One of the more perplexing challenges in the conservation business is measuring energy savings from a retrofit. Difficulty arises because it is impossible to measure savings directly. It is only possible to measure differences in energy use. This distinction may seem minor, but inevitably usages must be adjusted for various changes in conditions that also occurred.

Most people agree that an estimate of the energy savings from a space heating retrofit should include an adjustment for the differences in weather. The most common adjustment for space heating retrofits involves degree-days. Yet even this simple procedure is clouded by the choice of a degree-day base and by the unfortunate fact that space heating energy consumption is rarely directly measured; the usage typically comes from the home's gas meter, which also may include water heating and cooking energy. Since water heating is often as energy-intensive as space heating, the assumptions behind the division are crucial. Other retrofits can be even more difficult to measure.

That's why it is good news when standard evaluation tools are updated and improved. An advanced version of one such tool, PRISM, is described in "Advancing the Art of PRISM Analysis" (see page 19). PRISM has established a common analytical procedure (not to mention common technical vocabulary) for quantifying energy savings due to retrofits. To be sure, it can't be applied to all situations, but it sets a standard to which others can aspire.

Measuring energy savings is not just an academic issue. The absence of reliable, documented estimates of energy savings from weatherization programs was a major factor in Congress' unwillingness to sustain funding several years ago. At that time, Congress required the Department of Energy to measure energy savings from these programs. More recently, billions of dollars of utility demand-side mangement programs are facing similar scrutiny by state regulatory commissions because utilites are trying to be more "competitive." No longer are regulators content with engineering estimates.

Energy savings measurements have valuable applications. They help establish priorities, assure quality of service, and tell us when to move on. The extra cost and time needed to measure energy savings may seem high, but it is money well spent.