According to NIWA Christchurch & Wellington are about the same - approx
13-14MJ/m2 per day (averaged over a year).
(from the code of practice
<http://www.solarindustries.org.nz/documents/041224%20master%20SWH%20COP.pdf>)

This only works out to ~156 watts/m2 though so its not adding  up. I need
to figure out what I'm doing wrong.

(On a second look 156watts averaged  is 3.744kWh/day so not too far from
your figure of 3kW/hr/m2. On this basis savings would be
3.744 x 0.15 x 365 x 3.1 = $635/year at 100% conversion - still a long way
off what was indicated to me.)
The larger panel would save $840 a year which is getting closer but still
less than advertised.

As you note, I don't think the efficiency of the simple panel will get
anywhere near 100%.
The evacuated glass tube types do get up to 80-90% IIRC but convection
losses in the flat panel
types limit their efficiency particularly if the ambient temperature is
low.
 I guess "efficientcy" is really a misleading term. Losses will depend on
the
maximum temperature difference reached, while efficiency can be measured at
a very low temperature rise
where heat losses to the outside are at a minimum. The code of practice
(above)
only requires that a "measurable" temperature difference be used when
evaluating onsite.

I'd like to get a price on the evacuated glass types but suspect that they
may be exorbitant.
I did have a thought about making my own using old flourecent tubes and
home-made heat pipes but
figured I may have problem getting a good enough vacumn and maintaining it.
May not go down too well
 with "the boss" either unless  it's well hidden from view. Hail damage
could be a problem also.

I'd still like to have a play with the idea however when I get the time !!

Richard




> The panel size on the costing sheet is named differently from the
> ones on
> the
> glossy handout but they are :-
> 2 sqr m
> 3.1 sqr m
> 4.409 sqr m.
>
> I suspect the cost sheet is for the middle one & I'll need the
> larger
> variety (at least).

    3.1 x 3 kWh/m^2 * $0.15 x 365 = $600 pa saved.

Average of 3 kWh/m^2/day seems more likely for Christchurch.
Also panel area will be less than full size.
also panel efficiency < 100%.
Possibly far less for a non leading-edge design.




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