I presume some NAT'ers have 'heat pump' hot water heaters.
If you know nothing about them, they use refrigeration gas compression and expansion to extract warmth out of the air [even down to below freezing point] to heat water.
We own one, and it has just had some reliability problems, just 11 months old!
The service guys asked if it ran on night rate, 11.00 pm to 7.00 am, or as needed  - that is 24/7.
Interesting point is that night rate is 12c/Kw/hr, and day rate is 25c/Kw/hr.
Question - does any one know much about thermal efficiencies?
I guess that trying to figure whether it would be more efficient to run during the day, when of course the temperature is higher, but the cost of power is a little more than double, or at night, on the cheap night rate, is a very complex problem. There are so many variables!  But that I guess is what I'm asking!
Which is the way to go?

One thing I do know, the heat pump produces our hot water about 50% cheaper than the old heater coil did.