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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