Part VII: Solar ElectricityChapter 11. Grid-Tied and Off-Grid PV

Microinverters vs. String Inverters vs. Optimizers

How the three common power-electronics layouts behave in shade, and what fails when one unit fails.

Difficulty: Intermediate
Cost: Planning only

published

The electronics decide how an array behaves in shade and what stops when one part fails. Three layouts cover almost every grid-tied house array. A string inverter takes high-voltage direct current from a series group of modules and makes alternating current in one box, usually on a wall. Microinverters make alternating current at each module, so the roof wiring you see is already AC. Optimizers condition each module's direct current and still send it to one string inverter.

This page is the comparison. It is not a wiring plan, a string length, or permission to open the service panel. Array size still starts from the electric bill. The circuit itself is the hiring page.

Difficulty

Level: Intermediate
Time: An evening of choosing a layout, before any hardware is ordered
Skills: Picturing one shaded module and one dead box

Cost estimate

Planning only

Microinverters and optimizers cost more in electronics than a single string inverter and less in the pain of a shaded module. A failed central inverter is one replacement that stops the array. A failed micro is one module. Get prices on compatible sets. Mixing a random optimizer with an inverter that was not designed for it is not a discount.

Layout	Shade	Single failure
String inverter	One weak module can pull the series group down	The inverter takes the array with it
Microinverters	Mostly the shaded module	That module
Optimizers plus a string inverter	Often closer to the micro case	A module unit hurts one module. The inverter still hurts the array

Materials

Item	Qty	Notes
A shade sketch of the roof	1	Vents, trees, and the hours they cover a corner. Shade is why this choice exists.
A compatible set	Modules plus the electronics the maker lists	Voltage windows, connector types, and rapid-shutdown behavior have to match. The data sheet is the compatibility list.
A monitoring story	1	Per-module monitoring shows a shaded or dead unit. A single inverter number hides it inside the total.

How shade travels

Modules in a simple series string share one current. A shaded module cannot pass as much current, and the others are limited with it, except for what the bypass diodes can route around. Those diodes are a damage limit and a partial save. They are not a promise that one shaded corner costs only one module. If your roof has a vent, a dormer, or a tree, a single string is the layout that makes that shadow expensive.

A microinverter lets the shaded module sag while its neighbors keep working. An optimizer is built to do a similar job on the direct-current side and then hand a behaved string to the inverter. Products differ. Read how the one you are buying acts when a module is covered, and do not assume the word optimizer means the shade is free.

Electronics on the roof live in the heat. Heat shortens the life of the parts you cannot reach without a ladder. A wall-mounted inverter lives in a kinder place and is one box to replace. The trade is more boxes on the roof for better shade behavior and finer monitoring, or one box on the wall for simpler replacement and harsher shade behavior unless the roof is honestly clear.

String, microinverter, and optimizer. Shade and a dead box land in different places in each layout.

What else the layout changes

Rapid shutdown, so firefighters can de-energize a roof array, is required for many rooftop systems under current codes. Module-level electronics are one common way to meet it. A plain string with no module electronics may need another listed method. This is a code and product question for the electrician and the inspector, not a feature you skip because the array is small.

High direct-current voltage lives on a string and, in a different form, between optimizers and the inverter. Microinverter roofs carry alternating current at ordinary household voltage, which is still dangerous and still not a hobby circuit. None of the three is "safe enough to wire from a blog." Cold mornings raise the open-circuit voltage of the modules. A string that was fine on a warm afternoon can be over the inverter's limit at dawn in January. Whoever designs the string has to use the cold-weather number. This page will not pick that count for you.

Warranties differ, and the inverter warranty is often shorter than the module warranty. Budget a replacement. A layout you cannot monitor is a layout you will not notice failing until the bill does.

Mark the shade before you pick the box
A clear south plane can be a string. A plane with a daily shadow wants module-level electronics, or a smaller unshaded array.
Buy a matched set
Modules, inverters or optimizers, and connectors the makers list together. No mix-and-match connectors that merely look alike.
Ask how rapid shutdown is accomplished
The answer should be a listed method your inspector already sees. Not a promise to add it later.
Hand the circuit to someone licensed to build it
Interconnection, the service panel, and the string design are that job. Your job was the layout and the shade.

Difficulty

Cost estimate

Materials

How shade travels

What else the layout changes

Mark the shade before you pick the box

Buy a matched set

Ask how rapid shutdown is accomplished

Hand the circuit to someone licensed to build it

Further reading