You can't. But if you add together all the possible energy savings that all the different manufacturers claim, they total well over 100 per cent.
-- Some energy savings are independent of each other. Insulating one wall will not change the amount of heat lost through another wall or through a window, for example.
-- Some energy savings are not independent of each other. Improving your furnace may cut your heating bill by 20 per cent. Depending on how well your house is insulated, that could be 20 per cent of $1,500 or of $400. You could save either $300 or $80. They are both 20 per cent but they are not the same savings.
Percentage savings are valid, as long as nothing else changes.
To evaluate true savings, individual energy savings must be calculated in the proper order and then changed into dollars only on the bottom line. Calculate how much energy (not how much money) can be saved by sealing, insulating and ventilation heat exchangers. Subtract this from the present energy requirements of your home (which can be accurately calculated from your present heating bills). Then, subtract from this the percentage that can be saved by improvements to your heating system, giving you the energy needs of your rewinterized home. Convert this into dollars by looking at your new fuel costs. The difference between the new and the old is the true dollar savings you can expect from your rewinterized efforts. Unfortunately, it will never be more than 100 per cent of what you paid before.