The rise of string inverters

Are string inverters becoming the top choice for solar projects? It’s too early to tell, but more PV plant developers seem to be exploring this option.

A recent RatedPower report shows that string inverter simulations surged from 14% in Q3 2018 to nearly 60% by Q4 2023 — or 46% in just five years. This increase is most pronounced in Germany, Portugal, Italy, and China, where over 80% of projects were simulated with string inverters. Many other countries, including Poland, Greece, Indonesia, and Colombia, also favor this technology.

Adapted from “Preferred Inverter Type per Geography, Selected Regions” in RatedPower’s 2024 Trends Report: Renewable Energy and Solar Research, sorted by the percentage of string inverter simulations, from highest to lowest.

While string inverters are not the universal solution for every type of solar plant, there’s no denying their growing popularity. What’s driving this shift? Let’s explore their competitive edge.

What is an inverter?

Solar inverters convert direct current (DC) from solar panels into usable alternating current (AC) electricity. They also help stabilize voltage to deliver a consistent power supply. This prevents damage to electrical equipment and property.

How do string inverters compare to central inverters?

The main distinction between central and string inverters is their placement and configuration. A string inverter is usually located at the end of each PV string, distributed across the array, and handles fewer strings than a central inverter.

Arranged in a series similar to solar panels, string inverters historically have smaller capacities than central inverters; however, their increased capacity could be one of the drivers of their increasing popularity. This modular setup and distributed architecture can give solar developers more freedom to configure their systems to suit specific needs and optimize energy production based on site conditions.

On the other hand, a central inverter collects power from several PV strings and is positioned centrally among them. They have higher power capacities and combine power from multiple long strings of solar panels into a single location. This centralized approach simplifies design and maintenance but can limit flexibility.

Each inverter type comes with its own considerations regarding cost, efficiency, and maintenance.

When and why did string inverter technology gain ground?

Germany’s 1000 Roof Program in 1991 shifted the solar industry towards modularity to standardize components and simplify maintenance. During this time, microinverters were introduced to optimize power output at the individual solar module level. But there was a problem: their limited power capacity and high maintenance costs made them impractical for larger setups.

In response, the industry explored two development paths: larger central inverters were designed to capitalize on economies of scale and reduce inefficiencies for utility-scale projects, and smaller modular string inverters filled the gap between microinverters and larger units.

String inverters balance the benefits of both micro and central inverters with anti-islanding protection, a safety mechanism preventing them from operating independently from the grid during a power outage. They also solved the rigid design, inefficient MPP tracking, and DC arc hazards associated with early central inverters. As a result, string inverters reached widespread acceptance and are now used across solar projects of all scales, from small home setups to large utility PV plants.

How does a string inverter enhance solar power efficiency?

Modern string inverters are smaller, lighter, and more efficient because they use advanced power electronic components. Many of them are also transformer-less, further reducing their size and weight. This streamlined design minimizes energy losses and makes it easy to integrate them into different locations.

String inverters also use more Multi-Power Point Tracking (MPPT) algorithms that actively optimize power output at the string level, maintaining efficiency across long strings of modules.

Moreover, modern string inverters incorporate multiple MPPTs. This makes it possible to divide a long string of solar panels into smaller and more efficient segments. With this system design flexibility, PV plant developers can arrange panels in various orientations and elevations without compromising power output, a considerable advantage in complex installation scenarios.

What are the benefits of string inverters?

Cost-effectiveness

String inverters have a higher initial cost (lower cost per unit, but more need to be acquired), potentially being up to twice as expensive as central inverters. However, they offer greater efficiency due to having multiple maximum power point trackers (MPPTs) per unit, which can increase energy production by 4-8% compared to central inverters.

Adaptability

String inverters can handle various installation scenarios. They can effectively manage module mismatch issues in utility-scale installations that use single-axis trackers. They also work well for uneven terrains because string inverters are smaller, unlike central inverters that cast large shadows and affect nearby panels. They can be placed closer to the panels to reduce the risk of shading and improve overall performance.

Ease of maintenance

String inverters for larger systems make it easier to isolate operational issues. As there are more of them, if one has a problem, it can be dealt with while the other strings continue to operate.

Unlike central inverters, string inverters do not require additional housing.

What are the limitations of string inverters?

Lower efficiency at large scale

String inverters may not be as efficient as central inverters when deployed in large utility-scale plants, as they often experience greater losses due to the increased number of inverters and wiring connections.

Higher cost per unit

As mentioned above, while individual string inverters are typically less expensive than central inverters, the overall cost of a large system can increase due to the need for multiple inverters, additional balance of system (BOS) components, and wiring.

More complex installation, scalability, and maintenance:

Scaling a system with string inverters may require proportionally more inverters, each needing its own installation, maintenance, and monitoring. This makes scaling up less streamlined compared to central inverters.

Potential for Lower Reliability

String inverters, having more components, can be more prone to failure when compared to central inverters, which typically feature simpler designs.

Limited Power Output per Inverter

Until recently, string inverters were generally designed for smaller power outputs compared to central inverters, which are capable of handling larger, more uniform power outputs, making them more suitable for large utility-scale projects.

Notably, the capacities of both inverter types are starting to meet in the middle. Traditionally, smaller string inverters have grown from 1kW to 350kW each, while central inverters have shrunk from 4.5MW to as small as 700kW.

Should I use string inverters?

The right inverter depends on your project’s scale, budget, and layout.

Because they can efficiently handle high power outputs, central inverters shine in large-scale solar projects where peak power surpasses 100MW. However, they require dedicated housing and certified DC electricians for installation in some regions, which can push costs up.

String inverters are often more flexible and efficient for smaller residential and commercial applications and utility-scale setups in complex terrains. But even though they are easier and cheaper individually, deploying many string inverters across a large project can be more labor-intensive, expensive, and demands extensive planning.

RatedPower lets you accurately simulate different inverter types to determine which systems match your project’s requirements.