I did an experiment where I boiled two cups (500ml) of water in a kettle, and it took 1:30 minutes to reach around 98 C, average. However, when I boiled 4 cups of water, (1L) it only took me 2:30 minutes, when I expected it to be 3:00 minutes. Does this mean that the more water I boil, the faster it will reach 100 C (proportional to its volume, of course)? The kettle and the thermometer used were cooled down first before boiling another batch of 24.5 C water, and I did a few trials.
Can you tell me the reason for this? And is there an equation I can use, to figure out, for example, how long it would take to boil 6 cups?