Bad Solar Inverters

Why Some PV Installations Become an EMC Nightmare for Radio Amateurs

Solar power is a great technology. Unfortunately, not every solar installation is friendly to radio reception. For radio amateurs, shortwave listeners, and anyone trying to receive weak HF signals, a badly designed or badly installed PV system can become a wideband noise generator.

One common statement is that “transformerless solar inverters are bad.” That is understandable, but technically too absolute. The better statement is this:

Where the Noise Starts

A solar panel produces DC. The inverter converts that DC into grid-synchronized AC. This is not done by a smooth analog sine-wave generator. It is done with fast-switching power electronics, usually MOSFETs or IGBTs, switching thousands or tens of thousands of times per second.

Those switching pulses have steep edges. Steep edges contain harmonics. Those harmonics do not politely stop at the nominal switching frequency. They can extend into the HF spectrum and, in some cases, even into lower VHF.

For radio amateurs, that matters because HF reception often depends on hearing signals only slightly above the atmospheric noise floor. A PV system that injects common-mode noise onto long cables can raise the noise floor across multiple amateur bands.

The Real Problem Is Common-Mode Current

The most damaging interference mechanism is often not simple differential-mode noise between the positive and negative PV conductors. The real troublemaker is common-mode current.

In a transformer-isolated inverter, a transformer sits between the PV side and the AC grid side. The solar array can float more independently from the mains system. There is still some parasitic coupling, for example through capacitance between transformer windings, but the DC array is not directly part of the same high-frequency electrical structure as the grid side.

In a transformerless inverter, there is no full galvanic isolation between the PV side and the AC side. That does not mean that the positive and negative PV rails are simply hard-connected to live and neutral. That would be an overstatement. But it does mean that the PV array, inverter electronics, EMC filters, grounding system, and AC wiring can become part of one high-frequency common-mode system.

That is where things go wrong for radio. The DC string cables to the roof are often long. They may run in large loops. They may be close to metal frames, gutters, earth bonding conductors, coax cables, or antenna support structures. Once common-mode current flows on those cables, they stop behaving like harmless wiring and start behaving like unintended antennas.

Why Transformer Isolation Helps

A transformer provides galvanic isolation. It breaks the direct electrical path between the PV array and the AC grid side. That makes it much harder for common-mode current to travel from one side to the other.

This does not remove all coupling. A transformer still has parasitic capacitance between its windings. At high frequencies, even small capacitances can pass some noise. But the coupling is usually much smaller and easier to manage than in a transformerless system.

This is why the basic intuition is correct: for a radio amateur, a galvanically isolated inverter is often the safer choice, especially when antennas are installed on the same building, near the PV array, or near the inverter cabling.

But it is not magic. A poorly designed inverter with a transformer can still create interference. A well-designed transformerless inverter with excellent common-mode suppression, careful layout, and proper filtering can be acceptably quiet in practice.

Why Cheaper Transformerless Inverters Are Often Suspect

Transformerless inverters are popular because they are lighter, smaller, cheaper, and often more efficient. That is not automatically bad engineering. The problem starts when cost reduction also happens in the EMC design.

Typical weak points include undersized common-mode chokes, poor Y-capacitor strategy, compromised PCB layout, insufficient shielding, low-cost filters, poor bonding practice, or inadequate testing in real-world installations.

A CE mark or an EMC statement on paper does not automatically mean that a radio amateur will experience a clean HF spectrum. EMC compliance depends on the product, test conditions, installation, cable routing, grounding, and real-world enforcement. A device can look acceptable in a standardized test setup and still be troublesome in a sensitive radio environment.

Is a Transformerless Inverter Automatically Bad?

No. That would not be a fair technical conclusion without measurements.

A more accurate statement is:

A transformerless inverter is an EMC attention point for radio amateurs. It can be more sensitive to common-mode leakage currents and radiated HF interference through the DC strings, PV frames, grounding system, and AC wiring. Whether a specific inverter is problematic depends on its internal EMC design, switching topology, firmware, PWM strategy, installation layout, cable lengths, grounding, filtering, and distance to antennas.

So instead of saying “this inverter is bad,” it is better to say:

What Radio Amateurs Should Look For

The best solar inverter for a radio amateur is not simply “the most expensive one.” It is the inverter with the best EMC behavior in a real installation.

Still, some features and design choices are strong positive indicators:

• Galvanic isolation: An inverter with a low-frequency or high-frequency isolation transformer is usually easier to control from a common-mode point of view.
• Real EMC documentation: Ask for proper EMC test reports, not only a CE logo or marketing claim.
• Low conducted and radiated emissions: Pay attention to standards and test results for conducted and radiated emissions.
• Minimal rooftop electronics: Micro-inverters and optimizers can be useful for shaded roofs, but they also place switching converters directly under or behind the panels.
• Good installation practice: Even a good inverter can become noisy when the DC strings are routed badly.

Micro-Inverters and Optimizers Need Extra Attention

Micro-inverters and power optimizers are not automatically bad, but they can complicate the EMC picture. Instead of one central switching device, the installation may contain many switching converters distributed across the roof.

For a normal homeowner, that may be invisible. For a radio amateur with antennas nearby, it can mean multiple noise sources connected to a large rooftop wiring structure.

If shade management is not required, a simpler system with fewer switching devices can be a better EMC starting point.

Installation Quality Matters as Much as the Inverter

Even a good inverter can become noisy when installed badly. The positive and negative DC conductors of each string should run close together. They should not form large loops. Large loops are antennas.

DC cables should be kept as short and compact as practical. They should not run unnecessarily parallel to coax cables, antenna feedlines, receiving loops, shack wiring, or sensitive control cables.

Good bonding and grounding practice is essential, but grounding alone does not magically remove RF noise. In some installations, the grounding system itself becomes part of the common-mode current path. That is why cable routing, filtering, bonding, and physical layout must be treated as one EMC system.

Ferrites and Filters Can Help, But They Are Not Magic

Common-mode chokes and ferrites can help reduce RF current on cables, but they must be selected for the correct frequency range, current level, voltage environment, and installation location.

Filters on the DC side of a PV system must be rated for the high DC voltage and the installation category involved. This is not the place for random hobby experimentation. Many PV strings operate at hundreds of volts DC, and DC arcs are much less forgiving than AC arcs.

Never simply add capacitors across the PV input or from PV conductors to earth without understanding the inverter design, insulation monitoring, leakage-current detection, MPPT behavior, and safety implications.

The Practical RF.Guru View

For radio amateurs, the question is not only whether the inverter works electrically or whether the installation produces energy. The real question is whether the complete PV system remains quiet enough for weak-signal reception.

A transformerless inverter should therefore be treated as an EMC risk factor, not as an automatic failure. The correct approach is to look at the complete system:

• the inverter topology;
• the internal EMC filtering;
• the DC cable routing;
• the grounding and bonding system;
• the use of optimizers or micro-inverters;
• the distance to antennas;
• the actual measured noise floor before and after installation.

For a radio amateur, the safest choice is usually a well-documented inverter with galvanic isolation, careful cable routing, proper EMC filtering, and an installer who understands that long cables carrying common-mode current can become antennas.

Conclusion

The basic concern is valid: transformerless PV inverters can be riskier for radio amateurs because they may allow stronger common-mode coupling between the PV array, grounding system, AC wiring, and surrounding structures.

But “transformerless equals bad” is too simplistic. A bad solar inverter is an inverter with poor common-mode suppression, weak EMC filtering, noisy switching behavior, or an installation that turns the PV wiring into an unintended transmitting antenna.

A galvanically isolated inverter is usually the safer EMC choice for radio amateurs, especially when antennas are located on or near the same building. But the final result still depends on the total installation.

The best advice is simple: do not judge only by price, efficiency, or brand. Ask for real EMC information, avoid unnecessary rooftop switching electronics where possible, keep DC wiring compact, and measure the HF noise floor before and after installation.

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