Why a Solar System Sometimes Produces Less Energy Than Expected

Many homeowners expect a solar system to generate exactly the amount of energy stated in the brochure, this is almost never the case. This does not mean the system is performing poorly. A calculation is a model, while real life involves variable weather, overheating, equipment failures, engineering mistakes, dust on panels, and many other small factors that affect energy production.

On average, solar systems operate at about 70-85% of the rated capacity. Anything lower is a reason to contact support. Anything higher is rare.

Let’s look at where the energy goes.

How Generation Is Calculated

Theoretical Maximum

Two main parameters are used to estimate the potential of a solar installation:

• Solar irradiance on the plane of array (POA) – this accounts for system orientation, tilt angle, and location
• Nominal panel power under standard test conditions (STC)

The product of annual irradiance H_POA and installed capacity P_DC defines the theoretical energy output.

Performance Ratio and Energy Ratio

Two indicators are used to evaluate real system performance:

Performance Ratio (PR) – the ratio of actual energy output to the calculated value. PR is measured while the system is operating and answers the question:

“How efficiently is the system performing at this moment?”

With proper maintenance, 85% is a realistic target, although PR can be lower even when all requirements are met.

Energy Ratio (ER) – the ratio of total actual energy output to the calculated value, including downtime and failures. ER answers a different question:

“How much energy did the system actually produce over a period compared to the model?”

Now let’s examine the reasons why these indicators may decrease.

1. Shading

Nearby trees, adjacent buildings, and rooftop structures can cast shadows on solar panels, reducing their performance and overall energy output.

Shading even a small portion of a module limits the current that can flow through the string. In other words, if even part of a panel is shaded, the output of the entire string drops. Modern modules are equipped with bypass diodes that allow shaded sections to be bypassed, but the output from those sections is lost. Modules with microinverters or panels with optimizers can further reduce losses, but shading should still be avoided whenever possible.

The design must account for seasonal changes in the Sun’s position, as the same elements can cast shadows at different times of the year. To address this, engineers model the Sun’s path and shading in specialized software in advance. If panel rows are placed too close together, they can also shade each other—particularly during the morning and evening hours.

Shading is the most variable factor. With proper design, losses of 5-7% are considered acceptable. However, its impact strongly depends on site conditions and can reach tens of percent in poorly designed systems. Here are a few examples:

• Local shading (branches, antennas) – 2-5% losses when bypass diodes are present
• Long shade crossing a row of panels (e.g., from a pipe or parapet) – 5-15%
• Major design error (under a tree or near a tall building) – losses of 30-40% of expected generation

2. Soiling

Panel surfaces accumulate dust, sand, leaves, salt (in coastal areas), and bird droppings. As a result, part of the incoming sunlight is blocked and energy production decreases. In a real project in Malaga (Spain), soiling losses reached 4.4%. However, the impact increases significantly during dry periods. On average, in regions with long dry seasons, soiling causes around 5% output drop, while in areas with frequent dust accumulation, losses can reach 6-7%. In the same Spanish project, during extremely dry conditions, production dropped by up to 20%.

In rainy regions, losses are typically around 2%. Cleaning once a year reduces losses from 1.9% to 1.5%, twice to 1.3%, and three times to 1.2%.

Recommendations:

• Inspect and clean panels regularly, especially after dust storms or heavy leaf fall
• In dusty areas, arrange frequent cleaning, either independently or through a service provider
• Note that dust accumulates more on horizontal or low-tilt surfaces – this is one reason panels are installed at an angle

There are also panels with so-called self-cleaning coatings – special glass treatments that reduce (but do not prevent) dust accumulation and make it easier for rain to wash it off.

3. Overheating

The electrical power of a solar panel decreases as cell temperature increases. The temperature coefficient for silicon modules is about -0.35 to -0.4% per °C. This means that for every degree above the standard test temperature of 25°C, the panel loses up to 0.4% of its power. For example, at 60°C losses reach 12-14%, and at 75°C – about 20%.

In hot climates, rooftop temperatures can easily exceed 60 °C. When that happens, solar panels don’t perform at their peak, even if the sun is just as strong. That’s why systems often produce a bit less energy during the hottest months compared to cooler periods.

Newer panel technologies like TOPCon, HJT, and IBC handle heat much better. They have a lower temperature coefficient (around −0.25 to −0.30% per °C), which means they lose less efficiency as temperatures rise.

Light-colored roofing and ventilation gaps under the panels help reduce operating temperature.

4. Design Errors

Incorrect Orientation and Tilt

In tropical climates like Phuket, panel orientation plays a much smaller role than it does in more northern regions. Even if the system is rotated 90° away from the optimal direction—say, facing east or west instead of south—annual output typically drops by only about 10–15%. And unlike in higher latitudes, there’s very little difference between north- and south-facing panels here—both can deliver near-maximum production.

That’s why, in practice, engineers focus first on placing panels where they get the most unobstructed sunlight. If the roof orientation isn’t ideal, the solution is usually simple: adjust the layout or add a few extra panels to make up the difference.

Incorrect String Configuration

Each string of solar panels should contain the same number of modules. If one string has fewer panels, or if the panels differ in power, the entire system operates less efficiently and some energy is lost.

Incorrect DC/AC Ratio

The inverter must be properly sized relative to the array. Poor sizing can lead either to inverter clipping (when DC power exceeds inverter capacity and excess energy is curtailed) or to operation at low load, where efficiency decreases.

Overestimated Assumptions in Models

Using unrealistically high PR values (e.g., 90–95%) in proposals creates inflated expectations. A realistic value for a well-maintained system is 75-85%.

Electrical Losses

Losses in Cables and Connections

Cable losses: due to electrical resistance, part of the energy is lost as heat. Typical losses are around 2%, and with thicker cables and shorter runs – about 1%.

Connection losses: poor-quality contacts, fuses, and diodes can add up to 0.5% additional losses.

Inverter Efficiency

The inverter converts DC into AC with a certain efficiency. In real operation, modern inverters typically achieve 95-97% efficiency, corresponding to 3-5% losses.

Module Degradation and Mismatch

Over time, panels lose performance. At the beginning of operation, there is a small initial drop of 0.5-1.5% (light-induced degradation, LID). This is followed by annual degradation of about 0.5-1% per year.

Downtime

No engineering system operates 100% of the time. Some annual energy is lost due to inverter shutdowns, grid failures, repair delays, and maintenance activities.

Example Calculation

Below is an example calculation of total losses for a residential system in Southern Thailand. All values are indicative:

Adding typical values (e.g., 5% shading + 3% soiling + 10% overheating + 2% mismatch + 1.5% wiring + 5% inverter + 2% downtime) gives total losses of about 28.5%. This corresponds to a PR of 71.5%. Under better conditions (less shading, improved ventilation, higher inverter efficiency), PR can realistically reach 80-85%.

When Deviations Are Normal

Weather is never perfectly predictable, so actual energy production will always vary from estimates. Real solar irradiance can differ from long-term averages. During the cloudiest months—typically September to October—performance may drop noticeably, while in the dry season from December to March, systems tend to perform at their best.

As a rule of thumb, a PR in the range of 75–85% indicates a system is operating normally. A drop to 60–75% may point to issues such as shading, soiling, or overheating. Anything below 60% usually signals a more serious problem, such as an inverter fault or a disconnected string.

Conclusion

No solar power system converts 100% of available energy. Losses from shading, soiling, overheating, wiring, inverter inefficiency, and downtime collectively reduce performance to 70-85% of the theoretical potential. Models that ignore these factors can mislead system owners.

Professional system design always includes:

• Detailed shading analysis using 3D modeling
• Selection of optimal orientation and tilt
• Realistic PR assumptions (70-85% depending on conditions)
• Proper inverter sizing and use of high-quality cables and connections
• Consideration of soiling and overheating (adequate ventilation gaps, easy access for cleaning)
• Monitoring setup and regular maintenance

Following these principles helps minimize underperformance, improve financial outcomes, and avoid unrealistic expectations.

‍