Why Is My Solar Production Lower Than Last Year? 9 Common Causes

If your solar production is lower this year than last, you’re not necessarily looking at a failed system — but you shouldn’t ignore it either. Solar panels do degrade over time, but at a predictable rate of about 0.5% per year. If your production dropped 10%, 20%, or more in a single year, something else is going on.

This guide walks through the nine most common causes of dropping solar production, in roughly the order we find them when inspecting systems across Southern California. Most are diagnosable from your monitoring app. A few require a physical inspection. All of them are fixable if caught early.

Quick Answer: The 9 Most Common Causes

1. Soiling — dust, pollen, bird droppings, or wildfire smoke residue blocking light
2. New shading — a tree grew, a neighbor built something, a new structure casts shade
3. Failed microinverter or optimizer — one or more panels offline
4. String inverter problem — error codes, capacitor failure, or efficiency loss
5. Monitoring error — the system is producing fine, but you’re not seeing the data
6. Panel degradation — normal aging, accelerated by hot rooftops
7. Wiring or connector issues — corroded MC4 connectors are a leading cause
8. Weather differences year over year — fewer sunny days, more wildfire smoke
9. Utility metering or NEM configuration change — what you bill for has changed, not what you produce

How Much Production Loss Is Normal?

Before diagnosing a problem, it helps to know what’s actually normal. Solar panels degrade — this is expected and built into the original system design.

Typical degradation rates:

• Year 1: Up to 2-3% drop from initial output (called “light-induced degradation” or LID)
• Years 2-25: Approximately 0.5% per year on most modern panels
• After 25 years: Most panels still produce 80-87% of original output

So a normal year-over-year drop is small — about half a percent. If you’re seeing a 5%, 10%, or 20% drop, that’s not normal aging. Something specific is happening.

It’s also important to compare apples to apples. Solar production varies year to year based on weather. A cloudy or smoky year can produce 5-10% less than a clear one, even with a perfectly healthy system. The most reliable comparison is multi-year averages, not single-year changes.

Cause #1: Soiling (Dust, Pollen, Bird Droppings)

The most common cause of unexpected production drops in Southern California is soiling — physical material accumulating on the panel surface and blocking light.

What it looks like:

• Gradual decline over weeks or months
• Affects all panels similarly (if it’s general dust) or just specific panels (if it’s localized like bird droppings or a clogged gutter overflow)
• More noticeable in summer (fewer rain events to wash panels clean)

Expected production loss: Industry research from NREL (National Renewable Energy Laboratory) finds that typical soiling losses range from 3% to 15% in dry Southern California climates. Heavy soiling from agricultural areas, dust storms, or wildfire smoke residue can push that higher.

How to confirm:

• Visually inspect the panels (or have someone with a drone or ladder do it)
• Compare production before and after a heavy rain — if production jumps 5%+ after rain, soiling was a factor
• Note: California’s coastal marine layer can also deposit a thin film that’s harder to see than visible dust

What to do: Professional solar panel cleaning with deionized water and soft brushes is the safe option. Avoid pressure washing (damages seals) and abrasive scrubs (scratches anti-reflective coating).

Cause #2: New Shading

Solar panels are extremely sensitive to shading. A small amount of shade on one panel can disproportionately reduce output of an entire string of panels (with traditional string inverters) or just that one panel (with microinverters or optimizers).

Common sources of new shading we see:

• A tree that’s grown 5-10 feet taller since the system was installed
• A neighbor’s new construction, ADU, or solar installation
• A roof addition or new HVAC unit on your own roof
• Antennas, satellite dishes, or solar tubes added after installation
• Vegetation growing up the side of the panels (vines, climbing plants)

How to confirm: Check your monitoring app at multiple times of day. If specific panels (with microinverters) show steep production drops during certain hours, shading is likely. If you have a string inverter, you’ll see the entire string drop during those hours.

What to do:

• Trim trees if possible
• Consider relocating affected panels (a major repair, but sometimes worth it)
• If shading is permanent and significant, replacing string inverters with microinverters or DC optimizers can recover much of the lost production

Cause #3: Failed Microinverter or Power Optimizer

If your system uses microinverters (typically Enphase) or DC optimizers (typically SolarEdge), each panel has its own dedicated power electronics. When one fails, that panel goes offline — but the rest of the system keeps producing.

Symptoms in your monitoring app:

• One or more panels showing zero production while others are normal
• Specific panels consistently underperforming neighboring panels
• Error codes or “communication failure” warnings for specific microinverter serial numbers

Expected failure rates: Enphase quotes microinverter reliability at over 99.5%, but failures still happen — typically due to heat stress, water intrusion at the connector, or manufacturing defects. With a 25-panel system, expect at least one microinverter failure over the 25-year life of the system.

What to do: Microinverters are warrantied for 25 years. A qualified installer can pull serial numbers, file a warranty claim with Enphase or SolarEdge, and replace the failed unit. Labor is typically not covered by the manufacturer warranty (only the part), so you’ll likely pay a service call fee.

Cause #4: String Inverter Problems

If your system uses a central string inverter (typically SMA, Fronius, or older SolarEdge), the inverter is a single point of failure. When it has issues, you lose production from all the panels connected to it.

Common string inverter problems:

• Capacitor degradation: Internal electrolytic capacitors dry out over 10-15 years, reducing efficiency by 5-10% before complete failure
• Cooling fan failures: Inverters that overheat throttle their output
• Ground fault errors: Often caused by moisture intrusion or insulation breakdown
• Display or communication failures: Inverter is still producing but you can’t see data
• Internal arc fault detection: Inverter shuts down as a safety response

How to confirm:

• Check the inverter display for error codes
• Compare output against the inverter’s nameplate rating (an inverter rated at 7.6 kW should produce close to that on a sunny day with a full panel array)
• Listen — failing inverters sometimes hum, click, or buzz audibly

Average string inverter lifespan: 10-15 years (compared to 25+ years for the panels themselves). If your system is approaching that age and production is dropping, the inverter is a likely culprit.

Cause #5: Monitoring Error (Your System Is Fine)

Sometimes production hasn’t actually dropped — you just can’t see it accurately. This is more common than people realize.

Common monitoring issues:

• Internet connection at the inverter or gateway has dropped
• Monitoring platform login has changed (especially after installer bankruptcies — SunPower’s mySunPower portal shut down in September 2024)
• A new router or Wi-Fi password change has disconnected the monitoring gateway
• The monitoring gateway hardware has failed
• Software updates have changed what data is displayed

How to confirm:

• Check whether your electricity bill matches what you’d expect — if you’re still being credited for normal export amounts, the system is producing fine
• Look at the inverter display directly (most inverters show real-time and lifetime production locally, separate from cloud monitoring)
• If your system is producing but the app isn’t showing it, you have a monitoring problem, not a production problem

What to do: Monitoring reconnection typically takes 1-2 hours and restores visibility without changing the underlying hardware.

Cause #6: Panel Degradation

Panels do degrade over time. This is expected and built into the original system design — but it’s almost never the cause of a sudden, large production drop. Degradation is gradual.

When panel degradation matters:

• Systems older than 15-20 years showing accumulated degradation of 8-15%
• Panels exposed to extreme heat (hot rooftops without ventilation can accelerate degradation)
• Panels that have suffered hailstorm damage, micro-cracking, or PID (potential induced degradation)
• Specific panels with manufacturing defects (a few brands had bad batches in the early 2010s)

How to confirm:

• Compare individual panel output (visible in microinverter and optimizer monitoring) against neighboring identical panels
• If specific panels consistently underperform by 10%+ compared to their neighbors, degradation or damage is likely
• Thermal imaging can identify hot spots and cell-level defects (we use this in our diagnostic inspections)

What to do: Most panels are warrantied for 25-30 years of production. If degradation exceeds the warranty curve, you may have a valid claim with the panel manufacturer. We help orphaned customers file these claims when the original installer is gone.

Cause #7: Wiring and Connector Issues

This is one of the most overlooked causes — and one of the most common in older systems. The MC4 connectors that join panels together are weather-sealed plastic connectors. Over 10-15 years of UV exposure, temperature cycling, and physical stress, they can:

• Corrode internally (especially in coastal Southern California air)
• Develop high-resistance connections that waste energy as heat
• Loosen and create intermittent connections
• Fail completely, taking a section of the array offline

Industry data: Field studies have identified MC4 connector issues as one of the leading causes of underperformance in residential solar arrays older than 10 years. Many systems have measurable production losses of 3-8% from connector degradation alone.

Other wiring issues we find:

• Rodent damage to wiring under panels (squirrels are particularly bad in residential settings)
• DC arc faults from damaged or improperly torqued connections
• Junction box water intrusion
• Loose terminations in the combiner box or inverter

How to confirm: Most monitoring apps don’t show connector-level issues. This requires a physical inspection — typically thermal imaging on a sunny day, plus visual inspection of all accessible connections.

Cause #8: Weather and Environmental Differences

If you’re comparing this year to last year and last year was unusually sunny, you may not have a system problem at all — just a less-sunny year.

Factors that significantly affect annual production in Southern California:

• Wildfire smoke days. California has had multiple major wildfire years in the last decade. Smoke can reduce solar production by 10-30% on heavily affected days.
• Marine layer / coastal fog. Coastal Southern California systems can lose 5-10% production in years with persistent June/July marine layer.
• Rainfall patterns. Heavy rain years often have lower total production due to cloudy days, but cleaner panels.
• Temperature. Counterintuitively, hotter summers reduce panel efficiency (panels produce slightly less when very hot).

How to confirm: Pull weather data from the National Weather Service for both years. Compare cloud cover, total sunshine hours, and wildfire smoke advisories. If last year was clearer, the difference is likely environmental.

Cause #9: Utility Metering or NEM Configuration Changes

Sometimes the issue isn’t your production — it’s your bill. People often conflate “lower production” with “higher electric bill,” but they’re different things.

Common bill-related causes that look like production problems:

• NEM 2.0 to NEM 3.0 transition. If you added a battery, moved to a new house, or modified your system after April 2023, you may have moved to NEM 3.0 — which credits exported energy at much lower rates. Same production, much lower credit.
• Utility rate increases. If electric rates go up 15% but you’re using the same amount of grid power at night, your bill goes up even with stable solar production.
• Time-of-use rate changes. SCE and SDG&E have shifted peak hours over the years. Production patterns that used to align with peak rates may no longer.
• Increased household consumption. New EV, new pool pump, new air conditioning use. Same solar production, more home demand.
• Inverter clipping. If your inverter is undersized relative to your panel array (common in older designs), it caps production on the sunniest days.

How to confirm: Compare your monitoring app’s actual kWh production year over year — not your electric bill. If production is stable but your bill went up, you have a billing/usage issue, not a system issue.

How to Diagnose Lower Production: A Step-by-Step Process

If your production is down, here’s the order to work through:

Step 1: Compare year-over-year kWh production (not dollar bill)

Pull your monitoring data for the last 2-3 years. Look at total annual kWh — not your electricity bill. If kWh is stable, your problem is not with production.

Step 2: Check for offline panels or strings

If you have microinverters or optimizers, look at the panel-level view. Any panels showing zero or dramatically lower production point to specific hardware issues.

Step 3: Check inverter error codes

Walk to your inverter and look at the display. Note any error codes, warning lights, or alerts. Photograph them.

Step 4: Visually inspect for soiling and new shading

From the ground, look up at the panels. Are they obviously dirty? Has anything new been built nearby? Has a tree grown?

Step 5: Check weather data

Was the comparison year unusually sunny? Was the current year smoky or cloudy? Adjust expectations.

Step 6: Get a professional diagnostic inspection

For any production drop that can’t be explained by steps 1-5, a physical inspection by a licensed independent technician is the answer. A diagnostic inspection includes:

• Visual panel inspection (panels, frames, wiring, mounting)
• Thermal imaging to find hot spots, failed cells, and connector issues
• String testing with calibrated equipment
• Inverter diagnostic and error log review
• Monitoring data analysis
• Written report with findings and prioritized recommendations

Common Questions

How much does a solar diagnostic inspection cost?

For systems in Southwest Riverside County, ElectriCare provides a baseline visual inspection at no cost. A full diagnostic with thermal imaging and written report is quoted upfront based on system size — typically a few hundred dollars and worth the cost when production losses are costing you significantly more annually in lost generation.

If my installer is out of business, can someone else still do this?

Yes. ElectriCare services systems installed by any company, including SunPower, Sunnova, Titan Solar, and Freedom Forever — all of which are bankrupt as of 2026. See our orphan solar help page for more on servicing systems with defunct original installers.

Can I check my own system without calling a professional?

Yes for the easy stuff — visual inspection for soiling, checking the monitoring app, looking at the inverter display for error codes, comparing year-over-year kWh. Get professional help if you can’t identify the cause, if there’s an active error code, or if production is significantly off (10%+).

Should I be worried about my system catching fire?

Generally no. Modern solar systems include arc fault detection that shuts the inverter down when it detects a problem. But damaged wiring or failed connectors can sometimes overheat. If you smell burning plastic, see scorch marks, or hear unusual buzzing from the inverter or any junction box, turn off the system at the AC and DC disconnects and call a licensed electrician immediately.

How often should I have my system inspected?

Industry guidance suggests every 3-5 years for systems under 10 years old, and annually for systems older than 15 years. If you’ve had measurable production drops, after a major weather event (hailstorm, severe windstorm, wildfire), or if your original installer is out of business, schedule one sooner.

Will my warranty be voided if I have a third party work on my system?

Equipment warranties (panel, inverter, battery) are typically tied to the equipment, not the original installer. A qualified, licensed installer like ElectriCare won’t void them. Workmanship warranties from the original installer may have specific terms — but if your installer is out of business, those are usually no longer enforceable anyway.

The Bottom Line

A drop in solar production is almost always diagnosable and fixable. The key is to act before small problems become big ones — a failed microinverter caught early costs a few hundred dollars to replace under warranty. The same failure ignored for years can lead to additional failures, lost production worth thousands of dollars, and eventually expensive system-wide repairs.

If you’ve seen unexplained production losses, the fastest path to an answer is a written inspection report from an independent, licensed technician who works on all brands. That’s exactly what ElectriCare does, and we’ve been doing it in Southwest Riverside County since 2000.