Solar Panel Output Drop? How to Diagnose and Fix Efficiency Loss

You installed solar panels to save money on electricity. So why does your electricity bill look almost the same as before? Or why is your monitoring app showing output that seems lower than last summer?

Solar system underperformance is one of the most common homeowner frustrations — and it has a surprisingly logical diagnostic process. This guide walks you through every major cause, from the trivially simple (dirty panels) to the more complex (degraded cells, inverter issues), so you can identify the problem and know what to do about it.

Step 1: Verify You Actually Have a Problem

Before assuming something is wrong, establish a baseline. Solar output varies significantly with:

• Season — winter output in southern Australia can be 30–50% below summer output due to lower sun angles and shorter days
• Weather patterns — cloud cover varies year to year; a particularly cloudy month will show lower generation regardless of system health
• Consumption patterns — if you’re exporting less, it may be because your household is using more, not because you’re generating less

How to check accurately:

1. Open your inverter monitoring app (SolarEdge, Fronius, SMA, Enphase, Goodwe, etc.)
2. Find your total daily or monthly kWh generation — not consumption
3. Compare to the same calendar period last year
4. Correct for obvious weather differences if possible (check BOM historical solar radiation data for your area)

If you’re seeing a 10% or larger unexplained drop versus the same period in a prior year, you likely have a real problem worth investigating.

Step 2: The Soiling Check (Start Here — It’s Most Common)

Soiling (dirty panels) is the most common and most easily fixed cause of underperformance. Australian research consistently shows average annual soiling losses of 10–20%, with spikes above 25% after:

• Dust storms or strong inland wind events
• Extended dry periods (no rain to rinse even surface dust)
• Heavy bird season activity (nesting and roosting peaks in spring)
• Bushfire smoke events (fine carbon particulates settle on panels and resist rain washoff)
• Pollen season in high-pollen areas (Melbourne, Adelaide hills, coastal NSW)

Visual inspection: Go outside on a clear day and look at your panels from ground level. A thin uniform haze is fine (light dust). Heavy brown coating, bird-dropping islands, or visible debris accumulation are significant soiling events requiring cleaning.

Quick test: If panels are visually dirty, book or perform a professional clean, then compare output over the following 48–72 hours with comparable weather. If output recovers significantly, soiling was your problem — and it’s solved.

If output remains depressed after cleaning, continue to Step 3.

Step 3: Check for New Shading

Shading is the second most common cause of unexplained output drops, particularly on systems that have been running for 3–5+ years. Things that create new shading:

• Tree growth — trees that were clear of panels at installation may now be casting shadows for parts of the day
• New structures — a neighbour’s new pergola, second storey extension, or rooftop solar on an adjacent property
• Antenna, satellite dishes, HVAC units added to your own roof
• Bird nests under panels (create shading on portions of multiple panels)

How to check: Observe your roof at different times of day — particularly mid-morning and mid-afternoon — to see if anything is casting shadows on your panels. Shading even a small portion of a panel significantly reduces that panel’s output in a series-wired string.

If you find a new shade source that can be managed (e.g., a tree that can be trimmed), resolve it and monitor output over the following week.

Step 4: Check Your Inverter

Your inverter is the brain of your solar system — it converts DC power from panels into usable AC power and manages system operation. Inverter issues cause:

• Complete system shutdown (obvious — zero output)
• Reduced output throttling (less obvious — output lower than expected without apparent cause)
• Individual string or panel errors (MPPT issues in multi-string systems)

What to look for:

• Error codes or warning lights on the inverter display
• In your monitoring app, check if any strings or individual panels show zero output or significantly lower output than adjacent panels in the array
• Check inverter logs for any fault history over the past 30–90 days
• Verify the inverter hasn’t been throttled by your network distributor (in some states, DNSPs can impose export limits that change over time — check your connection agreement)

If you see error codes: Google the specific code with your inverter brand — most have published fault code databases. Many codes indicate transient faults (power grid fluctuation, brief overheat) that clear automatically. Persistent fault codes warrant a call to your installer or the inverter’s Australian support line.

Step 5: Check Individual Panel Performance (If You Have Module-Level Monitoring)

If you have microinverters (Enphase) or DC optimisers (SolarEdge), your monitoring app can show individual panel-level output. This is extremely useful for diagnostics.

Signs of individual panel problems:

• One or more panels consistently producing 20–40% less than adjacent panels in similar conditions
• A panel showing zero output while others are working normally
• Intermittent output drop on a specific panel correlated with time of day (suggests localised shading, not panel failure)

Panel-specific issues to investigate:

• Bird dropping hot spots — a dense dropping on even one cell creates a point shadow that can cause the cell to act as a resistive load, generating heat and damaging itself over time (“hot spot effect”)
• Physical damage — inspect panel surface for micro-cracks, which sometimes appear as spiderweb patterns visible in raking light or in thermal imaging
• Connector/cable issues — water ingress or mechanical damage to MC4 connectors can cause high-resistance faults

Step 6: Assess Degradation (For Systems 5+ Years Old)

All solar panels degrade over time — this is expected and is the basis of the performance warranty. Typical degradation rates for tier-1 panels are:

• Year 1: 2–3% (first-year light-induced degradation, LID — normal)
• Years 2–25: 0.3–0.7% per year

For a 10-year-old system, you’d expect 7–10% output reduction from degradation alone compared to Day 1. This is normal.

Abnormal degradation signs:

• More than 1% per year decline over 5+ years
• Visible yellowing or browning of cells (potential EVA discolouration — PID or cell damage)
• Any cell appearing significantly darker or lighter than neighbours (hotspot or bypass diode failure)

If you suspect abnormal degradation, arrange a thermographic (thermal) inspection — a thermal camera scan of panels under operation reveals hotspots and failed cells invisible to the naked eye. Some solar maintenance companies include thermal scanning in premium service packages.

Seasonal Adjustment: What’s Normal vs Abnormal

Season	Expected Output vs Peak Summer
Summer (Dec–Feb)	100% — benchmark period
Autumn (Mar–May)	70–80%
Winter (Jun–Aug)	45–65% (location-dependent; VIC/NSW lowest)
Spring (Sep–Nov)	80–95%

If your summer-to-summer (or autumn-to-autumn) comparison shows a 10%+ drop that isn’t explained by soiling, shading, or known weather differences, investigate further using the steps above.

When to Call a Professional

You should engage a CEC-accredited solar service technician when:

• Error codes persist after a standard reset
• Output remains depressed after cleaning and shading is ruled out
• You suspect physical damage to panels (cracking, delamination, discolouration)
• System is over 5 years old and you’ve never had a professional inspection
• You want a formal thermal imaging assessment
• You’re preparing to sell your home and want a documented system health check

Expect to pay $150–$350 for a professional inspection, depending on system size and what’s included.

Quick Diagnosis Checklist

Symptom	Most Likely Cause	First Action
Gradual decline over weeks/months	Soiling or seasonal variation	Clean panels, compare output
Sudden 50%+ drop	Inverter fault or string fault	Check inverter display/app for errors
Zero output	Inverter shutdown or complete string failure	Check inverter, check isolator switches
One panel/string low	Shading, soiling, or panel fault	Check for shading; clean; check module monitoring
Output okay in morning, drops midday	New afternoon shade source	Check for new obstructions to the west
Post-rain output lower	Mineral spotting from tap water on rain-wet panels	Check water source used; inspect for spotting

Conclusion

Most solar panel underperformance has a straightforward cause — dirty panels, a new shade source, or a minor inverter issue. Work through the checklist systematically before assuming expensive panel replacement is needed.

Start with cleaning. It solves the problem more often than you’d expect, and it’s the easiest, cheapest diagnostic step available.

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Last updated: April 2025. For system-specific diagnostics, consult your inverter manual and CEC-accredited installer.