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What does overplanting a solar farm mean?
Overplanting a solar farm involves installing more solar panels than necessary for producing the capacity (in DC terms) that the site is authorised to export to the grid (in AC terms).
For the producer, overplanting makes perfect sense. Weather conditions in the UK are highly variable, so solar farms often fail to reach their peak output. Additionally, solar panels degrade and become less efficient over time. Installing more panels than necessary ensures reaching the 49.9MW export limit more consistently. However, the legal question is – is that allowed?
Ross v Secretary of State for Housing, Communities and Local Government and Renewable Energy Systems Ltd
On 19 May 2025, the High Court handed down judgment in Ross v SSHCLG, a test case concerning the interpretation of government policy on overplanting.
The case involved Longhedge Solar Farm, which covered a total site area of 94 hectares, with a developable area of 63.5 hectares. The Planning Inspectorate approved the application on appeal. It included several planning conditions regulating the physical extent of the development, but did not specify the number of panels or their power rating. One condition restricted power export to the grid to 49.9MWac.
However, the DC capacity exceeded the AC export capacity by a ratio of 1.57. This excess served three functions:
- offsetting panel degradation;
- reflecting that panels do not reach their laboratory condition rating in normal operating conditions; and
- maximising the time the site was capable of operating at its full AC export limit.
The risk of exceeding the 49.9MWac export limit was avoided by inverter de-rating, also known as ‘inverter clipping’. The Claimant agreed that the planning condition, which limits the AC export capacity of the scheme, meant the application correctly fell within the Town and Country Planning regime.
Overplanting a solar farm: Judgment
In his judgment, Mr Justice Eyre confirmed that the National Policy Statement for Renewable Infrastructure EN-3 (NPSRI) does not prohibit overplanting to compensate for degradation in panel efficiency. He also found that overplanting for other reasons was not inconsistent with the policy wording. Indeed, overplanting to account for actual site conditions was quite consistent with the policy’s aim of maximising renewable energy generation.
The learned judge rejected the argument that designing around a 49.9MWac export limit artificially constrains the development. In doing so, he endorsed the wording at paragraph 2.10.56 of the NPSRI:
“AC installed export capacity should not be seen as an appropriate tool to constrain the impacts of a solar farm.”