Smurf I think we are talking about two different things here.
and regarding Hot water systems:
- Which is most efficient between rooftop Solar HWS or Quantum heat pump system?
- Can we direct the cold air from the quantum heat pump to some kind of refrigeration or air-con use... is there enough 'chill' to be useful ?
-dukey
There's nothing "wrong" with putting some panels on the roof if that's what you want to do.
But if your aim is to reduce CO2 emissions and you only have x $ to spend (either capital or annual cost) then there are a lot more effective ways to spend it that will save far more CO2 than solar panels on the roof. Simply buying green power from the utility being a lot cheaper and ultimately cleaner than DIY generation at home using solar.
As I said, there's nothing "wrong" with any of this. But if the aim is to cut CO2 then panels on the roof aren't the best way of doing it. Obviously there are some other benefits as you point out.
As for the hot water, it depends on location, whether you have cheap gas available and your hot water usage pattern. Also it depends on whether you want to save $ as first priority or if first priority is energy efficiency.
Solar with gas boost wins for energy efficiency under most circumstances. No real dispute about that one.
But if gas boosting isn't an option then it comes down to climate. If lots of sun then solar with electric boost will generally win. If it's a cooler climate (eg Vic, Tas) then the Quantum leaves any commercially available electric boosted solar water heater for dead.
Northern Australia expect to save 75 - 80% with solar. Vic / Tas it's realistically 50% at best.
Quantum expect to save 70 - 75% anywhere in Australia unless you live in the mountains.
Personally, I'd go for the Quantum for sure if frost is an issue. The solar units do have some answers for that, but they involve either loss of efficiency (heating the panels at night) or require ongoing maintenance (glycol based systems).
The Quantum and similar systems are a "drop in" self contained replacement for a conventional electric water heater. No panels anywhere and just the same maintenance as an electric tank - new anode every 5 years and perhaps a relief valve.
All that said, if you're going to be using electric boost or a Quantum unit then if you have gas available that's nearly as efficient as is (without solar). Not quite but it's a small difference and still a lot cleaner than a non-solar electric unit. Also cheap to buy, low maintenance etc too but not as cheap to run as any solar system.
So overall lots of choice here.
Outright worst choice (in terms of energy efficiency) - conventional electric
Best - solar with gas boost
In the middle and roughly the same (depending on climate) - solar with electric boost, Quantum and similar, conventional gas water heater.
If you just want cheap to run and fairly cheap to buy then I'd go for the Quantum or something similar. Only about $900 more than a conventional electric tank after the rebates and it requires no additional plumbing or wiring if the unit being replaced is outside.
As for cooling, yes you could use the Quantum in this way but it's not normally done in domestic situations. Just requires some ducting - not really profitable to pay for it usually but you could DIY quite cheaply if you're good with that sort of thing.
As for running costs, you'll need to know your local gas and electricity rates to work it out in $ but use the following consumption figures for a typical household. All figures are annual. The last figure in brackets is the approximate efficiency from fuel in ground to hot water at tap including all losses except in the hot water pipes after it leaves the heater.
Electric continuous (small tank) 5800 kWh (22%)
Electric off-peak extended hours 6300 kWh (24%)
Electric off-peak short hours (night only) 6800 kWh (23%)
Gas storage = 30,600 MJ (natural gas) / 612 Kg (LPG) (55%)
Gas instantaneous = 22,500 MJ (natural gas) / 450 Kg (LPG) (74%)
Electric boost solar (warm climate) = 1575 kWh (94%)
Electric boost solar (cool climate) = 3150 kWh (47%)
Electric heat pump (Quantum etc) = 1890 kWh (79%)
Gas boosted solar (warm climate) = 7090 MJ (142 Kg - LPG) (236%)
Gas boosted solar (cool climate) = 14175 MJ (284 Kg - LPG) (118%)
All the above are of course approximations for typical household usage. If you have 5 minute showers then it will be less. If you have 20 minutes showers, a spa and wash in hot water with 6 people living in the house then obviously your consumption will be higher.
Use your local gas, electricity and LPG rates and multiply the rate by the consumption to work out the cost.