6.1 Inverter Types

Quick Answer: Handbook v1.3 Learning Outcome 3.1.2 names five inverter families: domestic string, commercial string, central, micro, and hybrid. A string inverter takes one or several roof strings into a conversion unit, usually in a garage or plant room. Commercial string inverters are the larger three-phase, multi-MPPT cousins of that architecture. A central inverter gathers large sub-arrays into one high-power machine and is a more commercial pattern. A micro inverter converts at a module or small group. A hybrid converts DC to AC and can convert AC to DC, which is how a battery charges. This OpenExamPrep section is independent study material for that 2922-34 list; it is not a City & Guilds publication.

Why inverter type is a 2922 decision

Unit 301 places nine questions (30 percent) on Learning Outcome 3. Inverter type is not a catalogue preference. It decides where direct current (DC) from the modules becomes alternating current (AC), how many maximum power point trackers (MPPTs) you have created, how shade and split roofs behave, whether a battery can charge from the grid through the same cabinet, and how many generating devices the Distribution Network Operator (DNO) sees under Energy Networks Association (ENA) Engineering Recommendation G98 or G99. Choose the wrong family on a two-face roof and you either leave yield on the table or you connect two generators when one multi-MPPT string machine would have done the job.

The award is written for domestic and small commercial grid-connected photovoltaic (PV) work. That is why the handbook still lists a central inverter even though a typical house job will never hang a warehouse-scale cabinet on the garage wall. You must recognise the architecture so you do not call a 20 kW three-phase string inverter a “central” inverter simply because it is the only box on site, and so you do not call a garage string inverter a “microinverter” because it has a phone app.

Domestic string inverters

A domestic string inverter is the classic 2922 plant-room unit. Modules on the roof are wired in series strings. The string cables drop to a conversion unit in a garage, utility room, or plant cupboard. That unit runs MPPT on its DC inputs, converts DC to AC at 230 V nominal on a single-phase supply, and feeds the consumer unit through an AC isolator. One machine, one AC connection, one set of labels at the inverter.

One string is common on a simple south roof: for example twelve 400 W modules in series into a single-phase inverter of about 3–4 kW. Several strings land on the same inverter when the designer needs more current than one string should carry, or when two MPPT inputs are available. The architecture is still “strings to a remote inverter,” not conversion at each module.

Single-MPPT domestic inverters force every DC input they parallel onto one tracker voltage. That is acceptable when every module sees similar irradiance and the strings have matching module count and orientation. It is a poor default on a split roof. Multi-MPPT domestic inverters give each tracker its own voltage window, which is how a competent 2922 designer usually treats two clear roof faces without jumping straight to microinverters.

The failure mode is concentrated: one inverter dies and the whole array stops producing AC. Shade on one module still relies on bypass diodes inside that string. Those facts are why some specifications move to micros or to DC power optimisers. DC optimisers are not a sixth handbook inverter type. They are module-level DC converters that still feed a string inverter. If the paper asks for the 3.1.2 list, answer with the five named families.

Commercial string inverters

Commercial string inverters are not a different physics. They are larger three-phase, multi-MPPT cousins of the domestic string machine. Think tens of kilowatts, 400 V three-phase, several MPPT inputs, more than one string per tracker, still distributed around a roof rather than combined into one central block. A shop, farm shed, or school classroom block that still sits inside the Microgeneration Certification Scheme (MCS) MIS 3002 installer-scheme ceiling of 50 kWp DC is often a commercial-string job, not a central-inverter job.

2922-scale work uses them when the AC side is three-phase, when G99 is already in play because aggregate generation exceeds 16 A per phase, or when a single domestic-size chassis cannot cover the array without paralleling mismatched strings onto one tracker. You still land strings at inverters. You just have a bigger three-phase chassis, or several string inverters around the building.

Do not treat “commercial” as a licence to ignore domestic good practice. Isolation, DC connector rules, and G98/G99 notification still apply. The type name only tells you the scale and AC system of the string architecture.

Central inverters

A central inverter takes large sub-arrays — typically after combiner boxes — into one high-power conversion machine. MPPT, conversion and much of the protection sit at that central plant rather than at several string inverters around the roof. The architecture belongs to larger commercial and generating-station work.

On the 2922 list you must be able to name it. On a house survey it is usually the wrong picture. If your drawing shows many parallel strings combining into one 100 kW-class unit in a switchroom, you are describing central inversion, and you are leaving the “small solar PV” pattern this award is built around. MCS MIS 3002’s 50 kWp DC ceiling is an installer-scheme scope limit, not a City & Guilds size cap, but it is a useful reminder that 2922 teaching is not utility-scale plant.

Do not relabel a 15 kW three-phase string inverter as central just because it is “the only inverter on site.” Central means sub-arrays into one big inverter, not “there happens to be one cabinet.”

Micro inverters

A micro inverter converts per module or per small group of modules. The DC run is short. AC is what travels across the roof to a distribution point. That is module-level conversion: shade on one module does not collapse a twelve-module string’s operating voltage, mixed orientations are local, and monitoring is usually per module.

2922-scale jobs use micros when the roof is chopped by chimneys, hips and vents, when east and west modules are mixed on one array, or when the customer wants module-level visibility. The trade-off is more electronic devices in the weather, more AC connectors on the roof, and a different isolation story than a single garage isolator. Aggregate G98/G99 capacity still adds every micro’s registered output. Twelve 400 W micros on single-phase are not “twelve tiny jobs that each dodge 16 A.” They are one installation whose registered capacity is the sum.

Hybrid inverters

A hybrid inverter still converts PV DC to AC so the house and the public supply can use the array. What the handbook adds is the second power flow: it can convert AC to DC, which is how a battery charges from the grid and, on many DC-coupled products, how PV DC is steered into the battery without a second conversion box. Hybrid is an architecture and a power-flow statement.

Hybrid is not automatic proof that the house will stay on during a power cut. Island-mode backup needs a designed changeover so the inverter does not energise a dead public main. That safety distinction is the next section. Do not treat a “hybrid” nameplate as a backup certificate.

A hybrid can still be a string hybrid: roof strings into a garage hybrid. Some products pair a hybrid with DC optimisers. The handbook type is hybrid because of bidirectional conversion, not because of the brand on the case.

Type versus architecture versus 2922-scale use

Scenario: two roof faces, different orientation

A semi-detached house has 3 kWp east and 3 kWp west. The customer asks for “the usual garage inverter.”

If you series-wire east and west into one string, string current is limited by the weaker face all day. If you land both faces on one MPPT in parallel, the tracker still has to pick one voltage for two different current-voltage curves. Either way you have used a domestic string inverter as if the roof were one plane.

Three honest 2922 answers sit on the type list:

1. One multi-MPPT string inverter — east string on MPPT 1, west string on MPPT 2. Same plant-room unit, two trackers. This is the usual cost-effective answer when each face is itself unshaded.
2. Micro inverters on every module, or a micro per small group. Each module tracks its own maximum power point. This wins if the west face is treed after mid-afternoon, or if module-level monitoring is specified.
3. Two MPPTs is not the same as two micros. Two MPPTs still leave each face as a string: one cracked cell or one chimney shade on the east string still pulls that string. Micros isolate that mismatch at module level. Two separate string inverters would also give two trackers, but you then have two AC generators to connect and notify.

The exam trap is treating “string versus micro” as a brand war. The design question is how many independent MPPTs you have created, and whether remaining shade lives at string level or module level. A hybrid does not magically fix two orientations unless it also provides separate MPPTs (or you have used micros). Bidirectional battery charging is a different decision, taught as the hybrid type, then as functions in the next section.