High Court rules on overplanting of solar panels – a win for practicality and reality?

In a recent High Court judgment, Mr Justice Eyre dismissed a challenge to a planning Inspector’s decision to grant permission for the Longhedge Solar Farm. The case provides important clarification on the interpretation of national policy regarding solar farm development and the practice of “overplanting” solar panels.

Background

The case concerned a proposal for a solar farm covering approximately 223 acres near the villages of Hawksworth and Thoroton. The development would have an export capacity limited to 49.9MW AC (just below the 50MW threshold that would classify it as a Nationally Significant Infrastructure Project (NSIP)), but with an installed DC capacity of 78.54MW – giving a DC/MEC ratio of 1.57.

The local planning authority had refused permission, but this was overturned on appeal. The claimant, a member of a local action group, then challenged this decision on several grounds.

The Key Issues

The case turned on three principal questions:

1. The proper interpretation of the National Policy Statement (NPS) for Renewable Energy Infrastructure (EN-3) (the version of which that came into force 17 January 2024), particularly regarding “overplanting” of solar panels at footnotes 91 and 92. It should be said that whilst the proposed development did not constitute an NSIP and therefore did not require policy appraisal against the NPS, the Inspector concluded that because the farm’s export capacity was proximate to the NSIP threshold, having regard to the NPSs was appropriate.
2. Whether the Inspector’s application of that policy was flawed and whether the likely level of overplanting was reasonable.
3. Whether the Inspector failed to (a) adequately address all issues raised and/or (b) properly assess the proposal on a “worst-case” basis.

What is “Overplanting”?

“Overplanting” refers to installing more solar panels than would be needed for a solar farm with a particular maximum AC capacity.

It is addressed in EN-3 at footnotes 91 and 92 as follows:

91 “The combined maximum AC capacity of the installed inverters may only exceed the aforementioned thresholds for the sole purpose of overcoming reactive power consumption within the solar farm between the inverters and the connection point.”

92 “Overplanting” refers to the situation in which the installed generating capacity or nameplate capacity of the facility is larger than the generator’s grid connection. This allows developers to take account of degradation in panel array efficiency over time, thereby enabling the grid connection to be maximised across the lifetime of the site. Such reasonable overplanting should be considered acceptable in a planning context so long as it can be justified and the electricity export does not exceed the relevant NSIP installed capacity threshold throughout the operational lifetime of the site and the proposed development and its impacts are assessed through the planning process on the basis of its full extent, including any overplanting.”

In this case, the developer proposed installing panels with a total DC capacity of 78.54MW, despite the export capacity being limited to 49.9MW (in AC terms).

The developer explained this overplanting was to address three factors:

• module degradation (panels degrade over time);
• the difference between laboratory performance (STC rating) and actual field performance; and
• site maximisation (to achieve maximum export capacity for more of the day).

The Court’s Decision

The Judge rejected all six grounds of challenge, finding that:

1. EN-3 does not prohibit overplanting for purposes beyond module degradation. The policy explicitly mentions overplanting to address degradation but does not imply that other forms of overplanting are unacceptable. The judge noted that overplanting for site maximisation is consistent with the objective of maximising renewable energy generation.
2. Footnote 92 of EN-3 does not impose a separate “reasonableness” test for overplanting. The word “reasonable” in the phrase “such reasonable overplanting” is simply describing overplanting to address module degradation as being sensible or appropriate.
3. The Inspector had sufficient information to assess the impacts of the development, despite some flexibility in the final design. The conditions imposed, including requiring details of panel siting to be approved by the local authority, were adequate to control the development.
4. The Inspector was aware of and considered the potential energy loss from “clipping” (reducing voltage to panels at times of peak generation to stay within the export capacity), noting that the effect and countervailing benefit of maximising energy generation formed part of his assessment of compatibility with EN-3.

Practical Implications

This judgment provides welcome clarity for solar farm developers on several points:

• overplanting is not limited solely to addressing panel degradation but can legitimately be used to address STC rating differences and site maximisation;
• there is no separate “reasonableness” test for overplanting beyond the normal planning balance;
• in any event, it is advisable for developers to explain to the local planning authority in the first instance the proportion of panel areas attributable to overplanting and the associated generation benefits; and
• a further reinforcement of the principle that the 50MW NSIP threshold relates to export capacity (AC), not installed capacity (DC).

The case also confirms that developers can legitimately design schemes to fall just below the NSIP threshold – the export capacity of the development having been deliberately limited so as to achieve this. The Judge stated that: “If there are two potential statutory regimes governing developments of different kinds there is nothing improper in an applicant so arranging matters as to ensure that a proposed development does not cross the threshold which would bring it under one of those regimes“.

This decision will be welcomed by solar farm developers seeking to maximise the efficiency of their sites while remaining within the TCPA regime rather than the more complex NSIP process. However, this case highlights that keeping under the threshold does not mean permission will come easily. Despite the 49.9MW output, the Inspector rigorously interrogated the proposed development as if it had been an NSIP (assessing compliance with National Policy Statements) – throwing shade over the presumption that the grant of permission is more easily come by under the TCPA regime.

It is also worth noting how the implications of this case might apply to larger scale onshore generation projects (50MW>100MW) once the NSIP thresholds are increased under The Infrastructure Planning (Onshore Wind and Solar Generating Stations) Order 2025, which is to come into force later this year.

Evolving case law in this area has also been examined in recent Planning Law Blog posts on the cases of Galloway v Durham County Council [2024] and Drayton Manor v Stratford-Upon-Avon District Council [2025].