grid's voltage waveform and locks onto its frequency and phase. The inverter then pushes current in step with that waveform, and the size of the current sets how much power flows out.

By shifting the timing of its current slightly relative to the voltage, the inverter can also supply or absorb reactive power. Australian grid-connection requirements use this ability in response modes such as volt-var, which help steady local voltage when many homes export at once.

Hybrid inverters add a bidirectional battery stage, and in backup mode some can form their own voltage and frequency to supply the home while the grid is down. That is a different operating mode, described in how a hybrid inverter works.

Efficiency: where the losses go

Quality string inverters commonly reach peak efficiencies of around 97-98%, so only a few per cent of the energy is lost, almost all of it as heat. The losses come from two main sources:

• Conduction losses from current flowing through switches, inductors and internal wiring. They grow as output rises.
• Switching losses from the brief moments each transistor spends between fully on and fully off. They grow with switching frequency.

Efficiency is not constant. It is lower at very light loads early and late in the day, when the inverter's own control electronics use a larger share of the power, and it can dip when the unit is hot. Spec sheets therefore often show a weighted efficiency alongside the peak figure. The weighted figure averages performance across a range of typical output levels and is a more realistic basis for comparison.

Heat also sets a ceiling on output. If internal temperatures climb too high, the controller reduces power, known as derating, to protect components. Clear airflow around the heat sink and a shaded mounting position help an inverter deliver its full rating on hot days.

Transformer and transformerless designs

Older inverters often included a transformer that electrically isolated the panels from the grid. Most modern string and hybrid inverters are transformerless. They are lighter and more efficient, but without that isolation they rely on extra monitoring to detect insulation faults and leakage current, which is where several of the protection functions below come in.

Protection functions and what each one does

Each morning, before exporting any power, a typical grid-connected inverter runs through a series of checks, and it keeps monitoring for as long as it operates.

After a disconnection, an inverter does not jump straight back to full power. It waits for the grid to remain stable for a set period, then ramps output up gradually. This stops large numbers of systems surging back at the same instant after a network disturbance.

Error codes on an inverter screen or app usually relate to one of these protections. Tracing the underlying cause involves live DC and AC circuits and must be left to a licensed electrician or accredited installer.

Next steps

If an inverter shows recurring error codes or seems to underperform, the Inverter Inspection & Testing service ($199 per inverter) checks fault logs, firmware, ventilation, string voltages and currents, and grid settings. Where an older unit has reached the end of its life, Inverter Replacement (from $1,990 for a 5 kW inverter, supplied and installed) covers like-for-like units or an upgrade to a battery-ready hybrid from GoodWe, Fox ESS, Sungrow or Sigenergy. Both are on the energy market; prices are indicative and confirmed after a site assessment. To discuss the right inverter for your system, request a free assessment from Blue Energy Solar.

Frequently asked questions

Why does my inverter hum or run a fan?

Inductors and other magnetic components vibrate slightly as current switches through them, which can produce a faint hum that grows louder at high output. Some models also use fans to move heat away from the electronics, so fan noise on hot, sunny afternoons is normal. A sudden change in sound, such as rattling, clicking or grinding, is worth having checked by a licensed professional.

What is the difference between a pure sine wave and a modified sine wave inverter?

A pure sine wave inverter produces a smooth waveform that closely matches grid power, and all grid-connected solar inverters must meet strict limits on waveform distortion. Modified sine wave inverters are cheaper stand-alone units, sometimes used for camping, that produce a stepped waveform. That waveform can make motors run hot and some electronics buzz, and it cannot meet grid-connection requirements.

How long do solar inverters typically last?

As a common rule of thumb, string inverters often last somewhere around 10 to 15 years, although results vary with the model, installation quality and operating conditions. Heat is a major factor in electronic ageing, so a shaded, well-ventilated location helps. Warranty periods differ between manufacturers and models, so the terms on a specific quote are the ones to check.