Solar Panel efficiency and Cost

Catalog:

http://www.solarhome.com/catalog.htm
http://partsonsale.com/suntech.html

Compare:
Watts output and Price per Watts
http://sroeco.com/solar/learn-solar/solar-panel-comparison/

Services in California:
http://www.solarsourcepower.com/faq.html

Using the above two sites:
Sharp 230 DC watts panels are 40" x 65" = 3.33' x 5.4' = 18sqft/panel

Variable values:
PTC watt/sqft = 14 = 252 ptc watt/panel = @1.48KWh/yr/PTC watt = 373KWh/year/panel
cost per PTC = $3 = $756/ panel

Assumed fixed values:
1PTC = 1.48KWh
4.1 KWh = $1 saving
18sqft/panel
0.36 = 1.48KWHY/ 4.1

If 20 panels installed = $15, 120 = 7460 KWh/year = $1800 savings = ROI = 11.9% or 8.4years (16 yrs of free electricity if last 25 years or $28,800 of free money)

Highlight = variable values.
I.E. If PTC watt/sqft = 15 and cost per PTC = $2, ROI = $97.46 saving/ $540 panel = 18.0% ROI
$97.46 from (15 PTC/sqft x 6.5 [18sqft  per panel x 1.48 Kwh/yr / 4.1])
$540 from ($2/PTC sqft x 15 PTC/sqft x18sqft per panel)

Short-cut
ROI from 0.36 /$2/PTC
or
http://www.solar-estimate.org/index.php?page=solar-calculator