... have to be a good idea, right? The Energy Saving Trust said, on "You and Yours", the Radio 4 consumer programme, that they'd like to see the selling of tungsten lightbulbs phased out, so that only low energy ones were available in the fullness of time. If every house in the UK used three, then over their lifetime they would save enough energy to power the UK for a year (or something like that).
But hang on, correspondents pointed out. The truth isn't that simple. The low efficiency ones are less efficient because they generate more heat than light - a 100W lightbulb generates 80W of heat. If those watts of heat aren't being generated by lightbulbs, the house will be being warmed up less by the lightbulbs, which means that the heating will have to generate more heat to compensate. So there is no net benefit.
But hang on. The truth isn't that simple, either! Firstly, you have to take into account the environmental impact of manufacturing different sorts of lightbulbs. If the impact of making just one low energy lightbulb is lower than that of making the five or so inefficient ones, then this represents an advantage that needs to be considered. Also, for a significant proportion of the year - say six months - homes aren't heated using central heating. So for this part of the year, the heat energy that is produced is simply gradually dissipated - that is, used to directly warm up the atmosphere! - through open windows. Also, a lot of us have lights outside the heated part of the house - in porches, for example. In these cases, all of the heat energy is just lost in any case. And finally, you have to take into consideration the relative environmental impact of the energy sources used - is 80W of heat generated electrically in a lightbulb - with perhaps efficient generation but less efficient distribution - more environmentally friendly than 80W of heat generated through central heating.
The bottom line is that low energy lightbulbs are probably not as good for the environment as the manufacturers would like us to believe. But neither are they as "neutral" as the "You and Yours" correspondents were suggesting. It would take some careful research to get to the real truth of this question.