Energy efficiency is one of the more economically sounds way to sustainability. Real Estate accounts for 30% of energy consumption in most developed countries. It also accounts for more than 30% of GHG emission. In the last several years many research have been looking to energy efficiency technology. Very few of them include the future energy price forecast in their valuation. Very few of them include the fact that different energy source savings may have different value. This paper demonstrates that one euro of annual energy savings in different source has a very different value when we account for expected future cost. It also underlines the fact that energy source prices are more or less volatile depending on sources. This should imply different discount rate for investment in technology that involves savings of those energy sources with high prices volatility. Following this demonstration the paper support the idea that whoever assesses the value of energy efficiency have to make assumption on energy source saved, future energy prices, appropriate discount rate and maturity of the investment. When researchers and professionals omit to do those assumptions explicitly they do it implicitly. Hedonic models have been widely used by researcher to assess the value of buildings properties. We use forecast at the moment of the transaction to underline the integration of energy efficiency value in building valuation with hedonic models. We use a US database to investigate the relationship between building energy consumption and building value. We also investigate this relationship with rent. Results suggest that the energy consumption that impacts the rent value is different from those who impact transaction price. It appears that overall energy cost impact the rent value, but not the transaction price. In conclusion, the economic sustainability assessment of an energy savings investment must take into account energy sources and future price.