There are three architectures for converting module energy. The string inverter connects modules in series to one MPPT tracker per input. The microinverter converts to AC at each module and has its own MPPT. The optimizer tracks per module with a DC-DC converter and delivers regulated voltage to a string inverter. They change the rooftop DC voltage, rapid shutdown, shading behavior and monitoring granularity.
Three ways to convert module energy
A photovoltaic module delivers direct current and the grid needs alternating current. What changes between architectures is where the conversion takes place and where maximum power point tracking (MPPT) takes place, and that determines the wiring voltage, the behavior under shading and safety on the roof.
PV conversion equipment is tested against the safety standard IEC 62109-1 (general requirements) and IEC 62109-2 (particular requirements for inverters). Local codes take these standards, along with IEC 62116 and UL 1741, as the test reference for inverters in renewable energy systems. The set is covered in IEC standards for photovoltaic systems.
String inverter
Modules are connected in series forming strings, and each string reaches an MPPT input of the inverter. The inverter converts to AC in a single unit, located somewhere accessible (wall, technical room or on the array structure).
- DC voltage: that of the whole string, which can reach 1000 V or 1500 V; the DC wiring runs across the roof or the field to the inverter, sometimes after grouping the strings in a string combiner box.
- Tracking: one MPPT per input. All modules in a string share current; a shaded module or one with a different orientation affects the others in its string.
- Where it is used: from residential and commercial systems to utility-scale plants; for the latter, the comparison with the central inverter is covered in the guide on central or string inverter in solar plants. If the system adds batteries, the coupling is explained in hybrid solar system with batteries.
Microinverter
Each module, or each group of two to four, has its own inverter attached to the frame, and the outputs of all of them join in an AC circuit.
- DC voltage: only that of the module. In microinverter datasheets, the maximum input voltage is on the order of tens of volts; for example, the datasheet of a 60-cell Enphase IQ8 states 50 V.
- Tracking: one MPPT per module, so shading and orientation differences are not transmitted between modules.
- Wiring: the rooftop run is AC, with the manufacturer’s own connectors and trunk cables.
- Monitoring: produces data per module.
- Where it is used: mainly in small residential and commercial installations. Brands such as Enphase, Hoymiles and APsystems offer this architecture.
Power optimizer
It is a DC-DC converter connected to each module, which tracks the maximum power point per module and delivers a regulated voltage to the string, which ends at a compatible string inverter.
- DC voltage: the string keeps its voltage while the optimizers are active. They usually drop their output to a reduced value when their communication with the inverter is interrupted; the exact behavior and value are declared by the manufacturer in its datasheet.
- Tracking: per module in the optimizer and of the string voltage in the inverter.
- Compatibility: the optimizer and the inverter must be from the same family, because the inverter communicates with them.
- Where it is used: residential and commercial installations. SolarEdge is a manufacturer of this architecture, and others, such as Huawei, offer optional optimizers with their string inverters.
What changes between them
| Aspect | String inverter | Microinverter | Optimizer + string inverter |
|---|---|---|---|
| Conversion to AC | One unit for the whole string | One per module | One inverter for the string |
| MPPT | One per input | One per module | One per module plus the inverter’s |
| DC voltage on the roof | That of the string, up to 1000 V or 1500 V | That of the module | Regulated by the optimizers |
| Effect of a shaded module | Affects its string | Isolated | Limited |
| Electronics on the roof | None | One unit per module | One converter per module |
| Monitoring | Per inverter input | Per module | Per module |
| Replacing a unit | One accessible inverter | Access to the module | Access to the module or the inverter |
More parts on the roof mean more points to check and more equipment exposed to the weather; fewer parts mean lower data granularity. There is no single answer: it depends on the roof, the shading, the size of the system, ease of maintenance and the rapid shutdown requirement of the applicable code.
Criteria for deciding
The architecture is decided in the project with a short list of questions. How many orientations and tilts does the roof have? What fixed or moving shadows are there? How many modules are there? Who will access the equipment over twenty years, and how? What level of monitoring detail is needed? What does the applicable safety code require? Each answer tips the balance differently, which is why the comparison is made with the simulation of the specific roof and the datasheets of the candidate equipment, not with general rules.
Rapid shutdown and safety
The DC voltage on the roof is what concerns firefighters most. The architecture with per-module electronics (microinverter or optimizer) reduces that voltage without the inverter having to cut each circuit. The specific requirement depends on the standard applicable to the project. The guide on rapid shutdown in PV systems covers that requirement.
Interconnection protection
The conversion architecture does not change the interconnection requirements. Local codes ask every inverter for overvoltage, overcurrent and overtemperature protection, anti-islanding protection that disconnects after the grid voltage is lost, and a response to grid recovery. If the system grows, the protection requirements of the connection depend on the capacity and on the utility’s requirements; see also anti-islanding protection.
What we do at Tiza Group
Choosing the conversion architecture of a residential or commercial system is up to its designer. When the project needs interconnection design or commissioning of the electrical part, our primary engineering team works remotely, worldwide, with your project’s information. We also commission solar plants in Colombia, or elsewhere on request, with the interconnection protection required by the utility.
Related guides
- Rapid shutdown in PV systems
- Hybrid solar system with batteries: AC or DC coupling
- String combiner box: parts and functions
- Central or string inverter in solar plants: architecture
- DC-DC converters and inverters
- Anti-islanding protection
References
- IEC 62109-1 and IEC 62109-2, Safety of power converters for use in photovoltaic power systems.
- Enphase Energy, IQ8 microinverter datasheet (2021 version).
Frequently asked questions
Does a microinverter need additional rapid shutdown?
DC voltage is limited to that of the module itself and the rest of the rooftop circuit is AC. Manufacturers state in their datasheet whether the product is listed as rapid shutdown equipment under the applicable standards. Verify that declaration and your project's requirement.
Which architecture suits a shaded roof?
With partial shading or different orientations, module-level tracking (microinverter or optimizer) limits the effect of a shaded module on the others in its string. The real result depends on the roof geometry and the shading, and is assessed with the project simulation; no single architecture suits every case.
Do the interconnection protections change with the architecture?
Interconnection requirements depend on the installed capacity and the voltage level of the point of connection, not on whether modules are converted with a string inverter or with microinverters. Local codes require any inverter to have anti-islanding protection and a response to grid recovery.