[Besoeker]

Originally Posted by Richard

From a purely theoretical point of view it will always take more energy to boil more water than you need, let some of it cool down, then boil it again when you need it, compared to just boiling what you need. There may be other aspects in reality, such as the efficiency of the kettle, which may affect the actual answer. Or perhaps not. :P

I don't think the efficiency changes the answer.
Boil more than you need and the remaining water cools thus losing energy that was used to heat it.
A difference in efficiency will affect the rate at which heat is lost but it won't change the fact that heat energy is lost.
To re-boil the water that energy has to be supplied again.
But maybe it is a storm in a teacup.
Boiling a litter of water from room temperature costs about 1p at UK domestic rate. That's about the average cost of driving a family car around 30m. Boil the kettle 10 times a day and you have the equivalent of less than quarter of a mile.
I am not condoning wasteful use of energy - just trying to put it in some perspective.
Reducing driving by a mile a day would achieve greater savings than being parsimonious over the water in the kettle.
That doesn't mean you shouldn't do both, of course.