Seven months into owning my solar system, I noticed something I couldn’t explain. My Enphase Enlighten app showed a production figure for the past 30 days that was 14% below what my installer’s projection said I should be generating for that time of year. Weather didn’t account for it — Austin had been unusually sunny that September.
I spent a week assuming I was reading the projection wrong. Then I spent another week assuming September was just an outlier. By week three I opened the per-panel production view and found it immediately: panel 17 of 22 had been producing 31% below its neighbors for the entire month. The microinverter had quietly failed sometime in late August.
One warranty call later, it was replaced. But the experience made me realize how many solar owners would never have caught this — and how much production they’d silently lose over months or years.
Here’s how to tell if your system is actually underperforming, and what to do about it.
Step 1: Confirm It’s Actually Underperforming (Not Just Weather)
Before calling anyone, rule out weather. Solar production varies significantly with cloud cover, and a month that felt sunny can still have meaningfully more cloud cover than average.
Compare your production to the same calendar month last year (if you have the data) rather than the installer’s projection for the month. Year-over-year same-month comparison accounts for weather patterns better than comparing against a theoretical projection built on average irradiance.
The NREL PVWatts calculator lets you model what your system should produce for any month in any location based on long-term weather data. If your actual production is more than 10–12% below PVWatts’ estimate for that month, something warrants investigation.
Step 2: Check Per-Panel Data (If Available)
If you have microinverters (Enphase) or power optimizers (SolarEdge), open the per-panel view and look for outliers. Any panel producing more than 20% below its neighbors on a clear day is a signal — not definitive proof, but enough to flag for follow-up.
What your monitoring app shows you and how to interpret it is covered in detail in another post, but the short version: look for consistent underperformers across multiple clear days, not single-day anomalies that could be temporary shading.
If you have a string inverter with no per-panel data, your investigation starts at the system level — checking inverter error logs if available, and comparing string voltage readings to expected values.
Step 3: Check for New Shading
Trees grow. New structures go up. Something that didn’t shade your panels 18 months ago might now. Walk around your property in the early afternoon — 11am to 2pm — and look for anything casting shadow onto the panel array. Even a partial shadow on one panel in a string inverter system can drag the whole string’s output down.
Seasonal shading changes too. A tree that clears your panels in summer may shadow them in winter when the sun angle is lower. If your underperformance is seasonal, shading is a strong candidate.
Step 4: Check Inverter Health
For string inverter systems, the inverter’s display or monitoring portal typically shows error codes and event logs. Download the event history and look for fault codes, communication errors, or clipping events that correlate with your production dip.
For microinverter systems, the Enphase app flags communication errors at the panel level. A microinverter that “goes dark” in the monitoring data has likely failed — they don’t fail partially, they fail completely.
Step 5: Contact Your Installer or Manufacturer
If you’ve confirmed underperformance and can’t explain it with weather or shading, your next step is a service call. Most installers include some period of monitoring support in their contracts. Check your contract — “system monitoring” or “performance guarantee” language typically covers the cost of a service visit if underperformance is caused by equipment failure.
For equipment failures specifically (inverter, microinverter, panel), the warranty claim goes through the manufacturer. Your installer usually handles the warranty coordination as part of their workmanship agreement. Document everything: screenshots of monitoring data, dates when underperformance started, weather conditions for the relevant period.
The actual fix for my underperforming microinverter took 48 hours from my email to the installer to completed replacement. The detection was the hard part — and it only happened because I was looking at per-panel data with enough baseline context to recognize an anomaly.
Know your baseline. Check it periodically. The system pays for itself over 25 years, but only if it’s actually producing what it should.
— Allen
