Why an Array Never Makes Its Rated Power

Every photovoltaic array produces less energy than the sum of its panel nameplates would suggest. This is not a defect; it is the cumulative effect of several physical and electrical losses that occur between the sunlight hitting the panel and the AC power at the meter. Understanding these losses—collectively called the derate factor—is essential for setting realistic expectations, diagnosing underperformance, and designing systems that meet their contractual obligations.

This guide explains each major loss, its typical magnitude, and how to identify when a loss is larger than it should be. It also covers the regulatory context that affects how output is measured and compensated, which varies by location.

The Derate Factor: A Summary

The derate factor is the ratio of actual AC output to the sum of the panel nameplate DC ratings under Standard Test Conditions (STC). STC assumes 1000 W/m² irradiance, 25°C cell temperature, and an air mass of 1.5. Real-world conditions rarely match these, and additional losses occur in the system. The table below lists the major loss components and their typical ranges.

Note: These ranges are typical, not guaranteed. Actual values depend on site-specific conditions.

Temperature Losses

Solar cells lose efficiency as their temperature rises. The temperature coefficient of power for crystalline silicon is typically around -0.4%/°C. This means that for every degree Celsius above 25°C, the panel’s output drops by about 0.4%. On a hot summer day, cell temperatures can reach 60-70°C, resulting in a 14-18% reduction in power compared to STC. This is often the largest single loss.

Design implication: In hot climates, panels should be mounted with adequate airflow behind them to reduce operating temperature. High-efficiency panels with lower temperature coefficients (e.g., some thin-film technologies) may be preferable, but they often have lower overall efficiency.

Soiling Losses

Dust, pollen, bird droppings, and other debris accumulate on the panel surface, blocking sunlight. Soiling losses depend on local conditions: arid regions with little rain can see 5% or more, while rainy climates may see less than 1%. Snow can also cause losses in winter.

Diagnosis: If output is lower than expected after a dry spell, check for visible soiling. Cleaning panels with water (and a soft brush if needed) can restore output. In some areas, professional cleaning services are available.

Mismatch Losses

Mismatch occurs when panels in a string have different electrical characteristics due to manufacturing tolerances, partial shading, or different orientations. The weakest panel limits the current in a series string, reducing the output of all panels. Mismatch losses are typically 1-3% for well-matched panels, but can be higher if shading is present.

Design implication: Use panels from the same manufacturer and model, and avoid mixing different orientations or tilts in the same string. Use power optimizers or microinverters to mitigate mismatch, but these add cost.

Wiring Losses

Resistance in DC and AC conductors causes voltage drop and power loss. The National Electrical Code recommends keeping voltage drop below 2% for feeders and 3% for branch circuits. In practice, well-designed systems keep total wiring losses below 3%. Using larger conductors reduces losses but increases cost.

Diagnosis: If wiring losses are suspected, measure voltage at the inverter input and compare to the open-circuit voltage of the array. A significant drop indicates excessive resistance.

Inverter Losses

Inverters convert DC to AC with an efficiency that varies with load. Peak efficiency is typically 96-98%, but at low loads (e.g., early morning), efficiency can drop to 90% or less. Inverter losses are accounted for in the derate factor, typically 3-8%.

Design implication: Choose an inverter with high efficiency across a wide load range. Oversizing the inverter relative to the array can reduce losses at peak output, but may not be cost-effective.

Shading Losses

Shading from trees, chimneys, or neighboring buildings can cause significant losses. Even a small shadow on one cell can reduce the output of an entire string. The impact depends on the shading pattern and the use of bypass diodes.

Diagnosis: Use a solar pathfinder or shading analysis tool to assess shading at different times of the year. If shading is unavoidable, consider using microinverters or power optimizers to minimize its impact.

Degradation

Solar panels degrade over time, losing about 0.5-1% of their output per year. This is a normal aging process. Most manufacturers warrant that panels will produce at least 80-90% of their rated power after 25 years.

Design implication: Account for degradation when estimating long-term production. This is not a loss that can be avoided, but it should be included in financial projections.

What People Get Wrong: The Misdiagnosis

A common mistake is to attribute low output to a single cause, such as inverter failure, when in fact it is a combination of several losses. For example, a system that produces 20% less than expected might be suffering from 10% temperature loss, 5% soiling, 2% mismatch, and 3% wiring/inverter losses. Each loss is within normal range, but together they add up.

This is a natural mistake because it is easier to suspect a single faulty component than to consider the cumulative effect of many small losses. However, proper diagnosis requires measuring each component’s contribution, which is rarely done. Instead, use monitoring data to compare actual production to expected production based on weather and system characteristics.

Regulatory and Utility Context

Net metering, export compensation, and interconnection rules are set by state and utility, and they change. These rules affect how your production is measured and credited. For example, some utilities credit exports at the retail rate, while others pay a lower wholesale rate. Some have time-of-use rates that affect the value of solar energy. Always check with your local utility and state regulatory body for current rules.

Diagnosing Low Output: A Step-by-Step Approach

1. Check monitoring data: Compare actual production to expected production for the day and time. Many systems have monitoring that shows real-time output.
2. Inspect for soiling: Visually check the panels for dirt, debris, or snow.
3. Check for shading: Look for new obstructions like tree growth or construction.
4. Measure string voltages: If safe, measure the DC voltage of each string at the inverter input. Compare to the expected open-circuit voltage. A significant deviation indicates a wiring or panel issue. Never open a combiner box or disconnect energized connectors. This should be done by a qualified electrician.
5. Review inverter errors: Check the inverter display or app for error codes or reduced output messages.
6. Consult a professional: If the cause is not obvious, hire a solar professional to conduct a full system audit.

Conclusion

Understanding the derate factor is crucial for evaluating solar system performance. By accounting for temperature, soiling, mismatch, wiring, inverter losses, shading, and degradation, you can set realistic expectations and diagnose issues accurately. Remember that local regulations affect how your system is compensated, so always verify current rules. With proper design and maintenance, a solar array can perform reliably for decades, but it will never produce its rated power under real-world conditions.