1) Reduce usage first. For every $1 you spend on conservation, that’s about $4 less you need to spend on your solar system. It costs less to use less electricity than to make your own.

2) Research solar hot water. Solar hot water is ~ 85% efficient, as opposed to ~20% for solar electric. You get a better bang for your buck installing solar thermal than solar electric.

3) Install solar electric. Prices for solar panels have dropped dramatically this year as the demand in Europe has dropped and people are having trouble getting home equity loans in the US. You can get better pricing if you buy by the pallet (depending on the panel, 20 - 30 panels). You are right that you don’t need to generate all that you use, and if you do steps 1 and 2, you’ll be using less anyways.
References :
AltE Store - http://www.altestore.com/store/

I would not recomend buying a few panels here and a few panels there. You may save money; you may not. What will happen, however, is that you will end up with a system of mismatched panels that you wont be able to get the specified power out of.

For instance: if you have three panels in parallel
a 17v panel, a 23v panel, and a 26v panel = will add up to 17v

and if you have three panels in series
8A panel, a 9A panel and a 7.5A panel = will add up to 7.5A

The panel will the smallest current and the smallest voltage will control your power output. You will probably have to have 15kw in missmatch panels to power a 12kw system. The best way to ensure this doesnt happen is to get panels of all the same brand and model.

Panel prices are at an all time low thanks to the summer 09 international market; and there is a 30% federal tax incentive in the United States; so you have picked a good time to go shopping. I would recommend getting an inverter and the maximum amount of panels for that inverter. If you want to add panels at a later time I would add an additional inverter with its maximum number of panels.

Give Aurora Power & Design a call at toll free 877-337-2490 and we would be glad to give you an estimate.

Hope that helps!
References :
Im a Mechanical Engineer for Aurora Power & Design
http://www.aurorapower.net/

They can be used for:

Air conditioning (absorption system)
General Heating
Pool heating
Hot water
To power a stirling engine that can be used to run a generator and produce electricity. This may work well with an Air Conditioning system heat by product.

Vacuum tube collectors will work in colder weather or cloudy days.
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One thing to keep in mind is that it costs more to build an expandable system. If your thought was to first put in 1 kW, then add to it slowly, that’s going to cost much more than getting the whole thing at once.

12 kW is a large system for a house. If saving money is a concern, are there any conservation measures you can do first? If you haven’t done so yet, it’s worth getting a professional installer to look at your house and energy bills and bid on a system. You can always decline, but at least that will realistically size your system.

Also, large installers can get panels much more cheaply than a consumer can, no matter what, so it’s possible that it would cost less to just have someone provide the whole package than to buy parts, yourself.
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Trust me, wait about 5 years, they will be dirt cheap but still more expensive than buying Nuke energy.
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