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The latent heat of vaporization

Thursday, November 30th, 2023 at 08:48pm

After my recent posts about the cost of natural gas and how I want to heat and cool my home I got a couple of quotes on installing split system air conditioners in my bedroom and study, the paid the deposit for one which was scheduled to be installed on 20 December, just before Christmas.

Yesterday I got a phone call in the early afternoon saying that they had had some schedule changes and could they start the installation that day with it being completed the next morning. I said yes and long story short I now have air conditioning.

The cheapest way to install a split system is “back to back” which is where the outdoor unit is on the immediate other side of the wall to the indoor unit. This was ok for my bedroom because the outdoor unit would be in the gap between my house and garage, but not ok for my study as that would put the outdoor unit on the north facing wall leading to my front door. I was happy to pay the bit extra for the longer lines and labour to put that outdoor unit over next to the other one.

It is getting quite busy over that side of my house, there is the (now unused) central heating, the two new air conditioner units and soon there will be a new hot water tank and heater.

There is nothing special to say about the units I got, they are Fujitsu with decent energy ratings. I opted to not get any wifi interface as I planned to use an integration with Home Assistant that would send IR commands, in particular the ESPHome IR Remote Climate component.

While I could build and IR receiver/transmitter out of parts, the easier and quicker option was to order a couple of IR Controller Shields, they arrived last week and I started playing around with general IR control (that is another post about replacing the universal remote for my TV/etc) and specifically what ESPHome is like to use. Unsurprisingly it was pretty easy if you are familiar with Linux, Arduino and Home Assistant.

Now onto the hard part which is working out how I want things automated. With the central heating it was a simple thing of “make the whole house this target temperature”, but now the heating or cooling will be set for a specific room. Do I want to take the chill off my bedroom for when I get up, then turn off the bedroom unit while I am working in my study, then finally turning the bedroom back on for when I go to bed? Or just leave the bedroom unit off entirely, only using it on the really hot days?

It is also a convenient week to get it installed as despite tomorrow being the “start of summer” the weather has been below 20°C for the last couple and new few days, then jumping to 33°C mid next week. So I get to experience a range of temperatures and how the systems behave.

One other side effect was due to the layout of my study:

Study in Panorama (June 2023)

While they said they could possibly work over the desk if I cleared a space, I decided that a deep clean behind my desk was overdue so I moved all the NASA LEGO, the printer, the drawers and the desk extensions out so I could pull the desk away from the wall. Giving them room to install the unit and giving me access to clean out the dust and spiderwebs…

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The effect of solar panels

Thursday, November 16th, 2023 at 07:21pm

As I have been working through the cost of my electricity and the specific costs for hot water and other appliances I have also been reading up on what it would mean to get solar panels. A great starting point is Solar 101: A Guide To Buying Solar Power Systems and I have been getting information from other sites as well.

I am in the process of getting quotes and one thing common issue is that the shape of my roof isn’t the best. I might not be able to fit that many panels on the north facing section, nothing faces east, very little faces west, and then there is a nice big south facing section. I would definitely get 4.4kW of panels facing north and then optionally a second set facing south. Or even have a second set on the flat roof of my detached garage, but first I need to completely redo that old roof.

One aspect that caught my eye is whether or not a particular load can be completely handled by solar generation. Paying 22c/kWh overnight for power and then getting back 6c/kWh for unused solar during the day, means you are still paying out 16c/kWh. That cost would instead be zero if it were able to be moved to be during the day.

I have four large electrical loads, how easily would they fit within 4.4kW?

1. heat-pump hot water

I plan to leave a heat-pump hot water system on the typical schedule of 10am to 4pm, but in practice I expect it will from from 10am for at most two hours, drawing a constant 1kW for that time. In summer this should be trivial for any solar installation, and hopefully be possible in winter, but there are those cold dark and wet days…

2. dishwasher

Based on the usage I have been tracking so far a typical cycle of my dishwasher will consume around 1.1kWh, but spread over an hour and a half with sustained peaks of 2kW (not this is around 8.5A, so under the 10A rating of the cable, plugs, etc) which makes sense as it needs hot water for both the washing and then the rinsing. Again in summer this should be trivial to run when there is ample solar, then in winter the solution might be to run it in the afternoon when the sun is highest and the hot water has finished.

3. washing machine

I was surprised to see that the washing machine also consumed around 1.1kWh like the dishwasher, but as I showed in the previous post the pattern is quite different, with it running for much longer and only heating up the water (also at 2kW) in the early phases. Unlike the dishwasher that uses hot water for rinsing, the washing machine uses cold water for that. These differences aside I would treat this the same as the dishwasher and run it in the afternoon and not at the same time as each other.

4. reverse cycle heating and cooling

This one is still the most unknown to me as I don’t have it and I cannot compare it against my current gas central heating. I do know when the gas heating has been running and I know when I would like to have cooling:

  • Summer: Cooling to take the edge of the hot days and to make the really hot days livable
  • Autumn and spring: Heating in the early morning (I start working from home at 7AM) and late evening
  • Winter: Heating all though the day from early morning until late evening

The power usage for this system is hard to determine. For one model of system that I have been quoted the energy rating label says that for Melbourne it will use 920 kWh per year. How does that translate for my poorly insulated leaky house? This same model is rated at 3.5A when heating. So is that 0.8kW? I don’t yet know enough…

Do I need to know the detail now? Making these changes should mean that my overall grid consumption is lower so my monthly bills will be lower compared to my previous electricity and gas bills.

Something else I have realised is that the idea of running loads completely from solar means that I will be better off staying on a single rate plan instead of switching to a time of use plan. It doesn’t matter if the rate during the day is 22c/kWh or 19c/kWh if solar means neither is paid, but in the evening when there is no solar I would prefer to only pay 22c/kWh instead of 29c/kWh…

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Hungry appliances

Sunday, November 12th, 2023 at 07:57pm

After working through the cost of heating water my next target of investigation was appliances.

I am going to consider a fridge (or also a standalone freezer) to be part of the background usage. It is an appliance that needs to run all the time and be reliable because I do not want spoiled food. I am not considering trying to control it, if it is using too much power then the option is to replace it with a newer more efficient model.

Some other large kitchen appliances are the oven and hotplate. I know how much power they use and I know that I mostly use them around lunchtime which is already in “off-peak” classification. The smaller kitchen appliances such as the toaster and microwave are not used on a regular basis so I am not going to worry about those. The dishwasher I will come back to later…

Appliances for entertainment such as the television and speakers are used for specific purposes. I should check their standby usage, but otherwise I am not concerned.

I know the computers and monitors in my study are a large portion of my regular power usage, but they are essential, either they need to be on because I am working from home, or they are on because I am using them for personal projects or entertainment.

So now we come to the washing machine and are back to the dishwasher. These two applicances have pumps or motors, but also heat water which takes a lot of energy. This has been in the back of my mind for a long time, so in order to find out I went to Bunnings and picked up a “smart plug” that also included energy monitoring.

It worked and I was able to get data into home assistant, but this device needed an account on a cloud service to be configured and then the home assistant integration hooked in via that. Not the self contained approach I prefer.

I have yet to return this plug to Bunnings, but I after seem a couple of mentions in forums and other loactions I did order two Athom plugs with AU sockets. Although I haven’t looked at Tasmota for a while I knew of it, and importantly I knew it was MQTT based which I liked.

It took a couple of days for them to arrive and I set one up for my dishwasher and other for my washing machine.

I estimate that I run the dishwasher every two or three days, and this is its power consumption on the “speed perfect” and “half load” setting:

These days with working from home I find that I am only doing a load of washing once a week for clothes, then a dedicated wash for sheets or towels. This is my washing machine on an “intensive” cycle which is what I use for towels:

Quite different patterns. You can definitely see when they are heating water, and I think the irregularity with the washing machine is due to how it is detecting the motor usage, compared to a pump in the dishwasher.

But what does this tell me? Not really much…

But what might I be able to do with this information? Maybe something…

One thing I can do for both appliances is turn off the power using the smart plug to eliminate any standby power usage. I could also use a schedule in home assistant to enforce the time of use, not allowing them to run at certain times of day.

Depending on how fancy I want to get (simple could be x watts used for more than y minutes) I should also be able to add some form of indicator to know if the appliance is or has been running. If for some reason I have started a load of dishes in the afternoon I don’t want to always kill the power at 3pm, I would want it to complete the cycle and then turn off the power. A more useful automation I could configure is an alert to tell me (and remind me) when the washing machine has finished, because while dishes are ok to be left in the dishwasher, damp clothes in the washing machine need to be taken out and hung up.

As usual with these things, as soon as I dig into one aspect a bit I being up more questions and possibilities…

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Getting into hot water

Tuesday, November 7th, 2023 at 09:49pm

As mentioned in my previous post about electricity usage I now need to talk about how I get hot water.

My current system is as electric system up in my roof which is set up as a controlled load. This means that it is on a dedicated circuit that my meter will only power from 11pm until 8am at a reduced rate. It doesn’t appear to have ever been replaced, which would make it 54 years old…

The usage varies from day to day, but an average for a month is 200kWh, which at my current rate means I pay around $35 per month. On its own that isn’t a huge figure, but small things do add up over time.

My current system is electric resistance, however the future is in heat pumps. There are two brands available to me that get excellent reviews and those are Sanden and Reclaim, these also happen to be the most expensive options. I found it interesting that these are not an all-in-one unit like the cheaper options, but are a split system with a heating unit and separate tank, for my needs I will opt for the smallest tank at 160L.

So how do these compare for electricity consumption? It is hard to predict but there are many claims of up to four times more efficient. For my comparison I am going to be conservative and use two times as efficient, so 100kWh over the month instead of 200kWh.

A heat pump is about moving heat, so while they can still heat the water when it is cold outside, they are more efficient the hotter it is as there is less of a differential. So instead of heating the water overnight when it is coldest, it makes sense to run during the warmer day, eg between 10am and 4pm which is usually what they are set to.

So what does high efficiency and a different time of day mean for me?

Resistive200 kWhControlled Load17.60 c/kWh$35.20/month
Heat Pump100 kWhSingle peak rate22.66 c/kWh$22.66/month
Heat Pump100 kWhTime of use off-peak19.03 c/kWh$19.03/month
Heat Pump100 kWhSolar?free?profit?

So even with the higher rate compared to the controlled rate, the lower consumption easily covers the difference. Remember that I am being conservative with this estimate, so the actual results should be even better.

I am going to try to put monitoring in place so I can see how it performs, especially at the extremes of a 10°C day in winter versus a 35°C day in summer…

Update 13 November 2023:

After reading/watching some more posts/videos from people with heat pumps (such as Reclaim Heat Pump Hot Water Solar Energy Usage UPDATE – EEVblog2) I am thinking that my estimate above is way too conservative.

Both the Sanden and Reclaim systems are stated to draw at most 1kW when in use, and extrapolating from other people’s reporting that should mean it would run for an hour a day to heat up my water. Over a month that would be around 30kWh, not the 100kWh I used above. So what does that do to the monthly costs?

Resistive200 kWhControlled Load17.60 c/kWh$35.20/month
Heat Pump30 kWhSingle peak rate22.66 c/kWh$6.80/month
Heat Pump30 kWhTime of use off-peak19.03 c/kWh$5.71/month

Of course this is all still just a bunch of guesses, I won’t have any real data until I switch over, but the 1kW figure for the load does figure into thoughts on solar generation…

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Charting electricity

Sunday, November 5th, 2023 at 12:50pm

Continuing my thinking about how I heat and cool things in my house I have given my electricty bills the same treatment that I gave to my gas bills and produced another glorious Excel chart:

I have what is referred to as a “controlled load” which is that my electric hot water is on a dedicated circuit that only runs overnight (typically from 11pm until 8am) for a lower rate. I think I can see that there was a time when the consumption was fairly evenly split between hot water and everything else, but then once I started working from home the controlled load dropped a bit while the peak usage jumped up a fair amount.

I didn’t dwell too much on this data because there is a different data set that is much more detailed. If I want extreme detail I could go to the log of what I get directly from my smart meter, but an instantaneous power figure every eight seconds is a lot, and it only reports the peak usage, it doesn’t (though it did originally) report the controlled load.

The other data set I am referring to is the one that I can get from my electricity distributor as that is in a format of half hour blocks, essentially what the electricity retailer uses to calculate bills. Importantly this does include two sets, one for the peak usage and another for the controlled load.

I looked at this data in a few different ways but in the end summed up the half hour blocks for an entire month, this is for October 2023:

(Note that the peak and off-peak distinction is not real, I split them in the chart for something I will explain below, they are both currently billed as the same peak rate)

You can clearly see the controlled load turning on at 11pm to start heating up the hot water, then it drops off throughout the night as the amount of water that needs heating each day is different. Not sure what the bump around 5pm is caused by.

Looking at the peak usage there is a baseline visible overnight, then usage increases as I start working from home, a spike at lunchtime when I cook, followed by afternoon and then evening usage. There is no spike for dinner because that is just reheating the other half of the Hello Fresh meal that I cooked for lunch.

I was able to apply my current rates to this data and end up with the same amount as my bill for that month.

Another plan my electricity retailer offers is a time of use plan, where the usage is split into peak being 3pm to 9pm and then off-peak being 9pm until 3pm. The controlled load is still separate from this.

These rates are currently:

 Single rate with controlled loadTime of use with controlled load
Supply charge:107.03 c/day107.03 c/day
Peak rate:22.66 c/kWh29.48 c/kWh
Off-peak rate:N/A19.03 c/kWh
Controlled load rate:17.60 c/kWh17.60 c/kWh

I have the data, so what if I calculated the cost of these plans against my past usage?

 Single rate with controlled loadTime of use with controlled loadDifference
July 2023$152.97$152.52-$0.45
August 2023$141.26$140.01-$1.25
September 2023$128.33$127.40-$0.93
October 2023$130.48$128.46-$2.03

So there would be a slight benefit to switching to the time of use plan.

I haven’t been tracking the usage, but I think I do usually run the dishwasher and washing machine in the evening. It doesn’t make a difference while on the single rate plan, but that could be a big change if on time of use. For this I will find some plug in power meters that can report to Home Assistant (ideally MQTT based) so I can track it.

There are also some other options with other electricty retailers with a number of them offering three rates:

  • Peak – 3pm to 9pm
  • Off-Peak – 10am to 3pm
  • Shoulder – all other times

Then there is a retailer such as Amber which gives a half-hourly rate that is based on the live wholesale rate. I don’t know if any others that do this, and this is where you don’t want to just run loads like washing in off-peak, but automate it so that it runs when the live rate is low. I’m not at that point yet, but maybe someday.

Some other things I haven’t covered in this post are the impact of switching to heat pump hot water, running air conditioning all year round, and installing solar panels…

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