Racking my tiny brain, as I have done all this from scratch.
Your export mpan will be a new number (not the same as your existing Mpan) issued, I believe by your energy provider and this is where the fun starts, they all seem to charge different amounts for installing and running an export meter, and some pay a higher rate than others for your export also.
Also your FIT provider has to be your supplier if you have an export meter fitted, and they all have different priced supply tarrifs to add to the mixing bag also.
You really have to crunch some numbers to work out which provider will work out the best for the customer.
When I looked into all this for a large system, even though the customer is exporting over 50%, with the added costs of running the export meter the customer would have been ÂŁ200 per year worse off.
I managed to get around it another way, Tiny brain worked well that day...