How solar panel strings and MPPT work: a Perth buyer's guide

When reviewing a solar quote in Perth, you may see references to "two MPPT inputs", "one string of 12 panels", or "east-west split strings." These terms describe how panels connect to the inverter and how the inverter manages their output. Here's what they mean and why they matter for your Perth system.

What a string is

A string is a group of solar panels connected in series — positive terminal of one panel to the negative terminal of the next. Think of it like batteries in a TV remote connected end-to-end.

When panels are connected in series:

• Voltages add — a string of 12 panels at 40V each = 480V total string voltage
• Current stays the same as the lowest-current panel in the string
• The string behaves as a single electrical source to the inverter

A typical Perth residential system might have one string of 13 panels (one string into one MPPT input), or two strings of 6–7 panels each (one string per MPPT input, used for east-west split or two roof sections).

What MPPT is

MPPT (Maximum Power Point Tracking) is the control system in the inverter that extracts maximum power from each string.

Solar panels have a characteristic where maximum power is delivered at a specific combination of voltage and current — the "maximum power point." This point shifts continuously as irradiance changes (clouds, time of day) and temperature changes (cooler panels = higher voltage).

The inverter's MPPT circuit continuously adjusts the electrical load on the string to stay at the maximum power point, effectively "following" the optimal extraction point across the day. Without MPPT, the inverter would extract power at a fixed voltage and miss the peak output.

Most Perth residential hybrid inverters have 2 MPPT inputs — meaning two separate strings can be managed independently.

Why MPPT inputs matter for Perth roofs

Single orientation: one MPPT is fine

If all your panels face north at the same tilt angle, they all perform similarly throughout the day. A single string into one MPPT, or two strings into two MPPTs (for larger arrays), works correctly.

Mixed orientations: separate MPPTs are essential

If your system has panels on an east face and a west face — a common Perth design — they perform very differently. East panels peak in the morning; west panels peak in the afternoon.

If east and west panels are connected into the same MPPT string, the inverter has to compromise — it can't simultaneously track the morning east peak and the afternoon west peak. Output is significantly reduced.

Correct design: East panels go into MPPT 1; west panels go into MPPT 2. Each MPPT tracks its panels independently. This is why installer quotes for multi-orientation systems will specify separate strings per MPPT.

String length limits

Every inverter has voltage and current limits for each MPPT input. Panel specs list the open-circuit voltage (Voc) and short-circuit current (Isc). The installer designs string length so the string voltage stays within the inverter's operating range across Perth's temperature range:

• Cold morning = higher panel voltage — string design must ensure voltage doesn't exceed inverter maximum at minimum Perth temperature (~10°C on cold winter mornings)
• Hot summer noon = lower panel voltage — design must ensure voltage stays above the inverter's minimum MPPT range even at peak Perth temperatures

This is why string sizing is a design calculation, not a guess — CEC-accredited installers in Perth are trained to do this correctly.

Series vs parallel

Most Perth residential systems use series strings (higher voltage, lower current, thinner cables). Some larger systems use parallel connections to keep string voltage in range while achieving higher current.

For a standard Perth home (6.6–10kW), series strings are the norm. You typically won't encounter parallel connections in small residential systems.

What shade does to a string

This is the most practically important aspect of string design for Perth buyers with any roof shading:

When one panel in a series string is partially shaded, it limits the current of the entire string — because series connections share current. A string of 12 fully-lit panels can be reduced to the output of the one shaded panel's current level.

Most modern panels include bypass diodes that allow current to flow around a shaded cell group, limiting (but not eliminating) the impact. Some installers quote panel-level optimisers (SolarEdge S-Series, Tigo, Ampt) to address this more effectively.

For Perth homes with minimal or no shading — the majority of Perth suburban homes — this is largely theoretical. But for homes with partial tree shade, neighbouring buildings casting shadows, or chimneys, it is a real design consideration.

Questions to ask your Perth installer

1. How many strings and MPPT inputs does the proposed system use?
2. Are east and west panels on separate MPPT inputs?
3. Is the string voltage within the inverter's operating range at Perth's minimum and maximum temperatures?
4. If there is any shading, how does the design address the bypass diode behaviour?

A well-designed Perth string layout maximises annual output. The calculations are routine for CEC-accredited installers, but it's worth confirming the design matches your roof's orientation before accepting a quote.

Strings and MPPT inputs are the invisible plumbing of a solar system. For most Perth unshaded north-facing roofs, a straightforward single-string design works well. For east-west split roofs or any system with multiple orientations, separate MPPTs per orientation are essential. Ask your installer to explain the string design for your specific roof before signing.