Periodic support payments accredited RHI installations for complex systems in respect of which an application for accreditation was made before 24th September 2013
65.—(1) This regulation applies to an accredited RHI installation in respect of which an application for accreditation was made before 24th September 2013 and which does not fall within regulation 64.
(2) Subject to regulations 69, 71 and 72, periodic support payments in respect of each quarterly period must be calculated in accordance with one of the following formulae, as applicable—
(b)where the accredited RHI installation is generating heat from the combustion of biogas, 
where—
A is the tariff for the installation determined in accordance with regulation 59;
B is the heat in kWhth used by the heating system of which the installation forms part during the relevant quarterly period for eligible purposes;
C is—
the heat in kWhth delivered in the relevant quarterly period to the biogas production plant which produced the biogas which is combusted (other than heat contained in any feedstock used to produce that biogas); or
such proportion (as may be chosen by the participant and agreed by the Authority) of that heat, provided that the proportion is no less than—
where—
x is the heat produced by that biogas which is used for eligible purposes; and
y is the energy content of all the biogas produced by that biogas production plant;
D is the heat in kWhth generated by the installation during the relevant quarterly period; and
E is the heat in kWhth generated by all plants supplying heat to the heating system of which the installation forms part in the relevant quarterly period.