Brazil has gone through an important electricity generation crisis in 2001, but the country does not have as yet a legislation to improve building energy efficiency. The energy efficiency of Brazilian buildings can well be improved, as it was shown during the energy rationing period in 2001. This efficiency increase could usher in gains in quality for indoor environments, as well as lower investments in power generation facilities, including the emission of gases into the atmosphere, flooding arable land for reservoirs, etc. The current work briefly demonstrates the lack of planning that caused the electricity crisis, some results of multi-building studies and simulations of an existing office building of Rio de Janeiro. In this parametric case-study, we have simulated variations of the window-wall ratio (WWR) with different glasses and interior shade, using the natural light, aspects deemed to be of the utmost importance for a future Brazilian building energy efficiency legislation. The need of such legislation has been much increased as a result of the energy generation crisis and its consequences. To take advantage of the problems, in order to improve the quality of the Brazilians buildings, is one of our objectives. (C) 2003 Elsevier B.V. All rights reserved.