How to choose the right solar inverter for each installation

Choosing a photovoltaic inverter may seem straightforward: check the power rating of the solar panels, find an inverter with a similar capacity, and that is it.

In practice, several other factors must also be checked.

The inverter must be compatible with the power rating of the installation, as well as with the voltage and current of the modules, the number of panels connected in each string, the different roof orientations and, where applicable, the battery.

You do not need to know every model on the market to make an initial selection. The key is to answer a few basic questions correctly.

What does a photovoltaic inverter actually do?

Solar panels generate direct current (DC) electricity. However, homes, businesses, factories and the electricity grid mainly operate using alternating current (AC).

The inverter performs this conversion.

However, modern inverters do much more. Depending on the model, they can also:

• Control the output of the solar panels.
• Manage a battery.
• Limit the amount of energy exported to the grid.
• Keep specific loads powered during a power outage.
• Detect faults in the installation.
• Send production data to a monitoring platform.
• Coordinate several inverters within the same solar plant.

The inverter can therefore be considered the control centre of the photovoltaic installation.

First question: what type of installation is being planned?

The type of project quickly helps narrow down the available options.

A 5 kW home installation does not require the same inverter as a 500 kW industrial facility or a multi-megawatt photovoltaic plant.

Residential installation

Lower-power inverters are normally used in residential installations. They may be single-phase or three-phase, depending on the property’s electrical supply.

When a battery is to be installed, a hybrid inverter is normally considered.

Commercial premises or small business

Higher-power three-phase inverters may be used in shops, offices, agricultural facilities and small industrial buildings.

If the aim is to consume solar energy directly during working hours, a grid-connected inverter may be sufficient. When energy storage is also required, a hybrid solution should be considered.

Industrial installation

Industrial rooftops are often divided into different areas or module fields. For this reason, three-phase string inverters with several MPPTs are commonly used.

This type of architecture allows the installation to be distributed across several units and different sections of the roof to be controlled independently.

Large-scale photovoltaic plant

Utility-scale projects use high-power inverters designed to operate with large photovoltaic arrays, medium-voltage stations and advanced plant control systems.

Sungrow currently divides its solutions into residential, commercial and industrial applications, and large-scale projects.

Second question: will the installation include a battery?

This is one of the most important decisions.

Installation without a battery

If the installation will only generate energy for direct consumption or grid export, a grid-connected inverter can be used.

During the day, the inverter converts the energy generated by the panels and supplies it to the building’s loads. Any energy that is not consumed may be exported to the grid, provided that the installation and the electricity contract allow it.

Installation with a battery

If surplus energy is to be stored for later use, an energy storage system must be incorporated.

In residential installations and small commercial projects, a hybrid inverter is commonly used. This unit simultaneously manages the panels, the battery, the building’s consumption and the grid connection.

One example is the Sungrow SH3.0/3.6/4.0/5.0/6.0RS range, which consists of single-phase hybrid inverters from 3 to 6 kW. The series has two MPPTs, two string inputs, a maximum PV input current of 16 A per MPPT and 32 A in total, as well as monitoring through iSolarCloud.

For higher-power three-phase installations, the Sungrow SH15/20/25T range may be considered. This series offers power ratings of 15, 20 and 25 kW, three MPPTs, five string inputs and compatibility with Sungrow SBR and SBH batteries.

In any case, a battery should not be assumed to be compatible with an inverter simply because both operate at high voltage. The manufacturer must expressly confirm compatibility between the two units.

Third question: is the installation single-phase or three-phase?

The answer depends on the building’s type of electrical connection.

Single-phase connection

This is common in homes and small electrical supplies. In these cases, a single-phase inverter compatible with the installation’s power rating must be selected.

Three-phase connection

This is common in businesses, industrial facilities, agricultural operations and homes with high electricity consumption.

In a three-phase grid, energy is distributed across three phases. The inverter must be designed to operate with this type of connection.

Before selecting the unit, it is advisable to check:

• The type of electrical supply in the building.
• The contracted power.
• The maximum power permitted at the point of connection.
• Whether there are single-phase or three-phase loads.
• Whether loads need to remain powered during a grid outage.

A higher inverter power rating does not mean that the unit can be installed with any electrical supply. The grid type must be compatible.

Fourth question: is the roof straightforward or does it have several orientations?

This is where one of the concepts that causes the most uncertainty comes into play: MPPTs.

What is an MPPT in simple terms?

An MPPT is an inverter system that identifies the operating point at which the solar panels can generate the greatest possible amount of energy.

MPPTs can be thought of as independent lanes.

If all panels face south, have the same tilt and receive practically the same amount of sunlight, they can operate within the same group.

However, if some panels face east and others face west, they do not receive the same solar irradiance at the same time. In this case, they should be connected to different MPPTs.

MPPTs are especially useful when there are:

• Several orientations.
• Different tilt angles.
• Areas with different levels of shading that can be connected to independent MPPTs.
• Separate areas of the building.
• Different numbers of modules in each array section.
• Extensions completed in different phases.

Having several MPPTs is useful when areas with different orientations, tilt angles or shading conditions can be connected to independent trackers. However, this alone does not eliminate losses caused by shading within the same string.

MPPTs and string inputs are not the same

These two concepts are often confused.

For example, an inverter may have three MPPTs and five string inputs. This means that it has five physical string connections, but some of them share the same MPPT.

When several strings are connected to the same MPPT, the manufacturer’s specified conditions must be met. In general, the strings should use the same module model and the same number of modules, with equivalent orientation, tilt and solar exposure.

What is a string?

A string is a group of solar panels connected in series.

The current flows from one module to the next, while their voltages are added together.

For example, if 20 panels are connected in series, the inverter does not receive the voltage of a single panel, but the combined voltage of all 20 panels.

For this reason, the number of modules per string cannot be estimated. It must be calculated.

Why must the voltage be checked?

Every inverter has a maximum permitted voltage.

The voltage of solar panels changes with temperature. In cold conditions, the open-circuit voltage increases.

This means that a string that appears suitable when only the panel’s standard test data is considered could exceed the inverter’s maximum voltage on a particularly cold day.

To prevent this, the following must be checked:

• The module’s open-circuit voltage.
• The number of modules connected in series.
• The temperature coefficient.
• The minimum expected temperature at the site.
• The inverter’s maximum permitted voltage.
• The MPPT operating voltage range.

The calculated maximum string voltage must never exceed the limit specified by the inverter manufacturer.

Why must the current also be checked?

Voltage is not the only electrical limit.

Modern modules can operate at high currents. It must therefore be confirmed that the inverter can handle both the module current and the resulting current when several strings are connected in parallel.

An inverter datasheet normally distinguishes between:

• Maximum current per string.
• Maximum current per MPPT.
• Maximum short-circuit current.
• Number of available inputs.

A high-power solar panel is not automatically compatible with every inverter. Both voltage and current must be checked.

Must the power rating of the panels match that of the inverter?

Not necessarily.

A photovoltaic installation may have a total installed module capacity that is greater than the inverter’s nominal power. This is known as the DC/AC ratio.

For example:

• Installed solar panels: 120 kWp.
• Inverter power: 100 kW.
• DC/AC ratio: 1.20.

This oversizing can allow the inverter to operate closer to its nominal power for more hours of the day.

However, the module capacity must not be increased without carrying out the necessary calculations. Excessive oversizing may cause greater clipping losses and exceed the unit’s input limits.

The manufacturer specifies the electrical limits and the maximum permitted or recommended PV capacity for each model. Within these limits, the appropriate DC/AC ratio must be calculated for each project.

What does clipping mean?

Clipping occurs when the solar panels could generate more power than the inverter is able to convert.

Consider an installation with 120 kWp of modules and an inverter limited to 100 kW.

At certain times, the panels may attempt to deliver more than 100 kW. The inverter will limit its output to its maximum power, and the remaining energy will not be converted.

A certain amount of clipping may form part of the system design, but it must be calculated to confirm that the DC/AC ratio is suitable and that the resulting losses are acceptable.

In a conventional grid-connected installation, energy exceeding the inverter’s instantaneous conversion capacity is not used. In hybrid systems or systems with DC-coupled storage, the behaviour may vary depending on the architecture and the available charging capacity.

When is a higher-power inverter required?

As the size of the project increases, the inverter requirements also change.

Commercial and industrial projects

An industrial building may have different orientations, shaded areas, rooflights or separate roof sections. In these cases, having several MPPTs is often beneficial.

The Sungrow SG125CX-P2 is a 125 kW three-phase string inverter. According to the official documentation, it has 12 MPPTs, two string inputs per MPPT and a maximum input current of 30 A per MPPT. It also has an IP66 protection rating, a C5 anti-corrosion classification and can be integrated into iSolarCloud using the corresponding communication solution.

The 12 MPPTs allow a rooftop to be divided into different array sections. Nevertheless, the number of strings, the number of modules per string and their distribution across the MPPTs must be calculated for each installation.

Large-scale photovoltaic plants

Utility-scale plants use equipment designed to operate at much higher voltages and power levels.

The Sungrow SG350HX-20 is a string inverter for 1500 Vdc systems. The official documentation specifies up to six MPPTs, 75 A per MPPT, a maximum of 30 inputs and a declared maximum efficiency of 99.02%.

This type of inverter is intended for large-scale plants and requires a comprehensive assessment of the photovoltaic array, power blocks, electrical evacuation system and medium-voltage connection.

What is the difference between a grid-connected inverter and a hybrid inverter?

The main difference is that a hybrid inverter can integrate and manage a compatible battery, while also controlling energy flows between the modules, loads, battery and grid.

Grid-connected inverter

It converts the energy generated by the solar panels and supplies it to the loads or the grid.

If a power outage occurs, it normally stops operating for safety reasons, even when the panels are generating electricity.

Hybrid inverter

In addition to converting solar energy, it can manage a compatible battery.

Depending on the unit and its configuration, it may also power specific loads when the grid fails.

However, having a hybrid inverter and a battery does not automatically mean that the entire building will have backup power.

The following must be checked:

• Available power in backup mode.
• Battery capacity.
• Power demand of the loads.
• Motor starting surges.
• Distribution board configuration.
• Required accessories.
• Operation with unbalanced loads.

The backup circuit must be designed according to the customer’s actual requirements.

What environmental conditions must the inverter withstand?

The installation location also matters.

An inverter installed in a garage does not operate under the same conditions as one installed in an agricultural building, a coastal area or a plant exposed to dust and high temperatures.

The following should be reviewed:

• IP protection rating.
• Corrosion protection.
• Operating temperature.
• Maximum altitude.
• Cooling system.
• Required ventilation clearance.
• Direct exposure to sunlight.
• Accessibility for maintenance.

IP ratings and anti-corrosion classifications must be checked in the datasheet of the specific model. They should not be generalised across an entire product range.

Monitoring should not be overlooked

Monitoring makes it possible to see how much energy the installation is generating and to detect potential problems.

Depending on the project, the following may be required:

• Communication module.
• Energy meter.
• Current transformers.
• Data logger.
• RS485 communication.
• Ethernet or Wi-Fi connection.
• Plant controller.
• Zero-export system.
• Integration with an EMS.

Sungrow uses the iSolarCloud platform to monitor different residential and commercial product ranges. The required accessories depend on the inverter and the installation configuration.

Summary: which inverter solution should be considered in each case?

Type of installation	Solution to consider	Example
Single-phase home with battery storage	Single-phase hybrid inverter	Sungrow SH3.0–6.0RS
Three-phase home or small business with battery storage	Three-phase hybrid inverter	Sungrow SH15/20/25T
Commercial or industrial rooftop	Three-phase string inverter	Sungrow SG125CX-P2
Large-scale photovoltaic plant	Utility-scale string inverter	Sungrow SG350HX-20

This table is for guidance only. The final model should be selected after reviewing the modules, strings, electrical grid, and storage and monitoring requirements.

Common mistakes when choosing an inverter

When selecting an inverter, comparing its nominal power alone is not enough. These are some of the most common mistakes:

• Considering only the power rating: Two 100 kW inverters may have different numbers of MPPTs, different input current limits and different communication systems.
• Not calculating the number of modules per string: The number of modules connected in series must be calculated using the temperatures expected at the site.
• Confusing MPPTs with inputs: Having more physical inputs does not necessarily mean having more independent MPPTs.
• Failing to check the panel current: This is becoming increasingly important as the current ratings of photovoltaic modules continue to increase.
• Assuming battery compatibility: The battery must be expressly approved by the inverter manufacturer.
• Forgetting the accessories: An energy meter, current transformers or a data logger may be required to use functions such as zero export or centralised monitoring.
• Assuming that a battery provides full backup: Backup capacity depends on the inverter, battery, electrical distribution board and connected loads.

What information does the distributor need to recommend an inverter?

To make an initial selection, it is advisable to provide:

1. Module model and power rating.
2. Total number of panels.
3. Orientation and tilt.
4. Number of modules per string.
5. Total DC capacity.
6. Single-phase or three-phase grid type.
7. Required AC power.
8. Whether a battery will be installed.
9. Whether backup power is required.
10. Whether export limitation is required.
11. Project location.
12. Whether the installation is residential, commercial, industrial or utility-scale.

The more information that is provided, the lower the risk of selecting incompatible equipment or overlooking necessary accessories.

Advice on selecting Sungrow inverters

There is no single inverter that is suitable for every installation. The correct choice depends on the size of the project, the module configuration, the electrical grid and the functions required by the customer.