The principal amount of the loan shall in no event exceed the cost of the energy conserving improvements including the purchase thereof, cost of installation, architect's fees, interest during construction and such other miscellaneous fees and charges incident to construction as determined by the Commissioner. Nor shall the principal amount of the loan exceed the lesser of the following:

(a) An amount which can be supported by residual income, which is the amount of net income remaining after payment of all existing debt service requirements and deduction of proprietary earnings, as determined by the Commissioner. The computation of net income shall take into account the amount which will be saved in operating costs over the period of repayment of the loan as a result of the installation of the energy conserving improvements.

(b) An amount which, when added to the existing outstanding indebtedness, does not exceed the Commissioner's estimate of the value of the project after the energy conserving improvements are installed.