Diagnose Solar Production Drop

Goal

Diagnose why a solar system is producing less than expected. Customer notices drop on monitoring app + calls for diagnosis. Methodical investigation isolates cause.

Prerequisites

• Multimeter (DC voltage capable)
• Infrared camera (helpful but optional)
• Production history (from monitoring app)
• Pyranometer (irradiance meter) - for precise diagnostic
• 1-3 hours per investigation

Initial review (before going on-site)

1. Pull production history: today, week, month, year
2. Compare to design: was system producing at design level previously?
3. Weather correlation: was the drop on cloudy days only?
4. Time of day: drop at specific hours (morning, evening = shading)?
5. Inverter logs: any error codes, ground faults?

Symptom branching

Symptom 1: Sudden drop (yesterday OK, today not)

Cause 1: Equipment failure

• Inverter fault
• DC disconnect tripped
• Failed combiner / fuse

Diagnosis: check inverter display + status; check fuses.

Fix: replace faulty component.

Cause 2: Weather

• Cloudy / overcast day
• Compare neighbors' systems if possible

Wait for clear day; should recover.

Cause 3: Soiling event

• Pollen, dust storm, recent fire smoke

Fix: clean panels.

Symptom 2: Gradual drop over weeks/months

Cause 1: Soiling accumulation

• Dust, dirt, bird droppings building up
• Especially in dry climates

Fix: clean panels (see Maintenance SOP)

Cause 2: Shading changes

• Tree growth
• New construction blocking sun

Fix: trim trees; verify with customer; consider relocation (rare)

Cause 3: Panel degradation

• Some degradation is normal (0.5-1% per year)
• Excessive: faulty panels

Fix: identify under-performing panel; warranty claim if possible.

Symptom 3: Drop only at certain time of day

Cause: shading

• Morning shading from tree to east
• Afternoon shading from chimney/dormer

Solution: identify source; trim or remove if possible.

Symptom 4: One string of panels producing less

Cause 1: Cable issue

• Loose connection in string
• UV degradation of cable insulation
• Animal damage

Diagnosis: measure DC at each panel + at end of string.

Fix: replace bad cable, tighten connections.

Cause 2: Single bad panel

• Hot spot, internal damage
• Damage from hail (sometimes invisible)

Diagnosis: IR camera shows hot spot; or measure each panel individually.

Fix: replace panel (warranty if < 25 years old).

Cause 3: Bypass diode failure

• Internal panel diodes failed
• Single panel dragging down the string

Fix: replace panel.

Symptom 5: All strings producing equally low

Cause 1: Inverter issue

• Inverter not converting DC efficiently
• Inverter failure imminent

Diagnosis: measure DC in vs AC out; calculate efficiency (should be >95%).

Fix: repair or replace inverter.

Cause 2: Grid issue

• Inverter throttling due to high grid voltage
• Less common; check inverter logs

Fix: contact utility if grid voltage out of tolerance.

Symptom 6: Monitoring system shows no production but inverter shows production

Cause: monitoring system fault

• Lost internet connectivity
• App / server issue
• Hardware failure

Diagnosis: verify inverter is producing; check monitoring system separately.

Fix: re-establish internet; replace monitoring module if needed.

Diagnostic tools

Multimeter (DC):

• Test individual panel output (open-circuit voltage Voc)
• Test string voltage
• Test inverter input/output

Typical Voc: 35-50V per panel; 200-600V per string.

Clamp meter (DC current):

• Test string current
• Compares to design + other strings

Infrared camera:

• Identifies hot spots on panels
• Reveals internal damage invisible to eye
• thermal cameras adequate

Pyranometer:

• Measures actual solar irradiance
• Normalizes production to weather conditions
• Premium tool ()

Calculation: expected vs actual

System producing < 80% of expected after adjustment for:

• Weather (cloudy = less)
• Time of year (winter = less)
• Time of day (morning/evening = less)
• Soiling (dust = less)

= INVESTIGATE.

Expected daily output:

For 6kW system in mid-sunny region:

• Summer day: 30-40 kWh
• Winter day: 15-25 kWh
• Cloudy day: 10-20 kWh

Compare to monitoring app data.

When to call manufacturer

• Panel showing damage covered by warranty
• Inverter under warranty malfunctioning
• Mounting hardware failed
• Battery (if applicable) under warranty

Most panel/inverter manufacturers handle warranty directly. Document with photos + readings.

Production decline expectations

If a system is 5 years old + producing 85%, that's outside expected; investigate.

When customer is wrong about a "drop"

Sometimes customers misread monitoring data:

• Comparing winter to summer (drop is seasonal, normal)
• Today's production to a particularly sunny day (variation is normal)
• Monitoring app data is incorrect (server issues)

Educate without dismissing. Then verify whether real drop exists.

Common false alarms

• Cloudy day vs sunny day (huge difference)
• Smoke from wildfire reduces irradiance
• Snow on panels = 0% production (clear after melt)
• Heavy frost = reduced production (normal)
• App not syncing = appears as no production (data not real-time issue)

Always verify actual production at inverter before declaring a system problem.

References

• NABCEP diagnostic curriculum
• Manufacturer troubleshooting guides
• SEIA performance standards