Why Your Solar PV System Is Underperforming: The Impact of Shading on Solar ROI

You may have seen it in your solar proposal:

“Return on investment in 2–3 years.”

On paper, it looks compelling. In reality, many system owners later ask us the same question:

“Why isn’t my system producing what we were promised?”

Bills are lower — but not by as much as expected.

Production graphs fall short of projections.

And the ROI timeline quietly stretches from years into many more years.

In most cases, the issue is not the panels, the inverter, or even the weather.

It’s shading.

Shading: The Most Common (and Costly) Oversight in Solar Design

Shading is one of the single biggest factors affecting solar performance — and yet, it is often underestimated or oversimplified during the proposal stage.

Even partial shading can significantly reduce energy output. When left unaddressed, it directly impacts:

• Annual energy yield

• Bill savings

• ROI accuracy

• Long-term system performance

This is why at Stellar Industries, shading assessment is never treated as a checkbox exercise. It is a core part of responsible system design.

Why Shading Has a Bigger Impact Than Most People Expect

Solar panels do not operate in isolation. They are connected electrically in strings, meaning the performance of one panel can affect others within the same circuit.

This means:

• A small shadow can cause disproportionately large losses

• Production loss is often non-linear

• Losses repeat every day, every year — quietly eroding ROI

Even with Tier-1 panels and premium inverters, poor shading assessment can undermine the entire investment.

Roof Orientation: The Starting Point (Not the Conclusion)

Before evaluating shading, the first step is understanding roof orientation.

A simple way to assess this is by using satellite imagery (e.g. Google Maps) to observe roof direction and surrounding structures.

Example: Stellar Industries’ Rooftop

From satellite imagery, this roof is oriented north-west and south-east, which is generally favourable for solar production in Singapore.

However, orientation alone does not guarantee performance.

The sun’s movement — and how shadows interact with the roof throughout the day — is where real performance differences emerge.

Understanding the Sun’s Path in Singapore

The sun follows a path along the equator, rising in the east and setting in the west, with seasonal shifts north and south.

Because Singapore sits slightly north of the equator, this has practical implications for solar design.

South-Facing Roofs

South-facing arrays typically:

• Receive the most consistent sunlight throughout the year

• Perform strongly during midday, when solar irradiance peaks

• Deliver the highest annual energy yield

For this reason, south-facing roofs are often prioritised where feasible.

East- and West-Facing Roofs

East- and west-facing roofs:

• Receive sunlight for only part of the day

• Peak in the morning (east) or afternoon (west)

• Generally produces less total annual energy

These orientations can still be commercially viable — but only when shading is carefully analysed and system expectations are set correctly.

Common Sources of Shading We See on Rooftops

In real-world projects, shading most often comes from:

• Adjacent buildings (existing or future developments)

• Parapet walls

• Rooftop equipment (water tanks, HVAC units, vents)

• Trees

• Lift overruns or antennas

Some shadows appear only at specific sun angles, making them easy to miss without proper modelling. Others move slowly across the roof, affecting production daily without being visually obvious.

Can Technology Solve Shading Issues?

Technology can reduce shading losses, but it cannot correct poor design assumptions.

Common mitigation tools include:

Panel Optimisers

• Operate at panel level

• Reduce mismatch losses under partial shading

• Helpful, but not a substitute for good layout planning

Microinverters

• Each panel operates independently

• Shading on one panel does not affect others

• Effective for complex roofs, with higher upfront cost

Smart String Configuration

• Panels grouped based on similar shading behaviour

• Minimises losses without additional hardware

• Requires experienced system engineering

These tools improve outcomes, but they do not eliminate shading losses entirely. The most effective strategy is always avoiding shade wherever possible.

The Right Question to Ask Before You Install Solar

Instead of asking:

“How fast is my ROI?”

A better question is:

“How was shading assessed, and how conservative are the assumptions?”

A credible solar proposal should:

• Include shading analysis — not just system size

• Explain orientation trade-offs clearly

• Avoid overly aggressive ROI claims

• Be designed specifically for your site, not industry averages

Stellar’s Perspective

Solar is not a short-term product. It is long-term infrastructure.

When designed properly, a solar PV system should:

• Perform predictably for 25+ years

• Deliver stable savings year after year

• Meet expectations set at the proposal stage

When systems underperform, the issue is rarely the sun.

More often, it is:

• Shading that was underestimated

• Orientation that was oversimplified

• Design decisions driven by headline ROI instead of real-world conditions

At Stellar, we believe doing solar right means designing for reality — not best-case scenarios.

Because in the long run, honest assumptions outperform optimistic promises.