In considering the acceptability of a site proposed to accommodate ultimately additional power-generating capacity, the commission, in determining, pursuant to Sections 25514 and 25512, the conformity of the facilities proposed in the notice with the 12-year forecast of statewide and service area electric power demands adopted pursuant to subdivision (e) of Section 25305, shall base its determination only on such initial facilities as are proposed for operation within the forthcoming 12-year period. Additional facilities projected to be operating at the site at a time beyond the forthcoming 12-year period shall not be considered in the determination of conformity with the electric power demand forecast.