Is your Perth solar system underperforming? How to check and what to do
Perth solar systems should produce predictable output based on array size and orientation. When generation falls below expectation, whether gradually or suddenly, there are systematic ways to diagnose the cause. This guide explains how to check solar performance, common underperformance causes, and how to escalate to your installer.
Perth's solar resource is consistent enough that a correctly sized, correctly installed system should produce output within a predictable range. When monthly generation is lower than expected, or drops suddenly, it's a signal worth investigating rather than ignoring.
What "expected output" looks like in Perth
A 6.6kW north-facing rooftop system in Perth should produce approximately:
| Month | Expected monthly generation |
|---|---|
| December (peak summer) | 950–1,100 kWh |
| June (winter low) | 500–620 kWh |
| Annual total | ~9,500–10,200 kWh |
These numbers assume a north-facing array at roughly 25–35° pitch, with no significant shading, no faults, and a system with 98%+ inverter efficiency. Your installer should provide a system yield estimate at point of sale based on your specific roof orientation.
For a system oriented differently or with split east-west panels, adjust downward: an east-west split at 6.6kW typically generates 8,000–9,000 kWh/year. A south-facing component lowers it further.
The first check: inverter monitoring data
Open your inverter's monitoring platform:
- Sungrow: iSolarCloud app or web portal
- Fronius: Solar.web
- SolarEdge: monitoring.solaredge.com
Check the daily energy yield graph for the past 30 days. Clear Perth days should show a characteristic bell curve from sunrise to sunset: roughly 8am to 6pm in summer, 7am to 5pm in winter. The peak of the curve should correspond to system output approaching the inverter's rated capacity on full sun days.
Red flags:
- One or more strings showing no output (flat line on a clear day)
- Daily total significantly below comparable clear days
- Faults or alerts listed in the event log
If the monitoring shows normal curves with normal daily totals, your inverter is operating. The apparent underperformance may be a billing or data interpretation issue (see below).
Comparing against Perth solar resource data
Perth's Bureau of Meteorology (BOM) publishes historical solar irradiance data. A rough rule of thumb:
- Each kWp of installed solar generates approximately 4.5–5.0 kWh/day annually averaged in Perth
- A 6.6kW system → ~29–33 kWh/day annual average
If your monitoring shows average daily generation well below this range (e.g., a 6.6kW system averaging 20 kWh/day year-round), that's a 30–40% deficit worth investigating.
Exclude winter months when averaging: June and July average significantly lower than annual mean (closer to 18–21 kWh/day for a 6.6kW north-facing system), which drags down the annual average. Compare like periods.
Common causes of solar underperformance in Perth
Shade growth
Trees that were small at installation time grow. A tree branch that reached 3m at installation is now 6m, and shades the lower-left string in the afternoon. This is among the most common causes of gradual output decline in Perth over 5+ years.
How to identify: Shade-related drops appear in the afternoon generation data. On monitoring, the afternoon portion of the daily curve drops off faster than it should relative to a clear-sky model.
Resolution: Trim or remove the offending branch. Check with local council if the tree is on the verge. You may need council approval.
Inverter thermal throttling
An inverter mounted on a west-facing external wall in direct afternoon sun in Perth can reach internal temperatures that trigger thermal derating: the inverter reduces output to protect itself.
How to identify: Output peaks normally until early afternoon, then drops more rapidly than the solar resource would suggest. Check if this coincides with the sun reaching the inverter location.
Resolution: Add shading over the inverter (simple shed or overhang), or relocate to a cooler position.
Soiled panels
Perth's dry season means panels can go 6–10 weeks without rain washing. Significant dust accumulation reduces output by 5–10%.
How to identify: Output is consistently lower than clear-sky model during the dry season, then partially recovers after rain events.
Resolution: Clean panels before each summer season (October) and again in winter.
Fault on one string
If one MPPT string has a partial fault (damaged connector, failed bypass diode, faulty panel), that string's output drops while the other MPPT string continues normally.
How to identify: In monitoring, if the inverter shows two MPPT strings, one will show lower or no output. Most monitoring platforms (iSolarCloud, Solar.web) display MPPT 1 and MPPT 2 output separately.
Resolution: Contact your installer for a site inspection. DC connector inspection and string open-circuit voltage measurement will identify the fault.
Inverter partial fault
An inverter operating with degraded electronics may produce output but at reduced efficiency, or may regularly drop to low-power mode. Some faults are not reported clearly in monitoring.
How to identify: No obvious string issue but output is consistently 10–20% below expectation across all weather conditions. Check the event log for recurring non-critical faults.
Resolution: Contact the inverter manufacturer's service partner for a diagnostic.
Checking your bill vs your monitoring
A common source of "underperformance" complaints is a mismatch between:
- The monitoring platform (which measures DC generation)
- The Synergy bill (which measures AC import/export at the meter)
The difference is normal: Your inverter converts DC to AC with ~98% efficiency. Monitoring measures generation before this conversion; your bill measures consumption and export after. The 2% gap is normal.
Larger gaps can indicate: A faulty export meter, an incorrectly configured meter, or a solar monitoring system that's not properly calibrated.
Escalation: what to ask your installer
If your checks suggest genuine underperformance, contact your installer with:
- A period of monitoring data showing expected vs actual output (export from iSolarCloud/Solar.web)
- The month you're comparing to and the clear-sky days in that period
- Any specific events (faults, alerts) visible in the monitoring log
Most installers will conduct a site investigation. For systems still under workmanship warranty (typically 5 years), this should be at no cost.
Perth's consistent solar resource makes underperformance relatively easy to detect. If your system is generating 20%+ below expectation on comparable clear days, there's almost certainly a diagnosable cause. Start with monitoring data, rule out shade and soiling, then escalate to your installer with specific data rather than a general concern.
Money-relevant figures in this article are checked against primary sources. Here’s how we check our facts.
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