This thread has been thought provoking.
@J_D 2.0 had me thinking - regarding the value of a heat pump vs resistive heating hot water system.
I decided to run some numbers…
The web informs me that the average hot water usage is 76 L per person per day, and that the average household has 2.6 people. So, I thought I’d crunch some numbers to see where they land. (I’ll try and add sufficient detail to the calculations so anyone can customise the results to their circumstances)…
So, to heat 197.6 lt (2.6 people x 76 L) of water from 15C to 65C (50 degree rise) will take 41.36 MJ (ignoring all losses) per day. This equates to 11.5 kWh (in electricity usage).
Picking a rough typical cost, a kWh of electricity cost about $0.25 (but check your electricity bill for your particular rate). So, the daily cost of heating the hot water (sans losses) is $2.87 per day, or $1,050 per year using a ‘resistive heating’ hot water system (sans losses).
Heat pumps are quoted and something like 5x efficient - i.e. they produce about 5 or 6 times the heat energy that they consume by running the ‘pump’ to extract heat from the surrounding air. I’m going to assume that this is a ‘headline figure’ that is only achieved in ideal conditions, so for everyday circumstances I’m going to assume a factor of 4x (i.e. drawing 1 kW from the mains results in 4 kW of water heating).
Assuming a 4x factor, a heat-pump will heat the same quantity of water for $262 per year ($1050 / 4), or $210 per year if you use a factor of 5x (Reclaim claim x5 on their website).
So a heat pump will save the ‘average household’ $788 per year (using chrisp efficiency factor) or $840 per year (using Reclaim’s efficiency factor). My most recent quote for a Reclaim heat pump was $5,400 installed (up about $1,000 for 2 years ago!), so a heat pump would pay itself off in about 7 years (full cost), or if I assume that an equivalent replacement for the existing hot water system is $2,000 (like for like) then the heat-pump premium is $3,400 ($5,400 - $2,000), the payback period is 4.3 years ($3,400 / $788). So, as long as the heat pump lasts at least 4.3 years, I’m ahead! And every year beyond that I save $788 per year.
The above assumes that all the electricity is purchased from the grid. If one has solar panels (PV) then the calculation is more complex. But to get a bit of a handle on it, it is relatively easy to calculate the ‘worst case’ figures. If one manages to ‘self consume’ all the PV production needed to produce the hot water, the electricity ‘cost’ is the loss of potential income from selling that PV back to the grid. Assuming the present Victorian minimum FiT of $0.067 per kWh, using resistive hot water would ‘cost’ $281 per year (using the same assumptions above), but rather it is a loss of income of $281 per year than a true cost. Using a heat-pump instead would ‘cost’ $70 per year (using chrisp efficiency factors) or $56 per year (using Reclaim’s factor). So, the savings are relatively small reflecting the relatively low cost of (locally generated) PV energy. So, this does support
@J_D 2.0 ’s claim that heat-pumps may not be good value if there is an abundance of local PV generation.
To put this in to some sort of perspective or framework, and assuming a PV generation factor of 3.66 (Melbourne 3000 postcode), a PV system of 3.14 kW would be required to heat the hot water (sans all losses). So, 3.14 kW (about 10 panels) of the installed PV system would be needed to support a resistive hot water system, whereas the heat-pump would need 1/4 or 1/5 of that (0.78 kW to 0.63 kW - about 3 to 2 PV panels).
In reality, the real-world figure will fall somewhere between the two calculations above.
