String vs central inverters for ground-mounted plants

Solar modules produce direct current (DC); the grid runs on alternating current (AC). Inverters do the conversion, and they are among the most important — and most failure-prone — components in a plant. For ground-mounted plants, the main design choice is between central inverters and string inverters.

The two architectures

Central inverters are large units, typically housed in inverter stations, each converting power from many strings combined through DC combiner boxes. A plant has relatively few of them.

String inverters are smaller units distributed across the array, each taking a limited number of strings directly. A plant has many of them, often mounted on the module structures.

How they compare

Fault impact and availability

This is often the deciding factor for owners. When a central inverter fails, a large block of the plant stops generating until it is repaired, which can take days if specialist parts or technicians are needed. When a string inverter fails, only a small portion is affected, and a spare can often be swapped in quickly. Over a plant's life, this affects availability.

Performance under uneven conditions

Each inverter tracks the maximum power point of the strings connected to it. String inverters, with more tracking inputs per unit of capacity, can cope better when parts of the array behave differently — because of terrain, orientation, partial shading or mismatched soiling. On regular, uniform sites, the difference narrows.

Costs over the life of the plant

Capital cost is only part of the comparison. Owners should compare:

• Equipment and installation cost
• Cabling and civil works (inverter stations vs distributed mounting)
• Expected failure rates and repair times
• Spares strategy and cost
• Replacement or overhaul during the plant's life

Inverters generally have shorter lives than modules, so a mid-life replacement or major overhaul should be budgeted either way.

O&M implications

Central inverters need technicians trained on specific models and may depend on the manufacturer's service network. String inverters need logistics for swapping units and managing a larger fleet of devices. The O&M plan and spares strategy should match the architecture chosen.

Making the choice

There is no universal answer. Large, flat, regular sites with strong O&M support may favour central inverters; varied terrain, smaller plants or owners prioritising availability may favour string inverters. The decision should come from a lifetime cost comparison for the specific site.

Monitoring and data

Inverters are the plant's main source of operational data. String inverter plants generate many more data points, which allows faults to be located more precisely but requires a monitoring system that can handle the volume and turn it into useful alerts. Central inverter plants rely more on combiner-box monitoring to see what is happening within each block. Either way, monitoring should be designed so that underperformance is detected quickly and traced to a specific location.

Grid code and reactive power

Inverters do more than convert DC to AC. They must meet grid requirements for voltage and frequency behaviour, reactive power and response to disturbances. Compliance requirements are set by the relevant grid codes and connection agreements, and inverter selection must account for them. Check current requirements with the transmission utility at the connectivity stage.

Questions to ask

• What are the warranty terms, and can the warranty be extended?
• What is the expected lifetime, and what major maintenance is anticipated?
• Where are spares held, and what are delivery times for critical parts?
• Is there a local service network with trained technicians?
• How will the monitoring system present faults and losses?
• How does the design comply with current grid requirements?

Common mistakes

• Comparing capital cost only, without lifetime repair and replacement costs
• Assuming the manufacturer's service network is available locally
• Holding too few spares for a string inverter fleet
• Not planning for an inverter replacement during the plant's life

How we help

When we coordinate EPC partners, we help owners make sure inverter architecture decisions are compared on a lifetime basis and that warranties, spares and service arrangements match the choice.