String Inverter

ALMM and inverter compliance: MNRE’s current ALMM page publishes solar PV module and cell lists, not an inverter list. Verify the inverter’s exact model documents and applicable scheme or DISCOM requirements separately.

What is a string inverter

A string inverter is a centralised solar inverter that converts the combined DC output of one or more series-connected strings of PV modules into AC. A string is a chain of modules wired positive to negative in a row, adding up their voltages. The inverter sees the sum of those voltages at the input and runs MPPT on the whole string at once.

It is the most common inverter topology in Indian solar. Residential rooftops use single-phase string inverters in the 1 to 10 kW range. Commercial and industrial sites use three-phase string inverters from 10 to 100 kW. Utility-scale plants increasingly use 200 to 350 kW string inverters distributed across the field instead of central 1 MW units.

The defining trade-off is granularity. A string inverter optimises a string, not a panel. When the string is uniform in orientation, tilt and shading, the string inverter delivers near-microinverter yield at a fraction of the cost. When mismatched, the weakest panel drags the string down.

String inverter explained simply

Imagine a row of solar panels wired in a single chain on the roof. The chain produces high-voltage DC. One wire runs down to the inverter on the wall. The inverter is the single point of conversion for that whole chain.

Compare this with microinverters, where each panel has its own little inverter underneath. String is centralised, micro is distributed. Centralised is cheaper. Distributed is more flexible - see string inverter vs microinverter for how that plays out on a real Indian roof.

For a typical Indian rooftop with one clean south-facing slope, string is the obvious answer. For a roof with a tree to one side, a chimney casting shade for half the day, and two orientations, microinverters or DC optimisers earn their cost.

Why string inverters matter

Cost per installed watt. String inverters cost ₹2 to ₹6 per watt against ₹8 to ₹14 per watt for microinverters; see our solar inverter installation cost breakdown for real Indian numbers. Across an Indian rooftop market that grew from 8 GW to over 40 GW between 2020 and 2025, the cost gap drove almost all deployments to string. EPCs quoting these jobs typically run the numbers through quotation software built for solar sales teams.

Field serviceability. One inverter on a wall is easier to swap than 20 microinverters on a roof. Mean time to repair is hours, not days.

Scalability. The same architecture works from a 1 kW residential rooftop to a 100 MW utility plant. EPC standardisation lowers cost and trains a single workforce.

How a string inverter works

Internally the data path through a string inverter follows the same five-stage model as any modern PV inverter.

1. DC sensing. Voltage and current at each MPPT input are sampled at high speed.
2. MPPT. A control loop perturbs the operating voltage and observes power. The string is held at the maximum-power point of its I-V curve.
3. DC bus regulation. A boost or buck converter steps the string voltage to the DC bus level needed for inversion.
4. Inversion. IGBT or SiC switches chop the DC bus at 16 to 50 kHz. An LCL filter smooths the chopped output into a sine wave.
5. Grid synchronisation. A phase-locked loop matches grid voltage and frequency. Anti-islanding logic guards against islanding.

The string-specific part is the MPPT stage. Each MPPT input tracks one string. A 10 kW inverter with two MPPTs lets the installer point two strings in different directions, each tracked independently - a scenario covered in dual MPPT vs single MPPT. A 50 kW unit might have 4 to 6 MPPTs, allowing six different sub-arrays.

Technical deep dive

String sizing

String length is bounded above by the maximum DC input voltage of the inverter and bounded below by the MPPT lower window; our string sizing glossary entry and the solar string sizing and overcurrent protection guide cover the full EPC workflow.

Lower bound. At the hottest noon, panel Vmp drops. Indian rooftop modules at 60 degrees Celsius see Vmp around 32 V instead of nameplate 34 V. For a 100 V MPPT lower limit, minimum string length is roughly 100 / 32 = 4 modules.

Current. Per-string short-circuit current must stay under the inverter input current limit, otherwise current is clipped and yield falls.

MPPT count

DC oversizing

DC oversizing is the practice of connecting more PV than the inverter AC rating; see our DC oversizing in solar FAQ guide for the full ROI math. Indian conditions justify 115 to 130 percent ratio for most rooftops. The inverter clips midday output above its AC rating, a behaviour explained in detail in inverter clipping explained, but the day starts and ends earlier and later at the rated output, raising annual yield by 4 to 9 percent compared to 100 percent matching.

Topology and efficiency

Real-world applications

Indian rooftops. Single-phase 3 to 10 kW string inverters dominate residential. Three-phase 10 to 50 kW units cover small commercial and institutional rooftops.

C&I plants. 50 to 200 kW string inverters distributed across rooftops, carports and ground-mount arrays.

Utility solar. 200 to 350 kW string inverters in cluster boxes feeding pad-mounted transformers. Used in plants from 5 MW to 500 MW.

Solar carports. Multiple 10 to 50 kW string inverters under each shade structure.

Agricultural pumping. Three-phase string inverters with VFD output for direct PV-to-pump operation under PM-KUSUM Component B and C.

Limitations

String-level optimisation. One shaded panel drags down the whole string. Mismatch losses from soiling, tilt or partial shade hit harder than on a panel-level system.

Single point of failure. If the inverter goes down, the whole system stops. Microinverter outages affect only one panel at a time.

High DC voltage on the roof. Strings up to 600 V residential or 1500 V utility require careful DC isolation, conduit, and arc-fault management.

Limited monitoring granularity. Panel-level performance is invisible unless you add DC optimisers.

Inverter end-of-life is the system end-of-life unless replaced. Microinverters tend to outlive string inverters by 5 to 10 years.

Common problems

MPPT input current overload. Adding one more module per string in summer pushes Isc past the input limit. The inverter clips current, yield drops, and the user blames the panels. Our solar inverter troubleshooting guide walks through diagnosing these string-level faults.

DC isolator faults. Indian rooftops in monsoon corrode cheap DC isolators. Use IP66 or better, replace at 5 to 7 years.

Insulation resistance trips. A wet PV cable triggers the inverter’s Riso protection. The inverter does not start. Dry the cable, redo the gland, the inverter resumes.

Communication dropouts. RS485 or WiFi to the cloud fails silently. The user sees no data. Audit monitoring monthly.

Display showing PV power lower than measured ammeter reading. Common when MPPT lower window is set above the operating point on a partially shaded morning. The inverter stops tracking.

Best practices for string inverters

Match string length to inverter MPPT window at the worst-case temperatures for the site.

Use the manufacturer’s specified DC isolator. Cheap third-party DC isolators are the most common failure in Indian rooftops.

Mount in a shaded, ventilated location with the manufacturer’s clearance specifications.

Use Class II SPDs on DC and AC sides.

Earth the inverter chassis and the array frame separately, then bond at the main earth bar.

Plan DC oversize between 115 and 125 percent for Indian rooftops unless the inverter datasheet states otherwise.

Comparison: string vs micro vs central inverter

Indian market context

Common string inverter brands in India include Sungrow, Solis, Goodwe, Growatt, Solax, Deye, Huawei, Sineng, Fronius, SMA, Tata Power Solar and Luminous; EPCs evaluating vendors can cross-check options against our solar inverter manufacturers in India evaluation guide, or source stock directly through Heaven Green Energy’s solar inverter shop. Most three-phase utility-string inverters used in large plants are sourced from Sungrow, Sineng, Huawei and Solis as of 2026.

CEIG approvals are needed for systems above 10 kW in many states, with the inverter forming a key part of the single-line diagram.

Standards and certifications

Common mistakes

Putting east-facing and west-facing strings on the same MPPT. The string with weaker irradiance drags the other down by 8 to 15 percent annual yield.

Choosing too few MPPTs to save inverter cost. Two extra MPPTs cost less than the energy lost over five years on a mixed-orientation roof.

Ignoring temperature derate of Voc when sizing strings for Indian high-altitude sites like Leh and Spiti.

Skipping DC SPDs. Indian thunderstorm zones see lightning surges that fry inverter front ends.

Installing the inverter on a west-facing wall. The internal temperature rises, derate kicks in by 1 pm, and the user blames the panels for low afternoon yield.

Forgetting to record string layouts on the system handover document. The first service visit takes twice as long.

Key takeaways

String inverters are the workhorse of Indian solar. They give the best cost per watt for unshaded, single-orientation roofs and for utility-scale plants. The trade-off is string-level mismatch sensitivity, which can be managed with MPPT count, careful string sizing, and DC optimisers when needed. Sizing, mounting and earthing decide whether the inverter delivers full rated yield for 12 years or trips intermittently for 2 years.

Further reading

For how String Inverter plays out in real projects, these guides go deeper:

• Solar String Sizing & Overcurrent Protection
• Solar String Sizing Software: Complete 2026 Guide
• String Voltage vs MPPT Voltage: Inverter Guide
• Solar Inverter for Cold Storage, Sizing and VFDs

Sources

• IEC 62109-1, -2, Safety of power converters for use in photovoltaic power systems.
• IEC 61727:2004 and IEC 62116:2014, Utility interface and anti-islanding.
• IS 16221 (Part 1 and 2), Indian Standard for PV inverter safety.
• MNRE Approved List of Models and Manufacturers (ALMM), Ministry of New and Renewable Energy.
• CEA Technical Standards for Connectivity to the Grid (Amendment) Regulations 2019.
• EN 50530:2010, Overall efficiency of grid-connected photovoltaic inverters.
• NREL Technical Report TP-5D00-74465, Best Practices for Operation and Maintenance of Photovoltaic and Energy Storage Systems.
• Sandia National Laboratories Report SAND2018-12700, Comparative Analysis of String and Central Inverters.
• IRENA Renewable Power Generation Costs in 2024.