Efficiency is measured purely by "useful energy" output.
Don't know if anyone knows/remeber energy efficiency diagrams, big arrow on the left, pointing right as input, then a small arrow in the middle pointing up as energy wasted, and medium sized arrow on the right point right (following from input) represented useful energy.
As a percentage value, it's simply a factor of what useful energy you get from input energy. Tungsten lightbulb, 100W input, 30W waste as heat, 70W useful light (it's probably the reverse, I haven't looked it up), so it is 70% efficient.
The most common way of wasting electrical energy is through heat (look at your PC), but when that's the desired output, then surely it can be 100% efficient, since all energy is useful in generating heat.
To hell with the dynamics of the room, insulation and all that jazz, the OP wanted a 100% efficient heater, and that's basically, any heater as far as I can see.
