SolarIntermediate
Troubleshoot an Underperforming Array
Time
60–180 min
Steps
7
Pre-check
4 items
Skill
Intermediate
Scope
Diagnose why your solar production is below estimate, working from the safest checks to the most involved. Covers establishing a baseline, reading monitoring data and inverter error codes, finding new shading and soiling, verifying the AC side, and — only where safe — checking string voltage. Emphasizes the hard rule that PV DC wiring is energized whenever the sun is on the panels and cannot simply be switched off.
Safety
Read before starting
Photovoltaic modules produce dangerous DC voltage WHENEVER light hits them — there is no "off" switch for a panel. Never open or unplug DC connectors under load; doing so draws an arc that can cause severe burns and fire. Treat the rooftop as a fall hazard and never work the roof alone. Most of this guide is monitoring and visual inspection from the ground; only the optional voltage check involves the DC side, and it has its own safety gate.
Pre-Check
4 items · complete before you start0 / 32 complete
Steps
Establish the baseline — is it actually underperforming?
- Compare actual kWh over a recent period to the expected figure for the same season, not to nameplate.
- Account for weather: a cloudy stretch legitimately lowers output. Compare to a similar period last year if you have it.
- Expect seasonal variation — winter production is normally a fraction of summer, even on a healthy system.
- If actual tracks expected within ~10–15% once weather is considered, there may be no fault at all.
Read the monitoring data and inverter status
- Open the monitoring portal and check the inverter for fault or error codes — note any code and look it up in the manual.
- Compare panels or strings against each other: one panel/string flatlined or low while others are normal points right at the problem.
- Check whether the drop is sudden (a fault) or gradual (soiling, shading growth, or degradation).
- Look at the daily curve shape — a clean bell curve with a bite out of one side usually means shading at that time of day.
Tips
- Inverter and microinverter manufacturers publish error-code tables — match the code before touching anything.
Inspect for new or seasonal shading
- From the ground, look for shade that wasn’t there at install: grown trees, a new neighbor’s build, a new vent or antenna.
- Check at the time of day the production curve shows the dip — shadows move with the sun and seasons.
- On a basic string inverter, even one shaded panel can disproportionately cut the whole string.
- Note anything blocking the array; trimming vegetation is often the entire fix.
Check for soiling and clean safely
- Look for dust, pollen, bird droppings, leaves, or pollen film dulling the glass — soiling can cost several percent or more.
- Rinse from the ground with a hose and soft brush on a pole when possible; clean early morning or evening, never on hot glass (thermal shock) and never by walking the array.
- In dusty or low-rain climates, periodic gentle cleaning is normal maintenance, not a repair.
- Re-check production a day or two after cleaning to confirm the gain.
⚠ Warnings
- Do not pressure-wash panels or use abrasive pads — you can damage the anti-reflective coating and void the warranty.
Verify the AC side and inverter power state
- Confirm the inverter is actually on and not in standby/fault — check its display and status LEDs.
- Check the AC disconnect and the array’s breaker in the main panel; a tripped breaker stops everything.
- A microinverter/optimizer system that lost AC will have triggered rapid shutdown and produces nothing — restore AC and confirm it restarts.
- If the inverter won’t come out of fault after an AC-side reset per the manual, record the code; this is likely a service item.
Code notes
- NEC 690.12 module-level rapid shutdown means cutting AC de-energizes module-level electronics — useful for safe restart, but it also means "no AC" looks exactly like "no production."
Measure string voltage — only if trained and equipped
- This step touches the DC side. If you are not comfortable with high-voltage DC, stop here and call your installer.
- Measure DC string voltage at the inverter input terminals or a labeled DC disconnect — never by opening MC4 connectors in the field.
- Compare each string’s open-circuit/operating voltage to its neighbors and to the expected value; a string reading near zero or far low is your faulted circuit.
- A single dead panel in a module-level system shows in the per-panel data — use that instead of probing DC where possible.
⚠ Warnings
- Opening an energized DC connector or the combiner under load draws a sustained arc. This is the most dangerous mistake in solar service — it does not self-extinguish like AC.
Code notes
- NEC 690.7 governs maximum PV system DC voltage (commonly up to 600 V residential); 690.4 covers system installation and qualified-person expectations.
✅Continue Gate:Do you fully understand that PV DC conductors are energized in sunlight, that you must NEVER open or unplug DC connectors under load, and are you using a DC-rated meter on labeled terminals only? If not, do not proceed — call a pro.
Localize and document the fault
- Combine the clues: error code + which panel/string is low + when the dip occurs.
- Single panel low → likely a failed module, optimizer, or microinverter. Whole string low → string wiring, connector, or a string-level fault.
- Whole system low with no code → often soiling, shading, or a weather artifact you already addressed.
- Write down readings, codes, and photos before calling for warranty service — it shortens the visit.