Abstract: One of the difficulties in formulating effective residential sustainability policies is that it requires information on current resource consumption. It also requires predictions of future technological developments and the manner in which they may be used. These predictions facilitate the comparisons between competing policy scenarios. Much of the current discourse on developing more sustainable cities relies on current patterns of consumptions with little regard for the sensitivity of those results to very small changes in technology or behaviour. For example, the reported CO2 emissions reduction by switching from car travel to light rail is negated with the recent availability of electric cars. The apparent sustainability of competing alternative technologies may be erroneously reported because different forms of energy are often assumed to be comparable on their heating values alone, ignoring thermodynamic and practical limitations of converting between fuel types. Many authors have assumed that diesel (for transit buses) and electricity (for light rail) can be converted with 100% efficiency between each other based on heating energy content. They concluded that light rail was more energy efficient than buses. This paper questions those results. The reported efficiency benefits of light rail over transit buses or cars are conflated by the higher efficiency of electric motors compared with combustion engines. A different result is found where electric versions of these modes of transport are compared. In the US, travel by light rail and trolley bus was found to use 210 and 236 watt-hours per passenger kilometre travelled (Wh/PKT), respectively. Travel by electric cars and electric buses were found to use approximately half: 118 and 132 Wh/PKT, respectively.