Required size of solar electricity system: Would like recommendation

Buyer's quarterly electricity bill from last winter (in $, approximately): 450

Buyer's quarterly electricity bill from last summer (in $, approximately): 280

No. of people in the household/ premises on the solar power system: 4

Description of available roof space: The house is North facing. There are several trees around which cause shade most of the day on the Eastern facing side of the roof. So it is better to install the solar system on the Western side which is roughly 7m x 15m large - see photo. The photo was taken today at 3pm (12.3.2011) - you can get a good impression of the amount of shad at that time of day.

The buyer accepts only quotes that meet the following conditions:

Designer and installer need to be accredited by the Clean Energy Council.,
High quality solar panels with a minimum efficiency of 9% with less than 1% degradation per year (preferably less than 0.5%).,
Panels have a minimum Power Output Warranty of 80% for at least 20 years (preferably 25 to 30 years).,
Panels have a maximum negative tolerance of 5% on minimum power output.,
Panels have a minimum Material Warranty of 2 years.,
Warranties are issued by an Australian office and panels/ inverter are replaced on-site at no extra cost to the buyer (installer needs to highlight any variations in quote or bid 'comments').,
Panels are physically tough to withstand harsh weather such as hail storms (e.g. tempered glass).,
Panels have bypass diodes installed for better performance (installer needs to highlight in quote or bid 'comments' if they do not).,
Panels have a minimum Power Output Warranty of 80% for at least 20 years (preferably 25 to 30 years).,
All external cables will be in UV protected conduit.,
Inverter complies with the Australian Standard AS4777.,
Inverter has a minimum Warranty of 5 years (preferably 8 to 10 years).,
Inverter is transformerless for higher efficiency (installer needs to highlight in quote or bid 'comments' if it is not).,
Inverter has Maximum Power Point Tracking (MPPT) for higher efficiency (installer needs to highlight in the quote or bid 'comments' if it has not).,
Inverter is expandable if more panels will be added in the future (installer needs to highlight in the quote or bid 'comments' if it is not).,
Separate inputs on the inverter for each string of panels installed on different areas of the roof if they are facing slightly different directions.,
Installer will complete the necessary metering changes and paperwork for the local electricity supplier.,
Installer will handle all the rebate paperwork so buyer has to pay only the difference (installer needs to highlight in the quote or bid 'comments' if this is not included).,
Installer will include electrical hookup to the grid (installer needs to highlight in the quote or bid 'comments' if this is not included).,
Installer will provide at least one option for the buyer to monitor that the system is performing to its potential on an every day's basis.,
Installer will provide a written estimate of system electricity production.