Heat pumps seem like magic. You use a certain amount of energy (let’s go with 1kWh), and you use this to extract heat from the ambient air, and this allows you to apply a multiple (let’s go with 3kWh) of your original energy input.
This is significantly better than an element heater: in that situation, you can only apply a smaller multiple, depending upon the efficiency of the element.
Similarly, burning gas to heat up water is less efficient - you don’t quite get to apply all of the energy from the amount of gas you burn to the water. I believe boilers and element heaters are around 85% efficient.
This is also how reverse cycle air conditioners work - basically a transfer fluid is used to extract the heat from the ambient air, and either push this cooled air, or use the energy extracted to heat up other air to push around.
But heat pumps, unlike a gas heater, can use renewable energy as the source of heating water. This interests me greatly.
Currently, I spend around $100/month on gas - most of this is probably on my gas hot water, as a gas stove would use a small fraction. For my calculations, I went with 80% hot water, 20% stove. This would not reduce my gas bill to $20/month though, because of daily tariffs (the bane of low-consumption systems). Instead, an 80% reduction in usage would drop my bills to between $35 and $40 per month. Indeed, it would save me almost exactly $2/day.
But the other factor to take into account is how much more of my solar generation I would be using to run the heat pump. To do this, I increased my consumption measurements from my previous calculations by 4kWh (800W for 5 hours, picking the peak solar generation time).
This resulted in a projected increase in my power bill of about $1.10 per day.
So, having a heat pump would probably save me less than $1 per day.
With a $4200 installation price (including STC credits), this looks like a payback time in excess of 12 years. Which is far larger than the warranty period of the system - this is looking marginal.
If I double my PV system size, and install a heat pump, this should result in my energy costs reducing by around $3.40 per day. This is about $1240 annually, but then there are two capital costs to cover: closer to $10k. This reduces the payback to around 8 years.
Interestingly, this is better than the payback of me just adding more solar panels - I used a cost of $6000 for the PV upgrade, and $4000 for the Heat Pump here, rather than the $5k I used in a previous blog post.
But probably still not good enough, right now.