Interesting. Personally my first thought would be to go by the footprint of the array. (which you have calculated.)
I would always say that for a de minimis exemption the applicant should provide justification why they think they de minimis applies - eg measurements, photos. If an LPA were not offered those I can see why they might use the total panel area as a proxy on a precautionary principle, but if the applicant had calculated the footprint of impact - the amount of ground over-sailed by the array and thus impacted by shading and rain shadow - then I'd accept that.
I would point out though that temporary impacts are not exempt, merely regarded as retained if they are reinstated within 24mths. So I'd be asking whether the final array footprint is genuinely the only impact or if construction impacts will cover adjacent habitat - groundworks, storage of materials such that the habitat score would be reduced. If that takes it above the de minimis then BNG is required for the whole area including the temp impacted area (it needs it 10% too), though won't be further penalised by regarding it as 'lost' and then 'created' again, with a further temporal multiplier.
