When calculating the size of the solar HWS remember that for it to work efficiently - that is, for you to get the benefit of free solar energy to heat your hot water - the tank must be sized for at least a 2-day consumption qty.
That way if one day is sunny and one cloudy you will not need to boost till the third day.
If you have a small-ish tank and use it all up each day, then you need to have a fully sunny day EVERY day, otherwise you end up boosting almost every day which means you have an "off-peak HWS with occasional solar boost" rather than a "solar HWS with occasional electric boost".
See the difference?
So short showers to start with. 5-mins and a 3-star WELS-rated shower head, 9L/min flow rate, is only 45L, of which maybe 25L is hot water. Multiply by number of people in house. Add 20L/day for dishwashing. Multiply by 2, and this is the minimum size of the HWS you need.
eg: 2 adults, three children household
5 x 25L = 125L + 20L = 145L X 2 = 290L
Therefore the SMALLEST solar HWS tank you should plan for is a 315L (closest standard tank size), but I'd recommend a 350L as a better bet and would cope with visitors as well as the occasional bath.
You didn't say whether or not the house was NatHERS rated to the new BCA standard of 6 stars. I'd recommend re-designing for 8-stars, and doing without the ducted A/C.
Why? 8-stars will provide a much more even temperature house. Ducted cost mega bucks, around $7K upwards and from an energy efficiency perspective is unwise.
There is no need to heat/cool the entire house. Very wasteful.
Design living areas so that, rather than the modern "flow thru" design they have partitions and/or doors/folding screens/whatever in between them so you can *firstly* reduce the are required to be heated or cooled.
Then, and only if absolutely necessary, purchase a twin head split system designed so that only one head can be on at a time. This reduces the load on the system and means the system can have a smaller and therefore more efficient motor unit. PLace one head unit in the living room and one in the family-dining room. You don't normally use both at the same time.
A 'whole of house' ducted system probably is in excess of 6kW/h input current.
The multi-head split system as described might only need 2.4kW/h input.
See the difference?
With the ducted system you would probably be using that amount to simply cool one living area, as the compressor motor (apart from some of the newer inverter types) runs at a constant speed with a constant current draw.
Cooling the same living area with a single split system can use a third of the power.
And if money is tight, split systems can be bought separately and added to over the years, rather than ducted which needs to be installed all at once.
If down track you find you do want to cool bedrooms as well as licing areas, another multi-head split system, with enough head units for each bedroom, but with the motor sized for a maximum of two areas (kids always go to sleep before parents, and not all kids will be in bedrooms at same time, so ca still cool separately as needed).
Again, split system uses less current and only the actual rooms in use will be cooled, reducing load on system.
My brother had a ducted system installed, with 9 different zones and two different-located return ducts (way, way over-spec to a *standard* install) and the first summer they used it pretty indiscriminately and it added $500 to their power bills. Last summer, after concerns expressed and solutions suggested by me, they now use fans until the ambient temp gets to 35degC and cost dropped by two thirds.
And behavioural changes are free!! 
Factor those savings into the budget and get the solar system installed up front. If you have been using 10kW/day and expect this to increase slightly, then this will probably mean around 12-15kW/day (with solar HWS) so this would indicate a minimum Solar PV array of 3-4kW.
I'd recommend 4kW as this will provide additional capacity which will help negate fixed costs such as Service and/or access fees that form part o fthe electricity bills.
And if the mortgage won't stretch to all of the above, build a smaller house and use the savings to implement the energy efficiency measures.
Most people do NOT need the huge amount of wasted space incorporated into modern home designs. There is no real reason a 3-b/rm home needs to be larger than 150sq m. Plus garage.
The smaller the home, the less embodied energy is indorporated into the structure, the smaller your house's carbon footprint.
Posted Saturday 15 Jan 2011 @ 6:08:05 am from IP
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