Advice sought concerning a potential application for a large scale solar generating station: is the generating station threshold regarding 'capacity' considered to be the DC or AC figure.

The following advice was provided:
It is accepted that there is no definition of ‘capacity’ in either the PA 2008 or the Electricity Act 1989 (EA 1989), and therefore we are unable to provide a definitive answer to your question. However, we have provided the Planning Inspectorate’s section 51 advice, and we hope this information is of use to you.
We consider that the term ‘capacity’ relates to the sum of the ‘total installed capacity’ of a generating station and therefore refers to the ‘gross’ electricity generated in DC by solar farms, prior to being converted to AC. Our considerations and reasoning have been provided below.
It appears from the Parliamentary debate around the PA 2008 that the 50MW threshold was set at 50 (rather than a different figure) in order to correspond to the threshold for generating station consent under s36 EA 1989, which implies that the two thresholds should be considered in the same way. This is based on parliamentary discussion relating to the PA 2008, see Hansard, 16 Oct 2008: Column 827, comment by Lord Boyd of Duncansby: ‘the Infrastructure Planning Commission will deal only with generating stations producing above 50 megawatts, the same threshold as in the Electricity Act’.
In a previous Nationally Significant Infrastructure Project (NSIP) application under the PA 2008, the capacity threshold has been assessed as being the ‘gross output’, which we would therefore interpret for solar as the DC capacity. See, for example, paragraph 58 of the decision letter for the Progress Power Station, as follows: ‘The Secretary of State notes that discussion took place around the definition of “output” and whether this should be the gross output of the generating plant or the net output exported from the site after allowing for transformer and other losses. The ExA noted that it was important that the definition of output used is consistent with Directive 2009/31/EC and the CCR Regulations. The ExA argued that the purpose the Directive and CCR Regulations is to control emissions and that the scale of emissions is therefore determined by the gross output. The Secretary of State notes that the EA also considered that gross output should be used and therefore agrees with the ExA’s conclusion that a gross output definition would be consistent with the legislation’. This does not take into account transformer or other losses, which would seem likely to include any reduction of output lost in inversion from DC to AC for solar farms, for example. We also consider the gross output to be the same as total installed capacity, as this represents the sum of the output of the generating plant (in this case, solar panels).
The following statutory instruments also appear to be relevant:
Regulation 4 of The Developments of National Significance (Specified Criteria and Prescribed Secondary Consents) (Wales) Regulations 2016 (2016 No. 53 (W. 23)), which applies when determining whether a generating station will fall within the Welsh Developments of National Significance regime. This states that ‘“installed generating capacity” (“gallu cynhyrchu gosodedig”) means the maximum capacity of electricity generation (in megawatts) at which that generating station could be operated for a sustained period without damage being caused to it (assuming the source of energy used to generate electricity is available without interruption)’.
The definition of ‘total installed capacity’, within the Renewables Obligation Order 2015/1947 is very similar to the DNS definition of installed generating capacity: ‘ “total installed capacity”, in relation to a generating station or to generating capacity of any description, means the maximum capacity at which that generating station or generating capacity could be operated for a sustained period without causing damage to it (assuming the source of power used by it to generate electricity was available to it without interruption)’.
Whilst it is accepted that the above definitions are focussed on different statutory regimes, and so not conclusive, we consider them to be persuasive support for interpreting ‘capacity’ under section 15 of the PA 2008 as meaning the ‘total installed capacity’, or gross output of the generating plant. In the case of solar farms, we therefore consider this refers to the DC capacity. It is particularly noteworthy that the Welsh DNS definition is intended to apply to generating stations from 10 to 50MW, with those onshore generating stations above 50MW still being dealt with as NSIPs. This strongly suggests that this is how the PA 2008 capacity should be interpreted, in order to be consistent.
In your letter you queried which body is the enforcing authority for NSIPs, we consider the enforcing authority to be the local planning authority.