Help explaining power usage spikes

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Re: Help explaining power usage spikes

Postby Benny » Fri Dec 22, 2017 5:17 pm

Can you get a power chart rather than energy chart ? ie show kW rather than kWh ? This might better identify the appliance from its power use. A fridge/ freezer should be no more than 150w. I can't read that chart very well. but it its showing 600Wh used in 5mins, then the power of the device is 600*60/5 = 7200W or 7.2kW - way to much for a fridge/freezer and even too much for a heat pump. If its 10min intervals then its 3.6kW.
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Re: Help explaining power usage spikes

Postby Gordon-Loomberah » Fri Dec 22, 2017 5:35 pm

It is indeed difficult to read, and not all that useful, but I think they are 15 min intervals. ~600Wh in 15 mins is a rate of 2.4kW. Electric heater, oven, large electric hotplate, induction cooktop, electric jug, that's the sort of appliance that uses ~2.4kW. However, it could be made up of a number of lower power devices, which is why we need to wait for the 'Turn it off and see what happens' report ;)
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Re: Help explaining power usage spikes

Postby Thimsneb » Fri Dec 22, 2017 10:18 pm

Ok, apologies for the lack of follow up. This afternoon I isolated the breaker for my hot water heater (Dux Airoheat Heatpump). 7 hours later, no power spike. There you have it. Next question.. does that seem to be an exceptable consumption, or do I have a problem? I don’t know details at this stage of the HWU in terms of KW rating, etc. I’m pretty sure these only came in on one format. It’s approx. 8 years old, the fan motor had been hanged out about 2.5 years ago. 250 Ltr tank I believe. Any more info I can gather?
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Re: Help explaining power usage spikes

Postby Benny » Fri Dec 22, 2017 10:52 pm

So its using about 3kWh per day ? That seems fine fro a heat pump HWS. If its got efficiency of 4:1 then its making 12kWh of heat which will give 120L of hot water - seems about right if a few people are using it (showers) A bit high if you're on your own. Odd that it comes on at 1:30am - thats presumably just boosting up to compensate for heat losses - unless its getting cold overnight where you are ? Maybe improve insulation ?
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Re: Help explaining power usage spikes

Postby Thimsneb » Sat Dec 23, 2017 6:16 am

I would put it closer to 4.0-4.2kwh per day. There’s only 2 of us (avg. 2-3 showers per day between us). We live in Brisbane, temps last couple weeks haven’t dipped below 20-21 overnight, still kicking in regularly around 1:30-2:00am. I believe heat pumps perform optimumly in our type of climate. I shouldn’t complain too much, it’s still far cheaper to run than an electric storage.
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Re: Help explaining power usage spikes

Postby Gordon-Loomberah » Sat Dec 23, 2017 7:04 am

Can you out it on a timer, so that it can only operate during the day? It will have a higher COP then, and make use of your PV output, meaning you aren't using dirty grid power, and that works under normal circumstances for the best financial outcome.

However, if you are operating it at night on a tariff that is significantly less than your FiT, then it may be better to operate it mostly at night.

Either way, your storage tank should be insulated well enough for a day's worth of hot water for 2 people, so it shouldn't need to operate day and night.
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Re: Help explaining power usage spikes

Postby Thimsneb » Sat Dec 23, 2017 8:17 am

I like the idea of a timer. The unit is hard wired back to the switch board. So I couldn’t place a timer at the unit itself. That would mean a timer is installed in the meter box? I’ll speak to an electrician about options.

Running a heat pump off peak wouldn’t work would it? As the unit uses the ambient heat of the day to heat the water. So running off peak at night wouldnt be as effective? Is that logic correct?
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Re: Help explaining power usage spikes

Postby Gordon-Loomberah » Sat Dec 23, 2017 8:32 am

If you wanted to analyse it in detail, you'd need a COP vs ambient air temperature graph from the manufacturer. However, Brisbane is reasonably warm even in winter, so you'd expect a good quality heat pump would work at a reasonably high COP most of the time there. Sanden heat pumps, which use CO2 as the refrigerant, will operate effectively to well below 0C.

Do you know how much you pay for the power for the heat pump? What is your solar FiT?

Yes, an electrician would need to put the timer in the switchboard.
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Re: Help explaining power usage spikes

Postby Warpspeed » Sun Dec 24, 2017 8:36 am

I finally traced some weird irregular night time power peaks to the defrost cycle in my refrigerator.

This uses a 350 watt heating element, triggered when ice on the evaporator envelopes a thermistor temperature sensor. The defrost heater can run for typically 20 to 30 minutes at a time.
After a defrost, the compressor has to work extra hard for a short time to get rid of the extra heat so the total power used can be considerable above the normal temperature cycling.

This ice buildup is highly erratic, and probably depends on room humidity, and the number of refrigerator door openings. Its usually no more frequent than one or twice per 24 hour period, and is only really noticeable at night when loads are very small and otherwise fairly constant.

I suppose it could be much more frequent if your refrigerator door seals are leaking. Its not so much the entering warm air, its the humidity that builds up solid ice on the evaporator fins.
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Re: Help explaining power usage spikes

Postby jules » Sun Dec 24, 2017 10:29 am

Wow, that's interesting but rather scary. An hour of 350W per day is a significant amount of power.

You'd reckon that any fridge designer aiming for a high "star" rating would try to figure out a better approach.

Does your fridge have an efficiency rating? [and I wonder if defrost power has to be included in any efficiency rating?]
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