Responding to Adam McHugh's query, an early use of the concept is in Henry George, 1898, *Science of Political Economy*: "Value is but an expression of exertion avoided" (George, 1898, p. 249). The Federal Energy Regulatory Commission (FERC) has made good use of this concept, making electric utilities buy co-generated power from independent sellers at a price equal to "avoided cost." In each case the wording differs, but the concept is used. I can't guarantee George was the "first". Detail: George died in 1897; his *Science* book was published a year later. Mason Gaffney