We currently have an ailing 14 yr old solar hot water installation. A few plumbers have given us the same story: due to the age and general condition of the system, it makes more sense to replace it rather than attempt to repair it.

They've given us various options, all with advantages and disadvantages in terms of efficiencies, costs, performance and so on:

  1. A (new) Solar Hot Water system
  2. A Heat Pump Hot water storage system
  3. (Electric) instant hot water
  4. (Gas) instant hot water
  5. Gas storage

I have read that 1 and 2 are both fairly efficient, and - as I plan to get solar panels for electricity generation* - going with the heat pump version would free up some prime real estate on the roof for the panels.

Electric Instant probably wouldn't work for our household size, and the gas options would be cost prohibitive as we'd have to get gas connected to our property. (Plus... you know... it's gas).

My question is, which hot water system is 'best' in terms of sustainability, for a coastal, temperate location such as the east coast of Australia? Is there another (more sustainable) option than the two above? Are there other factors to consider here?

We live in a temperate, coastal environment. No snow in winter, and really hot summers.

*COVID-19 and personal commitments meant my original timeline for this was pushed back

  • Not a home owner, but my parents have a solar collector, combine with a large storage for hot water at theirs - plus the have a gas line for cold/cloudy times. Depending on where you're living, the collector plus a large enough storage might be enough to last through the year.
    – Erik
    Commented Jun 25, 2020 at 6:59
  • A couple of things to consider. Depending on the quality of water where you are, minerals from the water may be depositing on the inside of the water heater. With time these can build up rending the heater no longer usable, or repairable, requiring it to be replaced. Regarding instant hot water systems, they require a certain level of water flow before they start heating the water, so before you get any hot water from them they can waste a lot of water. by having the tap running to get hot water flowing.
    – Fred
    Commented Jul 6, 2020 at 9:30
  • Update - The responses to this question have varied wildly. It seems that location, local climate, install location (inside/outside/roof/ground/basement etc) and weather can all be a factor here. Keeping it open ended would be too broad, thus I have restricted this to the temperate climate of coastal Australia, which I believe is more reasonably scoped, and more closely aligns to the actual problem that I face.
    – Robotnik
    Commented Oct 7, 2020 at 0:03

4 Answers 4


Heat pump water heaters are powered by externalities

A heat-pump water heater warms water by chilling the room the water heater is in. On a sweltering hot day, that helps air conditioning load somewhat. But the rest of the time, it is adding cold and draft to a house you are trying to heat. So you pay twice: to run the heat pump itself, and then to replace the heat stolen by the heat pump. If your heat source is electric resistive heat, this is wholly worse than a normal water heater.

Further, the heat pump water heater will perform poorly if it is in a closed space. It will turn that space into a refrigerator, and then run inefficiently because it can't pump heat that doesn't exist. Of course all that inefficient pumping makes heat, and it will have that heat, at least. Hard on the machine and no cheaper than resistive heating.

Tanked heaters need to be kept at 60C/140F for legionella

In recent years, it was discovered that the bacterium which causes Legionnaire's Disease, among others, can thrive and reproduce in a water heater that is too cool. Because of that, the gold-standard advice of health departments is to set the knob at 60C/140F.

Meanwhile, we have the plumbing safety people saying that 60C/140F is too hot, and can cause scalding. That is also true. The only way to not find yourself in a dilemma is to have thermostatic mixing valves at every human-accessed hot water outlet. This is provided by most joystick faucets, and almost any modern style shower valve.

This higher temperature requires more standby power 24x7.

Electric tankless requires really big service

In Europe, they have "electric showers" which are an electric tankless water heater built into the shower and serving only the shower. These are modestly sized, on the order of 20-25A at 230V. It would not be hard for such an electric shower to support the bathroom faucets as well (though, not at the same time). The upshot is that electric demands for a single point of use are not extreme.

However, the public expectation is that a tankless is a "simple unit drop-in" for a tanked heater: that it installs in the same location and simply works the same way - i.e. all outlets can use water at once at high flow rates, until it runs out, only now, it doesn't run out! This is the expectation. The high flow rate expectation creates a serious problem that can only be solved by an ostentatiously large tankless heater - Americans are fond of 18KW, 27KW or even 36KW units. A typical American electric service is 24KW or 48KW. And since the huge heater runs at max power when it runs, a truly staggering amount of electric service must be provisioned for this heater.

This works much better with "point-of-use" heaters such as that electric shower. Imagine a house with four 30A heaters: one in each bathroom, one kitchen, and one for the laundry room. This is not provisioned the same as a 120A heater -- this is provisioned on the assumption that they won't all be running at once. This greatly reduces the house's design load to within the realm of the reasonable.

Note that most on-demand water heaters are fuel or inefficient resistive units - I'm not aware of any "heat pump" on-demand heaters.

  • I forgot to mention it in the question, but the hot water system would be installed outside in my case. We also have mild winters and hot summers here.
    – Robotnik
    Commented Aug 22, 2020 at 9:53
  • 2
    I've never heard of a heat pump which takes heat from a room. Sounds mad (as you imply). Mine - an air-source heat pump takes heat from outside air (even in the winter). The best (ground source heat pumps) take heat from below ground where temperatures are stable year-round, but they have a high capital cost.
    – aucuparia
    Commented Aug 23, 2020 at 23:00
  • @aucuparia its a heat pump water heater we're talking about. It's a new product type. Commented Aug 24, 2020 at 1:29
  • Interesting - may be a country-specific thing then: separate water heaters are very unusual in the UK, space heating and hot water are nearly always done by the same system.
    – aucuparia
    Commented Aug 24, 2020 at 6:51
  • 1
    I had a heat pump hot water system. The cylinder was relocated outside, so extra bonus of more cupboard space.
    – davidgo
    Commented Jan 7, 2021 at 23:16
  1. Electric Tank

The technology is tried and tested, and the tanks are cheap. Take the money you save and increase the size of your planned solar array. Tilt your (north) array string a little bit more towards the horizon to increase winter generation. Put your (300+L) electric tank on a timer so that it only comes on between, say 11:00 and 15:00. (Increasing solar self-consumption is better than any feed-in tariff that you're going to get nowadays.) Surplus power from your larger array can go to charging your EV or be sold back out to the grid.

There are fancy options, but you don't need fancy options.

Note: Any arguments about efficiency are moot when you generate your own power and have a regular surplus during the day.

Note 2: Heat pumps have noisy fans that can run for hours on end. (Solar) Electric tanks are completely silent. That may also matter to you.


Depends on where you live.

The problem with solar hot water in climates where real winter occurs every year is that you don't have enough sun in the winter. Thus, a solar hot water system always requires a backup, some source that is able to provide the heat. What is your backup? I suspect in most cases, it's electric resistance heating and that alone if less efficient than heat pump.

The problem with solar hot water, thus, is exactly the same as the problem with wind power or solar power. Many people have selected wind electricity as their electricity plan. When the wind stops and it's calm, they don't experience a power blackout. Thus, this simple tough experiment shows that wind power is a massive con. The same is true for solar power. If you do not have the possibility to adjust your electricity use according to the available renewable energy, your marginal energy comes from coal or natural gas.

However, if you have a heat pump based water heater, most likely it has some insulated storage. This storage allows you to run the heat pump when it's windy. It is often windy during the winters too, when there is not enough sunshine.

Thus, if you live in a cold climate, do by all means select the heat pump based system, but ensure you have a huge amount of heat storage built in. Adapt the heating based on the spot electricity price. When renewables are plentiful, electricity is cheap. You save money and the power comes from clean sources.

However, I wouldn't be surprised to find if in a hot climate solar hot water wins in terms of sustainability.


Electric is probably the best bet. YOU ALready get electricity, and electric water heaters have evolved alot since then to be vastly more efficient. Gas heating is pretty reliable and not to mention VERy hot if you like the heat or take long showers. Point is solar hot water is only efficient if you live in a climate where it's cool year round. Basically if you live below 40 degrees latitude. Solar panels must be angled based on what latitude you reside. If you live up north, passive solar is way more practical if it's built already. -electric: emission free

  • natural gas: Probably more efficient, certainly better temp quality.
  • solar: questionable economics

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