String Inverter vs Microinverter vs Power Optimiser: Which to Specify and Why for UK Residential Installations

Quick Answer: For a simple south-facing, unshaded UK domestic roof, a single string inverter is the most cost-effective and reliable solution. Where the roof has shading, multiple orientations, or more than one pitch, power optimisers (panel-level MPPT with a central inverter) or microinverters (AC conversion at each panel) significantly reduce shading losses and improve monitoring granularity. Power optimisers (SolarEdge, Tigo) are the most common UK solution for complex roofs; microinverters (Enphase) are preferred where string voltage concerns arise or where module-level safety shut-off is needed.

Summary

Inverter technology is the most significant specification decision in a solar PV design. The wrong choice for the roof configuration leads to lower yields, difficult fault-finding, and customer dissatisfaction. The right choice for a given roof maximises output, simplifies commissioning, and provides monitoring data that supports long-term performance verification.

This article compares the three main inverter architectures used in UK residential solar PV: string inverters, power optimisers (DC optimisers), and microinverters. Each has a different cost, complexity, and suitability profile.

Key Facts

Quick Reference Table: Inverter Architecture Comparison

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Architecture Relative Cost Shading Performance Monitoring Reliability Best For
String inverter (single MPPT) Lowest Poor on shaded strings String-level only Single point of failure Simple south-facing unshaded roofs
String inverter (dual MPPT) Low–medium Moderate (split strings) String-level Single point of failure Split orientation roofs
Power optimisers + string inverter Medium Excellent Panel-level Inverter failure affects all Shaded roofs; premium residential
Microinverters Medium–high Excellent Panel-level No single point of failure Shaded roofs; multi-orientation; safety-critical

Detailed Guidance

String Inverter: When It Is Sufficient

A standard string inverter (e.g., SMA Sunny Boy, GoodWe, Solax, Fronius Primo) is appropriate where:

How it works: Panels are wired in series to form a string. The string DC voltage is the sum of individual panel voltages (e.g., 20 panels at 40Voc each = 800Vdc string voltage). The inverter finds the MPPT of the string — but the MPPT is a single point for the whole string. If one panel produces less than the others (shading, soiling, fault), the string MPPT is pulled toward the weaker panel, reducing the output of all other panels in the string.

Single vs dual MPPT: Most modern string inverters offer two MPPT inputs. For a split-orientation roof (e.g., south-east and south-west slopes), putting each slope on a separate MPPT input allows each string to be tracked independently. This eliminates the worst of the cross-slope mismatch loss. However, it does not address shading within a single string.

Monitoring: String inverter monitoring (typically via manufacturer's cloud portal or app) shows total system output and string-level voltage/current. It does not show individual panel output. A panel fault may be hard to diagnose without panel-by-panel inspection.

Power Optimisers: The Most Common UK Solution for Complex Roofs

How they work: A SolarEdge or Tigo power optimiser is bolted to the back of each panel at the array. The optimiser performs MPPT for that individual panel and outputs a fixed DC voltage (typically 85V per panel regardless of shading or mismatch). The string of optimised DC feeds the central SolarEdge or compatible inverter.

Because each panel is independently MPPT-tracked, shading on one panel does not reduce other panels in the string. The system extracts maximum power from each panel individually.

Benefits:

Drawbacks:

SolarEdge vs Tigo: SolarEdge is a full system (optimisers + SolarEdge inverter; cannot mix with other inverter brands). Tigo energy optimisers can be used with compatible third-party inverters or as a retrofit on existing systems. Tigo also offers TS4-A-O (optimise only), TS4-A-F (flex — optimise, monitor, safety) for targeted deployment rather than whole-array MLPE.

Microinverters: Panel-Level AC Conversion

How they work: An Enphase IQ microinverter is bolted to the back of each panel. The microinverter converts the panel's DC output to AC (240V, 50Hz, UK) directly at the panel. Panels are then connected in parallel on the AC branch circuits — cables run down the roof in standard AC wiring (protected by an RCBO, run in conduit, etc.).

Benefits:

Drawbacks:

Enphase IQ8 islanding capability: The Enphase IQ8 microinverter (current generation) has the ability to form a limited microgrid during grid outage when paired with the Enphase IQ Battery. This is a differentiated feature for customers interested in backup power. Standard IQ8 installations without battery do not provide grid outage support (anti-islanding protection disconnects on grid loss as required by G98/G99).

Hybrid Inverters

For installations where battery storage is planned (either now or in the future):

Frequently Asked Questions

My customer has a south-facing roof with one chimney causing a 2-hour morning shadow across 4 panels. Should I specify power optimisers?

Probably yes. Four shaded panels in the morning, connected in series with 16 unshaded panels, can reduce morning string output by 15–25%. Whether this is economically justified by the optimiser cost depends on the array size and the severity of the shading. Run a shading simulation with and without optimisers to quantify the yield difference, then compare against the optimiser installation cost. For 4 panels in a 20-panel system, optimisers on just the 4 shaded panels (Tigo selective deployment) may be more cost-effective than optimising the whole array.

Is a more expensive inverter brand worth specifying?

For string inverters, brands like SMA, Fronius, and SolarEdge have strong long-term support and warranty claim histories. Budget brands (especially some Chinese brands at the low end of the market) may have shorter warranty periods and limited UK support. For a 25-year system, the inverter warranty and manufacturer longevity matter. The price difference between a budget and premium string inverter is typically £100–£300 for a domestic system — often justified by peace of mind and better monitoring tools.

Can I mix panel brands with a string inverter?

Technically yes, but it is not recommended. Different panel models have different Voc, Vmpp, and current specifications. Mixing panels in a string causes mismatch losses. For small replacements (replacing a damaged panel with the closest equivalent available), mismatch may be acceptable. For new installations, always specify matching panels throughout each string.

Regulations & Standards