We've done a number of energy-efficiency projects at our house over the last year. While it may help us lowering our own energy costs and usage, it'd be naive to think that it'll decrease overall energy consumption. Harry Saunders has done the research to explain this effect, know as "the rebound effect".
Varying degrees of rebound occur because the phenomenon works in several ways. Increasingly efficient technologies effectively lower the cost of energy, as well as the products and services in which it is embedded. This results in firms consuming more energy relative to other production inputs and producing more output profitably. Firms and individuals benefit from cheaper and more abundant products and services, causing them to find many more uses for these (and the energy they contain). A more efficient steel plant, for example, produces cheaper steel that, in turn, allows firms and individuals to afford to find more uses for the same material.