Solar Panels Producing Less Than Expected: How to Diagnose Underproduction

Understanding Expected vs Actual Production

If your system is producing less than promised, you need to verify that the promise was realistic, then diagnose what's actually wrong.

How Solar Production Is Calculated

System designers use industry-standard models to predict annual production. Here's what goes into the calculation:

Key variables:

• System size: Measured in kilowatts (kW). A 10 kW system is larger than a 5 kW system
• Location: Solar irradiance (how much sunlight hits) varies by location. Arizona gets more sun than Oregon
• Panel orientation: South-facing panels in the Northern Hemisphere get more sun than east or west-facing
• Roof angle: Optimal angle is usually your latitude (40° in Colorado, 30° in Texas)
• Shading: Trees, buildings, and structures reduce production proportionally
• Panel efficiency: Modern panels are 18-22% efficient. Older panels might be 15-17%
• Inverter efficiency: Inverters are typically 95-98% efficient
• Losses: Wiring losses, soiling (dirt), and temperature derating reduce output

Calculating Expected Daily Production

Simple formula:

System Size (kW) × Peak Sun Hours × Efficiency Factor = Expected Daily Output

Example:

• System size: 10 kW
• Location: Denver, Colorado (approximately 5 peak sun hours per day average)
• Efficiency factor: 75% (accounting for inverter losses, wiring losses, soiling, and temperature derating)
• Expected daily output: 10 kW × 5 hours × 0.75 = 37.5 kWh per day

Peak sun hours explained:

"Peak sun hours" is not the same as daylight hours. It's the equivalent number of hours at peak intensity. Denver averages 5 peak sun hours daily across the year (more in summer, less in winter).

Checking Your Installation Paperwork

Your installer should have provided:

• System specification sheet with kW rating
• Production estimate (usually "expected annual production" in kWh)
• Month-by-month production breakdown
• Shading assessment and losses calculation
• Warranty documentation

If you have this paperwork:

• Find the annual production estimate
• Divide by 365 to get expected daily average (accounting for winter and summer variation)
• Compare to your actual production over the same time period
• If actual is 10%+ lower than expected, something is wrong

Comparing Apples to Apples: Weather-Adjusted Comparisons

The single biggest variable affecting short-term production is weather. A cloudy month will produce much less than a sunny month.

How to account for weather:

• Use same-month year-over-year comparisons to eliminate seasonal variations
• Use multi-month averages (average of last 3 months) instead of single-month data
• Use annual production (January through December) as your best measure
• Check local weather records or solar irradiance data for your location
• Some monitoring platforms show weather-adjusted performance metrics

Common Reasons for Underproduction

1. Excessive Shading (Most Common)

• Trees, buildings, or structures casting shadows on panels
• Shadows during peak production hours (9 AM - 3 PM) have largest impact
• Partial shading on one panel can reduce output from an entire string by 75%
• Check your roof throughout the day. Where are the shadows?
• If trees grew since installation, consider trimming or removal

2. Dirt, Dust, or Soiling (Second Most Common)

• Accumulated dirt can reduce output by 5-20%
• Look at your panels from a distance. Are they visibly dirty?
• More common in dusty climates, near construction, or agricultural areas
• Professional cleaning typically costs $150-$300 and can restore significant output
• ROI is positive if production loss exceeds 10%

3. Roof Angle or Orientation Is Suboptimal

• Panels facing north produce significantly less than south-facing
• If your roof slopes east-west instead of north-south, production is reduced
• Non-standard angles reduce production proportionally
• This should have been accounted for in the design estimate
• Fixing this would require reinstalling all panels—rarely cost-effective

4. Equipment Failures or Degradation

• Individual panel failures reduce output from that panel or string
• Microinverter or power optimizer failures reduce proportional output
• Faulty wiring reduces current and power
• Check monitoring for individual panel production if available
• Look for error codes in your system

5. Installation Errors

• If panels are wired in an unusual configuration, output might be limited
• If system size is smaller than quoted, production will be lower
• Check your installation paperwork to verify the actual system size
• Physical panel count should match your system size

6. Inverter or String Limitations

• If inverter is undersized for the panels, output is clipped on very sunny days
• This is intentional in some designs to reduce cost, but production is affected
• Check if clipping is occurring in your monitoring data

Step-by-Step Diagnostic Process

Step 1: Establish baseline

Find your expected annual production from installer paperwork. Divide by 365 to get average daily expectation.

Step 2: Gather actual data

Pull 3-6 months of actual production from your monitoring app. Calculate the average across the same months from previous years if available.

Step 3: Account for weather

Check historical weather or solar irradiance data for your location. Compare your data month to a known-good month to see if weather explains the difference.

Step 4: Look for obvious problems

Check your roof for shading or visible soiling. Check your system for error codes or alerts in monitoring.

Step 5: Contact your installer

Provide them with: (1) Expected production from paperwork, (2) Actual production data for comparison, (3) Dates when problems began, (4) Any observations about shading or dirt, (5) Error codes if visible.

What You Can Do About It

Easy fixes:

• Clean the panels (professional or DIY if safe)
• Trim trees or vegetation blocking sun
• Clear leaves, debris, or bird nesting materials

Moderate fixes:

• Add power optimizers or microinverters to reduce shading effects
• Increase system size by adding panels if space allows
• Replace failed components under warranty

When to push back on installer:

• If production is consistently more than 10% below expectations
• If the design was clearly flawed (known shading not accounted for)
• If equipment has failed prematurely
• Your installer may be able to claim under their performance guarantee