[ajb]

I'm not sure that this is what jvdh had in mind, but any pure-heating device is by definition 100% inefficient. Any physical device which does work produces heat as well (well eventually. The energy may feed into other processes before it ends up as heat). A 100% efficient device is one that produces work equal to the amount of heat (I think - my thermodynamics is a little rusty). A pure heater produces heat without doing any work whatsoever with the energy. The most efficient heating would be done either by a heat pump (backwards air conditioner), or by doing some work with it.

Your point about heating only the rooms required is quite true, though. portable heaters may be more efficient than other extant kinds of heater. One other disadvantage is that many of them are air heaters, and the human body is more sensitive to radiant heat than air temperature. (It feels a lot better to be warmed by the sun, than to breath warm air).

There are portable radiant heaters, though far less of them around than there used to be. I think people see them as less safe, which I guess they are.

[dspeyer]

An electric heater is inefficient if the electricity was generated inefficiently.

Inefficient: burn methane at a central point running a steam engine generator, dumping tons of waste steam into the air, run the electricity to your house and turn it into heat there.

Efficient: pump methane to your house and burn it there.

More efficient: burn the methane at a central location running a steam engine generator, carry both the steam and the electricity to your house, heating it and running appliances.

The moral: live densely enough that steam pipes are practical.

[jvdh]

My thermodynamics is rusty as well, but from experience I can say that using a central heating system where hot water is pumped through feels a lot better and seems a lot more efficient than an electric heater.