System Loss Diagram
A System Loss Diagram is a visual representation of all energy losses that occur throughout a solar PV system—from solar irradiance striking the modules to the usable AC power delivered at the meter. It quantifies how shading, temperature, wiring losses, inverter efficiency, mismatch, soiling, and other real-world factors reduce a system’s theoretical maximum output.
In modern solar designing workflows, the System Loss Diagram is a core performance validation tool. It helps engineers and designers optimize layouts, improve performance ratio, forecast energy production accurately, and ensure realistic ROI projections across residential, commercial, and utility-scale solar projects.
It also plays a major role in proposal accuracy, enabling transparent and defensible production estimates inside professional solar proposals used by installers, EPCs, and sales teams.
Key Takeaways
- A System Loss Diagram maps every energy loss from sunlight to delivered AC power.
- Essential for accurate energy modeling, engineering validation, and proposals.
- Enables optimization across shading, temperature, wiring, and inverter performance.
- Critical for realistic ROI and defensible production estimates.
What It Is
A System Loss Diagram is a step-by-step breakdown of how energy is lost at each stage of a PV system’s conversion process. It typically starts with global horizontal irradiance (GHI), converts it into plane-of-array (POA) irradiance, moves through DC generation, inverter conversion, and ends at final AC output.
It is most effective when used alongside:
- Shadow Analysis for shading impact
- Solar Layout Optimization for tilt and azimuth decisions
- Stringing & Electrical Design for voltage and current behavior
Common loss categories shown include:
- Soiling loss
- Shading loss
- Module mismatch
- Temperature derate
- DC wiring loss
- Inverter clipping
- Inverter efficiency
- AC wiring loss
- System availability / downtime
A clear System Loss Diagram improves technical credibility, investor confidence, and customer trust.
How It Works
A System Loss Diagram works by starting with ideal theoretical solar energy and applying real-world loss factors step by step until final AC production is calculated.
Step-by-Step Flow
- Start with Solar Irradiance
- Global horizontal irradiance is converted into POA irradiance based on tilt, azimuth, location, and weather data—inputs defined during solar designing.
- Apply Environmental Losses
- Soiling losses
- Shading losses, identified using Shadow Analysis
- Snow losses (region-dependent)
- Apply Module Performance Losses
- Temperature derate based on cell temperature coefficient
- Spectral effects
- Module mismatch
- Degradation rate assumptions
- Calculate DC Output
- The system’s usable DC power is calculated under real operating conditions, not STC alone.
- Apply DC Wiring & Electrical Losses
- Losses depend on conductor sizing, cable length, and stringing & electrical design, often validated using the Voltage Drop Calculator.
- Apply Inverter Losses
- Inverter efficiency curve
- MPPT voltage window alignment
- Inverter clipping during peak irradiance
- Apply AC-Side Losses
- Transformer and combiner losses
- AC wiring losses
- System availability (maintenance and outages)
- Final AC Energy Output
- This final value feeds directly into solar proposals, financial models, and the Solar ROI Calculator.
Types / Variants
1. Basic System Loss Diagram
A high-level diagram showing major losses only. Common in early-stage proposals.
2. Detailed Engineering Loss Diagram
Includes inverter curve losses, nighttime tare losses, temperature-dependent behavior, and advanced derating factors.
3. Yearly vs. Monthly Loss Diagram
- Yearly: Long-term energy modeling and ROI forecasting
- Monthly: Seasonal analysis of shading, temperature, and irradiance
4. Bankable Loss Diagram
Used in financing and due diligence, with conservative assumptions aligned with industry standards.
How It’s Measured
Each loss is expressed as a percentage reduction relative to energy at that stage.
Common Measurement Units
- Irradiance: W/m²
- Energy: kWh
- Loss factors: %
- Temperature derate: %/°C
- Wiring loss: % or watts
Typical Formulas
Soiling Loss
[
E_{soiled} = E_{clean} \times (1 - L_s)
]
Temperature Derate
[
P = P_{STC} \times \left[1 + \gamma (T_{cell} - 25)\right]
]
DC Wiring Loss
[
P_{loss} = I^2 \times R
]
These calculations directly influence performance ratio and energy yield projections.
Practical Guidance
For Solar Designers
- Identify shading early using Shadow Analysis.
- Improve POA irradiance through solar layout optimization.
- Avoid clipping by validating DC/AC ratios with the AC Size Calculator.
- Minimize wiring losses using the Voltage Drop Calculator.
For EPCs
- Maintain consistent module cleaning schedules to control soiling losses.
- Reduce conductor losses with efficient routing and quality cabling.
For Developers
- Review all loss assumptions carefully when validating bankable energy forecasts.
- Align diagrams with financing-grade expectations.
For Sales Teams
- Use the System Loss Diagram inside solar proposals to clearly explain why real-world production differs from nameplate ratings.
Real-World Examples
Residential Solar (6 kW Rooftop)
A homeowner system shows 6% soiling loss, 9% temperature loss, and 2% wiring loss. Using shadow analysis, the installer repositions modules to eliminate tree shading, recovering 4% annual yield.
Commercial Solar (250 kW Carport)
The System Loss Diagram reveals inverter clipping caused by aggressive DC oversizing. Adjusting inverter capacity improves annual output and shortens payback.
Utility-Scale Solar (50 MW Tracker System)
A detailed loss diagram models snow, tracking inefficiencies, and availability losses—supporting bankable production estimates for financing.
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