This should have been picked up by your installer when they made the enquiry to the utility company prior to quoting. They should have then included the cost of the transformer upgrade in the quote, so that it would only attract 5% VAT, and be included in the grant fundable works (even though with a Proven 6 you would have 'maxed out' on the grant"

When an installer carries out a site visit, prior to quoting, grid connection capacity is one of the things they should be looking at to make sure that your proposed installation is viable.
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Windy Miller