For example have you ever notice the bathing habits for a young kid compared to when that kid turns into a teenager???  Basically their showering habit goes from perhaps a 5 minute shower to well over 25 minutes and this of course is multiplied if you have more persons in the house.  Therefore you water usage has changed however, your Off Peak setting hasn’t.

So, check to see if you have the correct Off Peak Setting and the correct size tank for the amount of usage you normally use.
This can be achieved by calling you supplier and asking them “what rate am I on?” or look at your monthly statement where it should clearly be visible.

As a guide:-

Off Peak 1    -    (6 hour run time)  est 12.00 midnight to 6.00am  = .06 cents
Off Peak 2   -   (18 hour run time)  est 12.00 midnight to 6.00pm = .10 cents
Normal        -   (rates vary through out the day) but as a guide = 0.17 cents

Otherwise call an Quality Electrics to check out your Hot Water Tank electrical elements as they may be on the way out...

As a guide:-

Computer on all the time = 3kWh / day
Fridge/Freezer = 1.2 – 3 kWh / day
Electric hot water = 5 kWh / day
Lighting 1kWh / day (if using fluoros, 4 times this for incandescent/halogen)
TV 1kW / day (3 hours per day + PVR on standby)
Washing machine 0.5 kWh / day
Microwave 0.02 kWh / day (1kW microwave on for 0.5 hours)
Evaporative cooler 1kWh / hour of use
Air conditioner 2 kWh / hour of use (for 6kW heat transfer capacity)

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