In planning for future energy expenditures, public agencies shall give priority to projects and resources which are cost-effective. Priority for future bond sales to finance energy expenditures by public agencies shall be given: First, to conservation; second, to renewable resources; third, to generating resources utilizing waste heat or generating resources of high fuel-conversion efficiency; and fourth, to all other resources. This section does not apply to projects which are under construction on December 3, 1981.