You could try adding an energy monitor to either your wholehouse feed, or just the HWC circuit. That is what I have and it is very easy to see when the large 3kW load is turned on/off.
I use the Owl CM160 currently (which I bought in from the UK). I am in the process of upgrading to the Open Energy Monitor project and am going to be using an Arduino with the EmonTX shield plus a number of CT clamps. There is a lot of reading on the OEM site (http://openenergymonitor.org/) but it is a very powerful/flexible solution.
A simple way - get a mechanical timer - the type with all the pins that slowly rotates. Connect it's motor terminals to the element. So the motor is on when the element is on. This will record the total hours that the element is on for.
But what exactly do you want to measure? Power used by cylinder, number of heating cycles, Time to heat from cold / time to reheat after a shower....., When the power company uses the ripple control to disconnect power to your cylinder, Something else again?
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