It’s probably true that energy improvements to Hall Elementary School in Portland would pay for themselves in a few years with energy savings.

But not if the building gets torn down first.

This is one example of the scenarios city officials have to work through before approving a $12.8 million renovation bond that promises to return $1 million a year in energy savings.

The program is a good idea. Ameresco, a Massachusetts firm, has conducted an energy audit on city and school department buildings with an eye at reducing the $8 million annual energy bill.

It would take about 13 years to pay off the loan, but even after paying interest and principal the city would still net $252,000 a year, reducing pressure on taxpayers.

Under the terms of the agreement with Ameresco, the company would cover any difference between its projections and actual energy savings.

But not all of the proposed improvements make sense, such as proposed investments in buildings that the city plans to tear down and retire.

This is a good time to match Ameresco’s proposed improvements with the city’s capital improvement plans to make sure that efficiency improvements are really efficient. Then the city should move forward with this ambitious and creative plan.

Energy efficiency will do more to reduce costs than any new power source or technology available. Portland should continue to refine this worthwhile proposal.