Do you know what voltage the step down transformer is outputting at?I use a step down transformer to deliver 1.5 kW into the immersion heater when there is sufficient 'spare' PV power available.
I'm just wondering if it's possible there's been some misunderstanding, and it's actually outputting at 110V, which counter intuitively would only give you 660W of power into the immersion, as explained in this calc I nicked via google.
[credit to unphased]Ohms Law V=IR and I=P/V
An immersion heater rated at 3kW on a 230V supply will use approx 13A of current. (I=3000/230).
So, using Ohms Law the resistance of the element would be 230/13= approx 18 ohms.
Using the same element on a reduced voltage using Ohms Law again 110/18=approx 6A
P=IV
6 x 110=660 watts. So if you reduce the voltage to 110V the power is reduced to approx 20%, not 50%. So the answer is 0.66kW not 1.5kW.
I'd suspect that at only 110V, and with only 660W of input that the majority of the heat output is then confined to the upper part of the immersion heater, though I'm not entirely sure on that.
If I'm right, then the same would also apply but to a lesser extent if it was stepped down to a voltage that did give 1500W output - ie the heat would be more concentrated in the upper section. I could easily be wrong about that though.