It doesn't really take more energy to heat the water; it's just that you're trying to heat it in seconds instead of minutes. If you think about the gas versions, you're using 30 minutes worth of a gas flame in a few seconds. That means that the flame will look more like a welding torch than a stovetop burner.
My electric one takes (4) 30 Amp 240V circuits, but it doesn't really show up as a noticable amount on my bill; so I'm not using a LOT of energy, I'm just using the same amount very very quickly.
