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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