6. Inverters

Learning objectives

• Explain the inverter’s two core jobs: DC-to-AC conversion and maximum-power-point tracking.
• Compare string, microinverter, optimizer, and hybrid architectures and where each fits.
• Read the inverter datasheet limits that bound your array design.
• Describe the grid-support standards a modern inverter must meet.

6.1 What an inverter does

Because PV is DC and the grid/loads are AC (Chapter 1.6), every grid-connected system needs an inverter. Its two jobs:

1. Conversion: switch DC into clean, grid-synchronized AC at the right voltage and frequency.
2. Maximum Power Point Tracking (MPPT): continuously adjust the array’s operating voltage to sit at the I-V curve “knee” (Chapter 4.4) where Vmp × Imp is greatest, as irradiance and temperature change minute to minute.

6.2 Inverter architectures

• String inverter: one central unit serves one or more series strings. Lowest cost per watt, simplest, and easiest to service. A shaded or mismatched module drags its whole string, and the array operates at higher DC voltage. The workhorse of ground mounts and unshaded roofs.
• Microinverter: a small inverter on each module; conversion happens at the module, so the array runs at AC and each module performs independently (great for complex/shaded roofs and per-module monitoring). Higher cost, more units on the roof.
• Power optimizer (with a string inverter): a module-level power electronics (MLPE) device on each module that conditions DC and does per-module tracking. A central inverter still handles the DC-to-AC conversion. A middle path: module-level optimization and monitoring with one inverter.
• Hybrid / storage inverter: integrates battery charging/discharging and often backup (islanding) capability alongside PV conversion. The backbone of the storage systems in Chapter 12.

⚠️ Microinverters and optimizers are MLPE, and MLPE inherently provides module-level rapid shutdown, which is relevant to the NEC 690.12 requirement covered in Chapter 22.

6.3 The datasheet limits that bound design

An inverter is a box of hard limits your array must respect:

• Maximum DC input voltage: the absolute ceiling; your cold-Voc string total (Chapter 4.5) must stay below it or you risk damaging the unit.
• MPPT voltage window: the range over which it can track; your hot-Vmp string total must stay above the window’s floor or the array falls out of tracking and loses output.
• Maximum input current / number of MPPTs / strings per MPPT: bounds parallel stringing.
• Rated AC output power and maximum AC current: sets the AC side and interconnection.

• CEC (weighted) efficiency: realistic conversion efficiency (~96–99% for modern units).

• DC/AC ratio (inverter loading ratio): arrays are commonly oversized relative to inverter rating (ratios ~1.1–1.3). Mild clipping of rare peak output is an intentional economic optimization, not a fault.

6.4 Grid-support and listings

A grid-tied inverter must protect the grid and the public. Core requirements:

• Anti-islanding: it must shut down if the grid goes down, so it doesn’t back-feed a “dead” line and endanger utility workers.
• UL 1741: the North American listing for inverters and interconnection equipment. Its supplements UL 1741 SA and the newer UL 1741 SB add advanced/smart grid-support functions (voltage/frequency ride-through, volt-VAR, etc.).
• IEEE 1547-2018: the interconnection standard defining how distributed resources must behave on the grid; UL 1741 SB tests to it. Internationally, IEC 62109 covers inverter safety.

Many utilities will not grant interconnection without proof of these listings.

6.5 Inverter architectures compared

 STRING                    MICROINVERTER              OPTIMIZER + STRING
 [mod][mod][mod]           [mod+µinv]                 [mod+opt]
   └──DC string──┐         [mod+µinv]──AC─┐           [mod+opt]──DC─┐
                 ▼         [mod+µinv]      ▼                        ▼
           [1 inverter]                [to AC panel]          [1 string inverter]
           DC→AC once                  per-module             per-module DC tuning,
                                       conversion             central DC→AC

Chapter 6 summary

The inverter converts DC to AC and tracks the array’s max-power point. String inverters are cheapest and best on clean arrays; microinverters and optimizers (MLPE) handle shade/complex roofs and add module-level shutdown; hybrid inverters add storage. Design within the inverter’s max DC voltage (vs cold-Voc), MPPT window (vs hot-Vmp), and current limits, and expect a DC/AC ratio above 1 with mild clipping. Compliant grid-tied units are listed to UL 1741 (SA/SB) and meet IEEE 1547-2018, including anti-islanding.

Practice Problems: Chapter 6

1. What are the inverter’s two core jobs?
2. A complex, partially shaded roof with many orientations: which inverter architecture fits best, and why?
3. Which architectures inherently satisfy module-level rapid shutdown?
4. What does a DC/AC ratio above 1 cause at peak, and is that a fault?
5. Which listing and interconnection standard must a grid-tied inverter meet, and what safety function shuts it down if the grid fails?

Solutions: Chapter 6

1. DC→AC conversion and maximum power point tracking (MPPT).
2. Microinverters (or optimizers): module-level MLPE isolates shade and handles multiple orientations.
3. Microinverters and optimizers (MLPE).
4. Mild clipping of rare peaks: an intentional economic optimization, not a fault.
5. UL 1741 (SA/SB) and IEEE 1547-2018; anti-islanding shuts it down on grid loss.