String inverter vs microinverters

A string inverter converts the whole array centrally: panels are wired in series into one or more strings, and a single unit — usually in a loft, garage or utility room — does the DC to AC conversion. Microinverters convert at each panel, on the roof, so each module works independently. Between them sits a third option: a string inverter with power optimisers attached to individual panels, giving module-level control while keeping central conversion. Shading, roof complexity, servicing access and monitoring requirements decide which is right. On a clean, single-orientation roof, the simplest architecture is usually the best one.

The three architectures

Plain string. Panels in series, one central inverter. Fewest components, nothing on the roof but panels and rails, and conversion happening at one accessible point. Efficient and well-proven.

String plus optimisers. A DC optimiser at each panel manages that panel’s operating point, then the central inverter converts. Shading on one module no longer drags down its neighbours, and you get per-panel monitoring.

Microinverters. A small inverter per panel converts to AC on the roof. Each panel is fully independent. Wiring on the roof is AC rather than high-voltage DC.

The string constraint that drives the decision

Panels wired in series behave like a chain: current through the string is limited by the least productive module. Shade one panel and the whole string suffers, not just that panel. Bypass diodes limit this but do not remove it.

Mismatched orientations produce the same effect for a different reason. Panels on an east slope and a west slope on the same string will never peak together, so one holds the other back — which is why split arrays are normally given separate strings or separate MPPT inputs.

Everything else in this comparison follows from that constraint and how each architecture handles it.

Where each one fits

Choose a plain string where the roof is a single orientation, essentially unshaded, and the array is a regular block. You get simplicity, fewer failure points and nothing extra on the roof. The saving is real and the performance is excellent.

Choose optimisers where there is genuine shading, where the array spans more than one orientation on the same string, or where per-panel visibility matters to you. Also worth considering where a roof is awkward enough that panel counts differ between sub-arrays.

Choose microinverters where panels are scattered across several small areas, where roof-mounted DC is undesirable, or where you want each module truly independent and are content with the servicing implications.

Failure and servicing

This is where the honest comparison lives, because all power electronics eventually need replacing.

A string inverter is a single point of failure — if it stops, the array stops. But it is one unit, usually indoors, and replacing it is a contained job with no roof access. When it fails, you know immediately because generation goes to zero.

Microinverters distribute the risk. One failure costs one panel’s output, and the rest of the array carries on. The downsides are that a failure is easy to miss without monitoring, and that fixing it means scaffolding or another access method for a single small component. Multiply that across a long system life and the access cost matters.

Optimisers sit in between: distributed components on the roof, plus a central inverter that is still a single point of failure.

Monitoring

Plain string systems typically report at string or system level. That is enough to see that the system is working and to compare year on year, but a single underperforming panel can hide within a healthy-looking total.

Optimisers and microinverters both give per-panel data. This is genuinely useful — it turns “output seems a bit low” into “panel nine has been down since March” — and it makes fault-finding much quicker.

Cost and complexity

A plain string is the least expensive and has the fewest components. Optimisers add cost per panel. Microinverters add cost per panel and put more electronics in a harsher environment.

There is a tendency to treat module-level electronics as automatically superior. They are not. They solve a specific set of problems very well, and on a roof that does not have those problems they add cost and components for little return.

Hybrid inverters and storage

If storage is planned now or later, a hybrid inverter handles solar and battery together on the DC side and is usually the cleaner route. Microinverter systems are AC by nature, so storage is added with a separate AC-coupled battery inverter. Neither is wrong; they simply lead to different retrofit paths.

We ask about storage plans at design stage precisely because the inverter decision is the one that constrains what is easy later.