Any subsidy granted under this section shall not in any financial year of an Electrical Supply Authority exceed half the amount which the Council, having regard to the capital cost of the electric line or the portion of the electric line in respect of which the subsidy is granted, considers is the annual cost of supplying electricity to the consumers served by that electric line or portion thereof. No subsidy shall be granted in respect of the whole or any portion of any electric line after the expiration of twenty years from the date of the payment of the first subsidy under this section in respect thereof.”