Why your system is underperforming and how to fix it

This guide covers the most common causes of solar system underperformance in Australia: soiling, shading, inverter faults, poor original design, and degradation. It explains how to diagnose each cause and what to do about it.

Key findings

• Soiling is the most common cause. Dust, bird droppings and debris can reduce output by 5 to 25% depending on conditions.
• Inverters typically last 10 to 15 years, the most likely component failure in older systems.
• New shading from tree growth or nearby construction is a common and overlooked cause.
• The most useful comparison is this month versus the same month last year, controls for seasonal variation.
• Poor original design (sized for the roof, not your usage) is structural, maintenance can't fix it.

How to tell if your system is actually underperforming

Open your inverter app and compare generation this month with the same month last year. If output is down 15% or more without an obvious explanation, something is wrong. A sudden drop to near zero on a clear day points to an inverter fault or tripped breaker. A gradual decline over months suggests soiling, growing shading, or degradation.

The four most common causes

1. Soiling

Dust, pollen, bird droppings and coastal salt block sunlight. Losses of 5 to 25% are common, higher in dry inland areas, near agriculture, and on flat-pitch roofs. A professional clean typically restores 5 to 15% of lost generation immediately. Most sites benefit from one to two cleans per year.

2. New or growing shading

A panel shaded for two hours per day loses a disproportionate amount of annual output. With string inverters, one shaded panel can drag down an entire string. Trim the source where possible; otherwise microinverters or optimisers on affected panels limit the impact.

3. Inverter fault or failure

The inverter is the hardest-working component and the first to fail. Your inverter app will usually show an error code. A system showing zero output on a clear day with no error code: switch the AC solar breaker off and back on after five minutes. If the fault returns, call a licensed technician. If your inverter is older than 10 years, replacement may be warranted.

4. Poor original design

If your system was sized for your roof rather than your usage, it may never have generated what you needed. This is not a maintenance problem, it's a design problem, and the only real fix is upsizing. A performance review against your actual consumption will confirm whether the system was ever correctly sized.