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Solar Production Anomaly Detector

Why is your solar system underperforming? Enter expected vs. actual production and recent events — get ranked probable causes with likelihood percentages, diagnostic next steps, and repair cost estimates.

System & production details
kW
kWh
kWh
Recent events (check all that apply)
Production Anomaly Analysis
5.0% underperformance — 550 kWh/year gap
Expected production
11,000 kWh/yr
Actual production
10,450 kWh/yr
Production gap
550 kWh/yr
Annual value of loss
$83/yr
System age
4 years
Expected normal degradation
~2.0%
Ranked Likely Causes
#1 — Panel soiling / dirt / bird droppings
90% likely
Dust, pollen, and bird droppings reduce light absorption. Peak soiling in dry climates (AZ, CA Central Valley) can cause 20-25% loss. Usually easiest and cheapest fix.
Typical loss: 3–25%  | Fix cost: $0–$200  | DIY possible
Next step: Clean panels with soft brush and deionized water. Compare production before and after cleaning.
#2 — Normal panel degradation (~2.0% expected after 4 years)
40% likely
All solar panels degrade slowly. After 4 years your system should produce about 2.0% less than rated output. This is normal and covered by the performance warranty.
Typical loss: 0.5% per year  | Fix cost: No cost  | Professional required
Next step: Compare your actual production to your original installer's production estimate year-by-year. If degradation exceeds the warranty threshold (usually 0.5-0.7%/yr), contact manufacturer.
#3 — Monitoring system miscalibration or CT clamp error
35% likely
Current transformers (CT sensors) that measure production can drift over time. A miscalibrated monitoring system can show lower production than actual output.
Typical loss: 3–10%  | Fix cost: $0–$150  | DIY possible
Next step: Cross-check monitoring data against utility meter readings. Ask installer to recalibrate or replace CT sensors.
#4 — New shading (tree growth, new construction)
30% likely
A single branch shading one panel can reduce string output by 10-30% due to microinverter or optimizer design. Gradual shading from tree growth compounds each year.
Typical loss: 10–40%  | Fix cost: $200–$2,500  | Professional required
Next step: Inspect roof from street at different sun angles (9am, noon, 3pm). Trim offending branches or add optimizers/microinverters to each panel.
#5 — Individual panel failure (microcracks, hotspots, PID)
20% likely
Failed individual panels reduce overall output by 5-15% per panel. Potential-induced degradation (PID) affects some older panels. Visible as dark spots or physical damage on thermal imaging.
Typical loss: 5–20%  | Fix cost: $300–$700  | Professional required
Next step: Request an IV curve trace or thermal imaging inspection from your installer. Individual panel replacement covered under manufacturer warranty if failure is a defect.
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How to Use This Calculator

Enter expected vs. actual production

Your expected production comes from your installer's original production estimate (in the proposal or monitoring system baseline). If you don't have this, use NREL's PVWatts calculator (pvwatts.nrel.gov) — enter your system size and location and it generates an expected annual kWh figure. Your actual production comes from your monitoring app or from your utility bill's solar production credits line. Use the most recent complete 12-month period for both figures to eliminate seasonal distortions.

Check recent events that may explain the gap

The recent events checklist dramatically changes the ranked causes. If you've had roof work done nearby in the past year, scaffolding or tarps may have caused temporary shading that's now resolved. If you haven't cleaned panels in 6+ months and you're in a dry or dusty climate (Arizona, Southern California, Texas), soiling is almost certainly the primary cause. Extreme weather events can cause damage that monitoring misses — a cracked cell may still produce some power but with reduced efficiency.

Read the ranked cause analysis and next steps

The calculator ranks possible causes by likelihood given your specific inputs. Each cause shows the typical production loss range, DIY vs. professional fix, estimated cost, and a specific next step to confirm or rule out that cause. Start with #1 and work down the list — most underperformance issues are resolved by the top 2-3 causes.

The Formula

Production Gap % = (Expected kWh - Actual kWh) ÷ Expected kWh × 100 Annual Loss Value ($) = Gap kWh × $0.15/kWh (average rate) Normal Degradation = System Age × 0.5% per year Cause likelihood adjusted by: - Gap % size (larger gaps point toward electrical issues vs. soiling) - Recent events selected (cleaning gap → soiling, tree growth → shading) - System age (older systems more likely degradation or inverter failure) - State/climate (dusty climates amplify soiling likelihood)

The 0.5%/year normal degradation figure is from NREL's analysis of thousands of residential systems — most quality panels degrade between 0.3-0.7% per year. After 10 years, expect about 5% less production than rated output. This is covered by the performance warranty (typically guaranteeing 80-90% of rated output after 25 years). A production gap significantly above the expected degradation rate points to an active problem requiring investigation.

Example

15% underperformance — California 2019 system

Sarah's 10 kW system installed in 2019 was expected to produce 14,000 kWh/year based on her PVWatts estimate. Last year it produced 11,900 kWh. She hasn't cleaned panels in 9 months and noticed a large oak tree in her neighbor's yard has grown significantly.

Expected production14,000 kWh/year
Actual production11,900 kWh/year
Production gap2,100 kWh/year (15%)
Annual value of loss~$315/year
Normal degradation expected~3% (6 years × 0.5%)

Ranked causes for Sarah's system

#1 — New shading from tree growth (85% likely): The oak tree has grown to shadow 2-3 panels during morning hours. Each shaded panel in a string inverter system can drag down the entire string. Trimming the offending branches or adding per-panel optimizers would recover this production.

#2 — Panel soiling (70% likely): 9 months without cleaning in a California agricultural area. Professional cleaning runs $150-200 and can recover 8-15% production. Sarah should clean first to isolate the soiling contribution before investing in optimizer hardware for shading.

After cleaning ($180) and trimming the tree (~$400), Sarah recovers approximately $280/year in lost production — payback on the cleaning and trimming cost in under 3 years.

FAQ

A production gap under the expected degradation rate for your system's age is normal — no action needed. As a rule: 0-2% gap for a 1-3 year system is normal. 3-5% gap for a 5+ year system is likely within normal degradation. 8-15% gap warrants investigation — soiling, minor shading, or a failed panel. Over 15% is definitely an active problem requiring professional attention — inverter fault, significant shading, or multiple panel failures. Also compare to weather: a gap that appeared after an unusually cloudy or smoky summer may be weather-related and worth comparing against your monitoring platform's peer comparison tool.
Start by checking the inverter's LCD display or indicator lights on a sunny day between 10am and 2pm — this is when the system should be producing near its peak. A green light and production reading means the inverter is operational. Error codes on the display (typically showing Err followed by a number) indicate a specific fault — look up your inverter manufacturer's error code list online. If the inverter display shows no output despite strong sunlight, it may be in protective shutdown mode (common after grid disturbances) — try resetting it by turning off the AC disconnect for 5 minutes, then back on. If error codes persist or the inverter doesn't recover after reset, call your installer or a licensed solar electrician for diagnostic service.
Yes, in dry dusty climates. A 2011 Google study on their Mountain View campus system found soiling costs of 1-7% per month without cleaning. In agricultural areas (Central Valley California, parts of Texas and Arizona) with dusty winds, heavy pollen seasons, or bird roost locations, monthly soiling rates can reach 3-8%. Over a full year without cleaning, cumulative soiling can reach 20-25% production loss. In rainy climates (Pacific Northwest, Southeast), rain naturally cleans panels and soiling losses are typically under 5% annually. The cleaning frequency sweet spot depends on your local soiling rate — in most areas, once or twice per year is sufficient.
Potential-induced degradation (PID) is an electrical phenomenon affecting some older solar panels where high voltage stress between the panel and its frame causes ions to migrate within the cell structure, permanently reducing efficiency. PID typically causes 10-30% production loss in affected panels and is more common in hot, humid climates and on panels manufactured before 2015 with certain cell designs. PID cannot be detected by visual inspection — it requires electroluminescence testing (a specialized camera that images the cell structure) or an IV curve tracer. If your monitoring shows some panels in a string consistently underperforming while others in the same string are normal, PID on specific panels is a likely cause. Some panel manufacturers offer PID recovery devices (voltage-reversal units) that can partially restore performance.
Four reliable sources for your production baseline: (1) Your installer's original proposal — this should include a year-1 production estimate in kWh. (2) Your monitoring system's baseline — platforms like Enphase Enlighten, SolarEdge mySolarEdge, and SMA Sunny Portal show expected vs. actual production comparisons built in. (3) PVWatts by NREL (pvwatts.nrel.gov) — enter your system size, location, tilt, and azimuth for a free annual production estimate. (4) Your utility's net metering summary — some utilities track your system's cumulative generation and can tell you if it's within expected range. If your monitoring platform shows you're within 5% of expected, the gap may be within normal forecast error margins.

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