I'm about to prepare some spaghettis and to do so I need 2 liter of boiling water.

For getting the water to certain temperature (lets say 100 Celsius degrees) from a starting temperature of 15 Celsius degrees, I might opt by pouring the 2 liters of water in one single action or might choose by adding in 4 times (1/2 liter each time) so I would start with 1/2 liter, when it starts boiling add another 1/2 liters and so on until I have the 2 liters boiling. 

What requires less energy for getting the 2 liters?

Assumptions:

1. There is only one pot size.
2. The time required for pouring additional water is 0s.
3. Pouring water despite it takes 0 seconds, does not spit any water and then there is no additional loose of temperature other than the one coming from mixing the water.
4. The surface of the pot distributes the temperature homogeneously.