Costs & Savings
Most people make energy investment decisions based on installed cost and expected savings. Unfortunately, these are not as simple to predict as they might first appear. Cost estimates depend on the quality of materials, quality of the contractor selected, and many design details. Savings for a combination of energy efficiency investments usually measures less than for each measure taken individually. And certain decisions require a fairly elaborate consideration of local weather and weather variability. Therefore, always check cost and savings estimates from several sources before making your final decision. For example, you might contact local utility companies, your service technician and equipment suppliers (or contractors) to estimate the likely costs and savings associated with making energy improvements to your home.
Remember also that the "potential" savings of any measure will depend on the effect other measures have had on reducing energy consumption. Percent savings cannot be added together!
If, for example, a resident had used 1000 units of energy, and could implement "energy efficiency" Measure "A" to reduce that by 20%, Measure "B" to reduce it by 15%, and Measure "C" to reduce it by 10%, the combination of all three measures would not reduce energy use by 45%!
In fact, the 20% applied to 1000 units would save 200 units. The 15% would now be applied to 800 units, and save 120 units. The 10% would be applied to the 680 units and save 68 units. The total "potential" savings would be 388 units or 38.8% -- not 45%.
Remember as well that the potential savings and payback of any single measure depends on the order in which it is applied. For example, if Measure "C" were applied first, it could save 10% of 1000 units or 100 units. If it were applied last, it could save 10% of 680 units, or 68 units. What is the real "potential" savings of Measure "C"? The answer is that it depends on what other measures have already been installed.