There are two ways that FIT payments are calculated.
The Generation Tariff
You get paid this fixed rate for the total amount of electricity you produce. There are different tariff rates according to the type of installation you have. You can find a detailed breakdown of tariff rates on the Ofgem website.
The Export Tariff
You get paid this fixed rate for the amount of electricity that you put back on the grid. If you don't have an export meter then your export amount is estimated to be 50% of what you generate (except for hydro installations at 25%). If you have an export meter, you will be paid for exactly how much you generate.