Solar string wiring explained: how series circuits affect your Perth system

If you have a string inverter system — the most common solar setup in Perth homes — your panels are wired in series into one or more "strings." Understanding how strings work explains why a single shaded panel can drag down the output of the whole string, and helps you interpret your monitoring data and evaluate whether shading mitigation (power optimisers or microinverters) is worth the cost.

Why panels are wired in series

A solar inverter converts DC electricity from the panels into AC electricity for your home. It needs a specific voltage range to operate efficiently — typically 150–600V for residential string inverters.

A single 60-cell residential panel produces roughly 40V open-circuit. A string of 10 panels in series produces ~400V. Wiring panels in series (positive terminal of one to negative terminal of the next) adds voltages while keeping current constant.

The inverter operates its Maximum Power Point Tracker (MPPT) within this voltage range to extract maximum power from each string. One MPPT input handles one string — or sometimes two strings if they're identical in size, orientation, and shading exposure.

What happens when one panel is shaded

In a series circuit, current must be the same throughout the string. The weakest panel dictates the current of the entire string.

Think of it as a garden hose kink: the kink restricts flow through the whole hose, not just at the kink point.

Example:

• 10 panels in a string, each generating 8.5A in full sun
• One panel is partly shaded — it can only generate 4A
• The entire string's current drops to 4A
• A string that should produce 3,400W now produces ~1,700W

The shaded panel acts as the bottleneck. The panels before and after it in the string are producing normal voltage but the current restriction means they can't deliver their full power.

Bypass diodes partially compensate: Most solar panels include bypass diodes in the junction box, which allow current to flow around a severely shaded cell. This limits the impact to the shaded panel's output rather than the entire string — but bypass diodes only kick in for severe shading (entire panel in deep shade), not partial shading.

How this shows up in your monitoring data

If you have string-level monitoring (most string inverters provide this), you can see:

• Lower-than-expected string voltage in the afternoon when the neighbour's chimney shades two panels: the string voltage drops as bypass diodes activate around shaded panels
• Uneven performance between strings: If you have panels split across north and west faces, the strings will show different output profiles — west string peaks later in the day
• Early morning or late afternoon dips: Shade from trees, aerials, or neighbouring buildings that only affects one string

If you have two strings with identical panel counts and orientations but one shows consistently lower output than the other, that's a diagnostic signal — something is shadowing one string (or there's a panel fault, wiring issue, or inverter MPPT problem).

Multi-MPPT inverters: partial solution

Many modern string inverters have two MPPT inputs, allowing you to run two independently optimised strings. This is useful when:

• You have panels on two different roof faces (north and east, or north and west)
• One face gets afternoon shading that the other doesn't

With dual MPPT, the shaded string doesn't drag down the unshaded string — each is tracked independently. You still have the shading problem within each string, but it's isolated.

Power optimisers: panel-level MPPT

A power optimiser (SolarEdge, Tigo) is a DC-DC converter attached to each panel. It performs MPPT at the panel level, then feeds a fixed voltage into the string to the central inverter.

Effect on shading: When one panel is shaded, the optimiser adjusts only that panel's output, delivering its maximum available power independently of the others. The other panels in the string are unaffected.

Cost: $50–$100 per panel in addition to standard inverter cost. For a 22-panel system, $1,100–$2,200 additional cost.

When worthwhile in Perth: Shading analysis is the key. If your roof has intermittent shading from trees, chimneys, or neighbouring structures affecting 2+ panels per string, optimisers typically pay back through recovered generation. If your roof is unshaded, optimisers add cost with minimal benefit.

Microinverters: panel-level AC conversion

A microinverter (Enphase, APsystems) is a small inverter mounted on each panel that converts DC to AC at the panel itself. There is no string — each panel operates completely independently.

Effect on shading: Complete independence. A shaded panel produces its maximum possible output; unshaded panels are completely unaffected. This is the maximum shade tolerance available.

Monitoring: Microinverter systems provide panel-level monitoring as standard — you can see the exact output of each individual panel.

Cost: 10–30% higher than a comparable string inverter system.

When worthwhile in Perth: Complex roof geometries with multiple faces and orientations, or significant tree shading. Also appropriate when you value granular monitoring.

Practical guidance for Perth installations

Assess shading before choosing inverter type: Ask your installer to conduct a shade analysis (physical assessment at your property, ideally with a solar pathfinder tool or drone-based shade mapping). A string inverter is the lowest cost and most appropriate choice for an unshaded north-facing Perth roof. Power optimisers or microinverters are justified by shading evidence, not by general "better technology" marketing.

Read your monitoring strings data: After installation, check your inverter's MPPT input data during clear days. If two identical strings show meaningfully different outputs on a clear day (>5% difference for more than a few days), investigate whether one string has a shading or fault issue.

String sizing matters: A string must stay within the inverter's MPPT voltage range across all temperatures. Perth's summer heat means panels at 60–70°C surface temperature — the panel voltage drops significantly. Ask your installer to show you the string sizing calculation for your system.

String wiring is the default for Perth string inverter systems and works well for unshaded roofs. Understanding that series-connected panels share a current constraint helps you interpret monitoring data and evaluate shade mitigations. The cost of power optimisers or microinverters is justified by specific shading, not by default.