I appreciated the contribution above.  It was helpful.  And eventually led me to another estimating 'calculator,' at Kyocera.  See http://www.kyocerasolar.com/products/pv_calculator.html .

I thought I had responded a couple of days ago, but, I don't see my posting.  I apologize for the delay to say 'thanks.'

I'm working to put in a solar PV system at my residence here in Texas (Dallas).  Using what I've learned so far, and my current KWH annual consumption, and including assistance from Kyocera's calculator, the most cost effective system I can put in appears to be right at at 3.5 KW.  Anything larger and the payback diminishes.

Still, with the prevailing market install price here at $6.00/watt, payback period still goes out to double digits, at 12 years.

Here are the details.

Optimum economic system size: 3.5 KW (nameplate)
AC rating: 3.0 KW

Assumptions:
Utility: Oncor
Rate: $0.12/KWH
Annual Electric Bill: $3400
Annual Energy Consumed: 28.3 KWH
Annual Rate Escalation: 6% (actual, on avg, going back to 2002)
Financing: 5 year simple interest note at 7%, with 10% down payment

System Cost Before Any Rebates/Incentives: $21.0K
Less Total Incentives: $12.3K
Net Cost: $8.7K

Energy purchased from utility: 23.6 KWH
Energy produced by solar system: 4.8 KWH (17% of total need)
Excess solar energy produced: 0 KWH

Electric bill savings: $570 first year (17% reduction)

Loan payments: $1.9K annually (for 5 years)

Time period to break even point: 12 years

Not included in the analysis are maintenance costs, and inverter replacement at around 15 years.

Best regards,

Bill