Solar developments and “overplanting”

The issue of “overplanting” was considered in the case of Ross v Secretary of State for Housing, Communities and Local Government and Renewable Energy Systems Ltd [2025] EWHC 1183 (Admin) where the court rejecting a statutory review challenge to an Inspector’s decision to grant planning permission, on appeal, for a 49.9MW solar farm in Nottinghamshire.

The primary issue for the court was the proper interpretation of the provisions of National Policy Statement for Renewable Energy Infrastructure EN-3 (“EN-3”). Whilst National Policy Statements are pertinent to the determination of Nationally Significant Infrastructure Projects (NSIP) under the Planning Act 2008, rather than the determination of applications for developments under the Town and Country Planning Act 1990, the Inspector had explained that because the capacity of the Solar Farm was so close to the 50MW threshold for a NSIP he considered that EN-1 and EN-3 were material considerations. The court agreed and the specific interpretation of EN-3 were the provisions in relation to the “overplanting” of solar panels.

To explain; solar panels generate electricity in direct current (DC) form. That is converted by inverters to alternating current (AC) for transmission to the National Grid. The inverters also apply voltage to the panels and the level of voltage applied determine the power measured in watts which would be generated by the panels at any given time. The DC capacity of the Solar Farm was anticipated to be 78.54MW. However, the AC capacity to be transmitted to the National Grid was to be limited to 49.9MW. The inverters are pre-set to ensure that the maximum AC capacity never exceeded 49.9mw. This limitation was to be achieved by “clipping” which was to take the form of reducing the voltage applied to the panels at times of greatest potential generation. The maximum DC capacity of 78.54MW would be the consequence of the proposed “overplanting” of panels. “Overplanting” is an arrangement whereby more solar panels are installed than would be needed for a solar farm with a particular maximum AC capacity. “Overplanting is to address three factors”.

• Solar panels degrade over time with the consequence that the electricity generated from a bank of a fixed number of panels will reduce over time (“module degradation”).
• The maximum output of a solar panel is determined in laboratory conditions (“the STC rating”) but the actual output of any given panel in the field will be less than in laboratory conditions.
• The combined effect of the configuration of the site and of fluctuations in the level of sunlight over the course of the day and of the year means that more panels would be needed to ensure an export capacity of 49.9MW for a greater part of the day and of the year (“site maximisation”)

Renewable Energy System’s evidence was that 16 – 22% of the proposed sizing was to address module degradation; 8% to address the difference between the STC rating of the panels and their actual performance; and 8 – 14% to the desire for site maximization.

EN-3 explains that Overplanting is required in order to take account of degradation in panel array efficiency over time (ie module degradation) and such reasonable overplanting should be considered acceptable in a planning context so long as it can be justified. No mention is made of the other two factors, STC rating and site maximisation.

The court found that the Inspector was correct to conclude that overplanting which went beyond that necessary to address module degradation was not inconsistent with EN-3; provided the overplanting was justified and account was taken of the full effects of the development (including overplanting).

The court rejected all six grounds of challenge, the first and second of which were the interpretation of EN-3 and reasonableness of the level of overplanting. The other grounds dismissed were:

• The alleged failure to control the density of panels or to assess the proposed development on a proper worst-case basis.
• The alleged failure to consider the wasted energy and its environmental consequences
• The adequacy of the EIA Screening Opinion
• The Inspector’s alleged failure to consider the Impact of the proposed development on Skylarks and Yellowhammers.

This is an important case for solar developments since it clarifies the position on overplanting in the context of interpretation of EN-3. It also provides an overview of how solar development applications should be the determined.