Hot Water Energy Cost Comparison

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Hot Water Energy Cost Comparison

Postby Tracker » Thu May 12, 2011 7:17 pm

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As empty-nesters, we have relatively stable energy usage.
We also have an unusual situation, where we can actually compare "Alternative Hot-Water" heating sources.
We have a Heat Pump, and Instantaneous Gas and I can disable either, or use them in tandem.

Should you be interested, I have been spending time and running the two and logging the data.

Fortunately, these days, the old gas bill is now understandable, and you are able to convert daily usage into simple Dollars and Cents, so here are the results..

The average daily consumption for hot water is..
........................................................OffPeak......Shoulder.......PEAK
Gas............0.5meter units @ $0.62/unit = .. 31c............31c...........31c
Electricity.....2.9meter units @ $0.08/unit = .. 22.5c.........40.5c........107c

Now, it's important to note that this is based on full use of OFF PEAK TOU electricity prices and both Electricity and Gas are discounted by 7%.
I have included for comparison, the Heat-Pump electricity costs IF I were to let it run during other times.
What I would like to do is to disconnect both the gas and the Heat-Pump and use the normal Resistive element for practical comparison.
The Heat-Pump energy usage is also based on an ambient temperature above 15DegC.
The energy usage WILL be higher during the colder nights. It would naturally be better to run the Heat-Pump during the warmest hours of the day, but this also corresponds to drastically higher TOU rates.

Has anyone else had the chance to undertake some real testing and price comparisons.?
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Re: Hot Water Energy Cost Comparison

Postby Gordon-Loomberah » Thu May 12, 2011 7:37 pm

Convert the HP and Gas to MJ and let us know which is the most efficient ;)

I've been running my solar hot water on direct solar power... hasn't cost me a cent since I installed it :)
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Re: Hot Water Energy Cost Comparison

Postby Tracker » Fri May 13, 2011 8:26 am

Gordon-Loomberah wrote:I've been running my solar hot water on direct solar power... hasn't cost me a cent since I installed it


Undoubtedly, Solar is better.
One thing that I would like to know, is the actual Electricity Used to RUN the solar hot water system.. ie. Pump the water through the solar-collector.
Given that the sun shines during the day, and during the day, the tariffs for electricity are much higher, I would be most curios what THAT energy might cost.
Do you have a plug-in meter that could record the average daily usage.

The average gas consumption is 18.8MJ and the average Electricity is naturally 2.9KwH..

Naturally, I am only interested in the COST, and I would not be installing a Solar-HWS, having the HP and the GAS currently installed and working.. I had thought of it, but the cost to undertake the experiment is unjustified.

Challenge to one and all with Solar-Hot-Water.. Can you measure the average daily electricity usage ?
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Re: Hot Water Energy Cost Comparison

Postby Gordon-Loomberah » Fri May 13, 2011 8:49 am

Tracker wrote:
One thing that I would like to know, is the actual Electricity Used to RUN the solar hot water system.. ie. Pump the water through the solar-collector.


That is also zero, the tubes are connected to the tank, all under pressure, so there is no circulation pump.
Of course I have to pump water up the hill for house pressure, not being on mains water here, but that doesn't apply for people in town, and hot water use is only a tiny fraction of the water pumped, most of it goes on the vegie garden/weed patch ;)

The average gas consumption is 18.8MJ and the average Electricity is naturally 2.9KwH..


2.9kWh = 10.44MJ, so the gas is ~80% less efficient, as far as energy used to produce a given amount of hot water.
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Re: Hot Water Energy Cost Comparison

Postby Tracker » Fri May 13, 2011 9:59 am

Gordon-Loomberah wrote:That is also zero, the tubes are connected to the tank, all under pressure, so there is no circulation pump.

But - that is not the norm, is it ?
I understand that most "Remote Tank" systems use a pump to circulate the water.. We often see questions about strange things happening with solar HWS and pumps running or not, and at wrong times...

2.9kWh = 10.44MJ, so the gas is ~80% less efficient, as far as energy used to produce a given amount of hot water.

I would love to do the same measurements with a "Storage" gas HWS. I have never liked the thought of all the heat that is blown out of the Instantaneous (Rinai) HWS
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Re: Hot Water Energy Cost Comparison

Postby munter » Fri May 13, 2011 11:07 am

Well - a 40W circulation pump running for 8hrs a day will use 320Whrs. I presume they actually run at a lower duty cycle so the total consumption will be less than that figure.

In terms of waste heat from gas hot water. Our gas consumption dropped by over a third when we switched from storage to instant. I presume that the heat transfer losses in operation were more than compensated for by the avoidance of standing losses 24hrs/day. The former is far more visible though so might appear to be more wasteful.
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Re: Hot Water Energy Cost Comparison

Postby Tracker » Fri May 13, 2011 4:51 pm

[quote="munter"]Our gas consumption dropped by over a third when we switched from storage to instant./quote]

Now that is interesting.. I was thinking that the difference would be minimal, as the storage losses would equate to the inefficiency of the rapid-heating Instant heater..
Functionally , I prefer the Instant, as the water is not sitting at "Warm Temperatures" where bacteria can flourish, and I don't like storage water to be so hot that bacteria is killed.

It's a shame that we can't have a gas-consumption device (plug-in) like we have for electricity.

Thanks for your thoughts..
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