String vs Central Inverters at 500 kW–2 MW: Which Architecture for Your Roof

For most factory roofs between 500 kW and 2 MW, string inverters are the safer default. One failed unit costs a slice of output, monitoring reaches each string, and a replacement comes off the shelf. Central inverters earn their place on one large, uniform, easily reached array whose owner has an engineered service arrangement. The roof layout, not the catalogue, should decide.

What actually differs between the two architectures?

A string inverter converts the DC from a few strings of modules, usually one section of a roof. A 1 MW plant therefore carries a fleet of units, commonly in the 50 to 100 kW class, hung on the structure or a wall near the array. Their AC outputs are combined in an AC distribution board and taken to the transformer or the LT panel.

A central inverter takes the DC from many strings through combiner boxes and converts it in one large cabinet, often a single unit of several hundred kW to a few MW, placed in a room, shelter or skid. DC cables run long; AC cabling is short. Some central units are built from parallel power stacks, so ask the vendor in writing what one stack failure does to output.

Everything else follows from that difference in granularity: how much generation one fault takes out, how finely you can see losses, how the DC cable is routed, and what the year-12 replacement looks like. Our glossary entry sets out the same trade-off in short form.

How much generation does one failure cost?

Take the planning figure we use across the site, about 120 units per kW per month in north India, which is roughly 4 units per kW per day. A 1 MW plant then produces around 4,000 units a day on average. These numbers are illustrative; the repair times are assumptions you should replace with the vendor's service-level commitments.

• String fleet of ten 100 kW units. One unit fails and is swapped from a spare in two days. Output lost is about 10 percent of 4,000 units for two days, so roughly 800 units.
• One central 1 MW unit. It fails and the service engineer arrives in seven days. Output lost is the whole plant for seven days, roughly 28,000 units.

The central owner can narrow that gap with a spare stack or board on site and a service contract with a stated response time. The string owner can widen it by not holding a spare. The architecture sets the exposure; the contract decides how much of it you carry.

String or central: which factors move the decision?

The two cells that surprise owners are the DC cable and the AC cable. A central design is not simply cheaper per kW once the long DC home runs, the combiner boxes and the inverter room are added. A string design is not simply dearer once the central owner has priced a spare stack and a service contract. Ask for both as complete, line-by-line proposals before comparing.

What does the roof tell you?

Factory roofs are rarely one clean plane. Expect two to eight blocks with different heights, skylights, vents, parapet shadows and sometimes different pitches. Each change in orientation or shading is a reason to give that section its own MPPT input. String units do this by default. On a central inverter the same effect needs more, smaller inputs, which erodes the cost advantage.

Roof access matters too. A string inverter on the roof must be reachable for service and sited away from the hottest roof zones, because inverters derate when they run hot. A central inverter keeps the electronics on the ground, which helps service and heat but adds the long DC run through the building, and DC cable is the part that deserves the most care for fire safety. Our note on rooftop solar DC fire safety covers routing and isolation.

Which architecture for which size and roof?

This is a starting point, not a verdict. A shadow study, the roof's load capacity and the DISCOM connection voltage can all move a given plant to the other column. Our size pages show how this plays out at 500 kW and 1 MW.

What rules apply whichever you pick?

Inverters for solar PV fall under MNRE's Solar Systems, Devices and Components Goods Order, 2025, notified on 27 January 2025 under the BIS Act and effective 180 days after publication (PIB, 29 January 2025). It requires inverters to conform to the latest Indian Standards and carry the Standard Mark under a BIS licence. BIS's registration-scheme page lists the inverter standards as IS 16221 (Part 2) for safety, IS 16169 and IS 16270 for islanding-prevention testing.

Large units have a separate path. BIS lists a gazette notification of 5 August 2025 for inverters above 200 kW, and MNRE has extended implementation for that class on a self-certification basis; reported end dates have moved more than once. Most central inverters are above 200 kW, so ask for the BIS licence number or the self-certification declaration with the test reports, and confirm the current date on the MNRE site before you sign.

Grid connection is separate. The CEA's technical standards for connectivity of distributed generation set the protection behaviour the DISCOM checks, including anti-islanding. Under the central Electricity (Rights of Consumers) Rules, net metering runs up to 500 kW or the sanctioned load, whichever is lower, per a 2021 amendment (PIB). Above that, plants usually go gross metering, net billing, captive or open access, and the DISCOM's own conditions apply. Protection settings must be recorded at commissioning because they are checked at inspection.

How Secured Engineers helps

• We design the plant in-house, including the inverter architecture and the DC and AC layout for your roof.
• DISCOM net-metering filings and CEIG approval are prepared in-house, so the inverter protection settings and the single-line diagram agree.
• MEPF work and solar sit under one contract, which helps when the inverter room, DG interlock or cable routes touch other services.
• We work from offices in Noida (Delhi NCR) and Ludhiana, with 535+ projects across 18+ states and 89.77% repeat clients.
• On multi-shed factory roofs our default is string inverters, as on our 500 kW page; we price a central option when the array is uniform.

To see how this applies to your roof, read our inverter supply and installation service, then message us on WhatsApp, call +91 70099 85817 or ask for a free proposal with both architectures priced line by line.