Daylight and Sunlight Assessment Solar Panels

Daylight & Sunlight Assessment Solar Panels

A Daylight and Sunlight Assessment for Solar Panels is essential to ensure performance and viability, especially in dense urban settings.

As the UK intensifies its efforts toward net-zero carbon targets, solar energy technologies like photovoltaics (PV) and solar thermal systems are playing a pivotal role in achieving more sustainable buildings. With planning policy increasingly focused on renewable energy generation and climate resilience, integrating solar panels into the built environment requires careful attention to their access to sunlight.

This article provides a comprehensive overview of how solar energy installations are evaluated within the context of daylight and sunlight planning assessments, based on the most recent BRE guidelines (BR209:2022).

The Importance of Sunlight for Solar Panels

Solar panels, whether photovoltaic or solar thermal, rely on uninterrupted solar radiation to function effectively. Any new development that overshadows existing panels or limits future potential installations can severely impact system performance. Therefore, assessing potential losses of solar radiation due to overshadowing is an important part of planning and environmental design.

Solar energy assessments are usually integrated into the broader Daylight and Sunlight Assessment, which also considers impacts on internal daylight for residents, the sunlighting of gardens and public spaces, and rights to light. BRE’s 2022 guidance expands on these principles by dedicating a section specifically to solar radiation loss for panels and passive solar buildings.

Overview of Solar Energy Systems and Overshadowing

1. Photovoltaic (PV) Panels

Photovoltaic systems convert sunlight into electricity. They are highly sensitive to overshadowing. The efficiency of PV panels depends on both direct solar radiation and diffuse sky radiation. Even partial shading of a single panel can lead to a disproportionate reduction in the energy yield, especially in systems using traditional ‘string’ inverters.

2. Active Solar Thermal Collectors

Solar thermal panels use the sun’s energy to heat water or air. Though slightly less sensitive to shading than PV systems, their effectiveness still depends on adequate solar exposure, especially during cooler months.

3. Passive Solar Buildings

These are designed to optimise natural heating by sunlight, typically through large south-facing glazed areas. Overshadowing such elements can undermine the building's energy efficiency.

Key BRE Recommendations: Assessing Loss of Solar Radiation

Minimum Acceptable Performance Ratios

These thresholds reflect the annual total solar radiation received with and without the new development, as outlined in Section 4.5.9 and Table 2 of the BRE Guide (BR209:2022). If the calculated ratio of solar radiation post-development falls below these advisory levels—0.90 for panel tilt angles between 0°–30°, 0.85 for 30.01°–59.99°, and 0.80 for 60°–90°—it indicates a substantial reduction in solar gain.

This could compromise the effectiveness of solar installations. Such outcomes are deemed material impacts under the BRE guidance and typically trigger a requirement for mitigation strategies—such as redesigning building massing, stepping back upper floors, reducing height, or reorienting the proposed development to limit shadowing.

In some cases, this could lead to planning refusal if the overshadowing is deemed unacceptable and cannot be reasonably addressed.

Annual Probable Sunlight Hours (APSH) Test

As an initial screening tool, BRE suggests calculating the Annual Probable Sunlight Hours (APSH) at the centre of each panel location. According to Section 4.5.8 of BR209:2022, if the APSH falls below 90% of its existing level following development, a more detailed assessment of annual solar radiation is warranted to determine if the impact is significant.

Considerations for PV Systems: Technology and Design

Shading Sensitivity and Inverter Technology

String Inverter Systems: In these setups, all panels are electrically linked. If even one panel is partially shaded, the output of the entire string can drop significantly.

Module-Level Power Electronics (MLPE): These mitigate the effects of shading by allowing each panel to operate independently. MLPEs include optimisers and microinverters and are recommended where shading is likely to vary across the system.

Optimal Orientation and Tilt

According to BRE, in the UK:

The ideal orientation for maximum irradiance is due south.

The optimal tilt angle is approximately 35°.

A south-east or south-west orientation may result in around 6% performance reduction, while east- or west-facing panels may see a 21% drop from the optimum.

Planning and Design Implications

1. Impact on Existing Solar Installations

When planning a new development near existing solar panels, a key consideration is whether it causes a significant loss of solar access. If the performance ratio falls below BRE's thresholds, the impact is considered material and could be grounds for refusal or require design alterations.

2. Safeguarding Solar Rights

While there is no absolute “right to sunlight” in planning law, the BRE guidance provides a robust technical framework for evaluating impacts. Increasingly, local authorities are acknowledging the material impact on renewables as a planning consideration.

3. Future-Proofing Solar Potential

Designers should ensure that new developments don’t compromise the solar potential of:

Adjacent rooftops

Neighbouring rear gardens (which may house ground-mounted panels)

Passive solar façades

It is good practice to set buildings back from the southern boundary to prevent overshadowing future installations.

How Daylight and Sunlight Assessments Help

A comprehensive Daylight and Sunlight Assessment for Solar Panels will typically include:

3D shadow studies at critical times (e.g., 21 March)

APSH analysis pre- and post-development

Site-specific solar radiation modelling (including both direct and diffuse light)

Assessment of the cumulative impact of multiple obstructions

Technical commentary on BRE guideline compliance

Specialist software is used, and results are compared to BRE’s advisory limits to assess acceptability.

Implications for Developers, Architects, and Consultants

Failing to consider overshadowing impacts on solar panels can:

Delay or derail planning applications

Trigger objections from neighbouring residents or businesses

Undermine the sustainability credentials of a scheme

Conflict with local and national energy targets

Therefore, early-stage integration of solar assessments into planning applications is essential. Collaboration between architects, planning consultants, and daylight/sunlight experts helps to avoid expensive redesigns and ensure policy compliance.

Conclusion - Daylight and Sunlight Assessment Solar Panels

The integration of solar technology into our built environment is vital for a low-carbon future. However, these technologies depend on long-term, unobstructed access to sunlight. A Daylight and Sunlight Assessment for Solar Panels ensures that both existing and future solar installations can operate efficiently and in harmony with new development.

By following the BRE 2022 guidelines and best practices outlined above, planners and developers can create schemes that balance density, aesthetics, and sustainability—without compromising solar energy performance.

Need Support with a Daylight and Sunlight Assessment?

At Anstey Horne, our experienced team of daylight and sunlight specialists provides expert reports for planning, neighbour negotiations, and solar viability. Whether you're installing rooftop PV or assessing a new residential scheme, we offer tailored support based on BRE best practice and real-world experience.

Contact us today to learn how we can help safeguard your solar investment and maximise the development potential of your site.

For more information on all aspects of Daylight & Sunlight Assessments for planning see the collection of articles on our blog page.

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