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

  • String inverter — a single centrally-located inverter; all panels in a string are connected in series; the inverter converts the combined DC string voltage to AC; simplest and cheapest architecture
  • MPPT (Maximum Power Point Tracking) — the inverter's algorithm for tracking the optimal operating point of the solar array; a string inverter has one or two MPPT inputs; a power optimiser has one MPPT per panel
  • Power optimiser (DC optimiser) — a small device fitted to each panel; performs MPPT at the panel level and outputs a fixed DC voltage to the string; a central string inverter converts the regulated DC to AC; SolarEdge and Tigo are the main UK suppliers
  • Microinverter — a small inverter fitted to each panel; converts DC to AC at the panel; panels are connected in parallel on the AC side; Enphase is the dominant UK supplier
  • Shading loss — in a conventional string inverter system, shading on one panel reduces the output of all panels in that string (weakest link effect); power optimisers and microinverters mitigate this by providing panel-level MPPT
  • Module-level power electronics (MLPE) — the collective term for power optimisers and microinverters; provide panel-level monitoring and in some implementations module-level safety shutoff (rapid shutdown)
  • Rapid shutdown (NEC 690.12) — a US National Electrical Code requirement; reduces DC string voltage to <30V on demand; relevant to UK commercial installations under IET guidance; microinverters achieve this by default (AC only on roof)
  • SolarEdge SafeDC — SolarEdge's trade name for their safety shutdown feature (power optimisers drop string voltage to 1V on inverter shutdown)
  • IQ8 Microinverter (Enphase) — Enphase's current microinverter series; capable of limited operation during grid outage (islanding-free under normal operation; Enphase IQ8 can support Enphase IQ Battery microgrids)
  • Hybrid inverter — a single inverter that handles both solar PV and battery storage; increasingly specified where battery storage is planned simultaneously or as a future upgrade
  • Warranty comparison — string inverters: 10 years standard (extendable to 20 years for premium brands); SolarEdge optimisers: 25-year product warranty; Enphase microinverters: 25-year product warranty

Quick Reference Table: Inverter Architecture Comparison

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
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:

  • The roof is predominantly south-facing (or within 45° of south) with a single pitch
  • There is no significant shading (shading loss <4% on the MCS assessment)
  • The array is a single string or can be divided into two strings on the dual MPPT input
  • Budget is the primary constraint

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:

  • Panel-level monitoring: the SolarEdge monitoring portal shows the output of each individual panel in real time; faults are pinpointed to the module level
  • SafeDC safety shutdown: on AC disconnection, the SolarEdge inverter signals all optimisers to reduce their output voltage to 1V; the DC string becomes safe to handle (important for emergency services and maintenance)
  • Flexible string design: optimiser-based systems can have long strings and mixed panel orientations on the same string with minimal loss
  • 25-year optimiser warranty: longer than standard string inverter warranty

Drawbacks:

  • Higher installed cost: optimisers add £30–£60 per panel to the system cost
  • More components on the roof: additional points of potential failure; however, optimiser failures are panel-specific and don't take the whole system down
  • Still requires a central inverter: a central inverter failure affects the whole system

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:

  • No high-voltage DC on the roof: the highest voltage present on the roof is 240V AC (equivalent to a standard socket outlet); this is a safety advantage, particularly for properties with fire risk concerns or where installers prefer AC-only roof wiring
  • Full panel-level independence: a failing panel or microinverter does not affect adjacent panels at all
  • No single point of failure: the failure of one microinverter reduces output by one panel, not the whole system; other panels continue unaffected
  • Enphase 25-year product warranty
  • Suitable for complex roofs with multiple orientations, shading, or mixed panel types

Drawbacks:

  • Higher installed cost: Enphase IQ8 microinverters are significantly more expensive per panel than a SolarEdge optimiser system
  • More complex AC wiring: the AC branch circuits (one AC circuit per row of panels typically) add wiring and conduit runs on the roof that a string DC system does not have
  • Inverter conversion efficiency: each microinverter has a slightly lower conversion efficiency than a central string inverter; this is marginal (typically <1%)

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):

  • A hybrid inverter (e.g., GoodWe ES, Solax Hybrid, SolarEdge Home) handles both solar PV and battery simultaneously in a single unit
  • Specify a hybrid inverter where battery storage is on the customer's horizon, even if they don't want it at installation time; this avoids the need for a second inverter later
  • Hybrid inverters are generally AC-coupled or DC-coupled; DC-coupled is more efficient (see solar battery storage installation)

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