With respect to any limitation on the amount of any loan which may be made, insured, or guaranteed under this subchapter for the purchase of a dwelling unit, the Secretary may increase such amount by up to 20 percent if such increase is necessary to account for the increased cost of the dwelling unit due to the installation of a solar energy system (as defined in subparagraph (3) of the last paragraph of section
1703
(a) of title
12) therein.