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199 posts

Master Geek

  # 1397510 30-Sep-2015 18:51
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Bee: Does she have a dehumidifier on for most of the day or something else like that???

Dehumidifiers typically only run the compressoer on a duty cycle of 15-20%. If you look at the mandatory power rating label, you'll see there are two ratings - one for compressor and one for the fan and any reheating element. Normally the compressor will be 150W and the fan maybe 40W. So using those figures the hourly consumption rate is 150W x 15% + 40W = 62.5W. 16 hours continuous running is then 1kWh - $0.27. What's more, they pay for themselves in cheaper heating due to the reduced specific heat of the air - not only does it require less heat to raise the temperature, it feels warmer at the same temperature due to the air not drawing heat away from your skin as quicky.

We also found our oven used a lot of power even when we weren't using it so that gets switched off at the wall...

Not possible unless it has a fault. It's just running a clock.

Bee: Of course NEVER switch the hot water heating off unless you are away for a week or two because that uses more power to reheat the water and can kill the element...

This is patently false and an often repeated myth due to misunderstanding of thermodynamics. Firstly, the thermostat already turns the element off and on and the temperature drops due to heat loss or HW use and the incoming cold water needing heating. The heat loss from the water is related only to the temperature difference between the water and ambient air - the water temperature doesn't drop to stone cold as soon as the HWC is switched off which is the only scenario in which turning the HWC off would use more power to "reheat".

If the thermostat temperature is set lower (shouldn't be less than 60°C to prevent bacteria forming and to kill any that has) the rate of heat loss is also lower - trying to maintain a higher ΔT takes more energy. Switching it off entirely when you don't need the HW immediately available lowers the average temerature differential which reduces the rate and consequently total losses. It's only losses you're ever compensating for with the heating. So turning the HWC off manually, whenever you like, will only ever lower your energy consumption for HW - it cannot increase it.

199 posts

Master Geek

  # 1397512 30-Sep-2015 18:54
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ubergeeknz: I think I've figured it out already.  She said she uses the heater for "1-2 hours a day" but the graph looks like it was on at least 12 hours yesterday.

IME people are notorious for under-reporting their usage, like people who claim they only have a 3-minute shower.


3344 posts

Uber Geek


  # 1397898 1-Oct-2015 12:15
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Bee: Of course NEVER switch the hot water heating off unless you are away for a week or two because that uses more power to reheat the water and can kill the element...

Not true.  As the temperature drops due to heat loss from the cylinder, the /\T between water and outside air will drop and therefore the rate of temperature drop will drop.  

Therefore the effort to reheat the water to the set temperature when you switch it on will be marginally less than the effort to keep it up to the set temperature the whole time.

However the savings are minimal if your cylinder is well insulated.  The outside of our cylinder (in both houses) is dead cold and the graph tells the story as well : our hwc barely uses power except when cold water needs to be heated (ie. when hot water is used)

640 posts

Ultimate Geek

  # 1398073 1-Oct-2015 15:18
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SQLGeek: The SmartNow product looks like exactly what I need as Meridian don’t have a smart 3-phase import/export meter. The only problem is the SmartNow site says they are no longer in stock. Does anyone know where to get these or an alternative?

We are in the same position. 

Saw this on their site:


We are expecting the following stock arrivals:


Current Cost EnviR - 15th-20th October 2015


Current Cost Gateway - 15th - 20th October 2015


Current Cost OptiSmart - March 2016

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