Is it cheaper to boil water in a ketle by halfs or fulls?

Question

E.g. is it cheaper (electricity) to boil 5l of water twice or 10l at once? or would it be the same etc? Which is better for the environment?

Answer 1

Boil it all at once. If hot water has been running at the sink already, start out with hot water. It will take the same amount of energy to raise a given amount of water to boiling.