Understand the distinction
A proposed saving usually depends on assumptions about the starting condition, household use and the planned work. Without those assumptions, a percentage can be difficult to interpret. A forecast is not a guarantee of an identical future bill.
Put it in your home notes
Ask for the baseline period, included changes and basis of the calculation. Distinguish energy quantity from monetary cost and retain any uncertainty or exclusions. Avoid comparing several estimates until their scope and units are clear enough to describe side by side.