How much roof space does a solar power system require?

Below are some numbers I came up with. I would appreciate any input from real professionals in this area.
Assumptions:
average daily electric power usage of a small house - 19 kWh
average daily number of sunshine hours - 5hrs
output of a solar panel during sunshine - 190 W (Evergreen ES-190 solar panel)
sunshine - you have to be able to see shadows on the ground to obtain the specified solar panel output)
dimensions of a solar panel - 5.150 x 3.125 feet (Evergreen ES-190 solar panel)

dimensions of a Sanyo 200 watt panel: 4.325 x 2.93 feet

To cover the daily energy demand in this case you need:
19,000 kWh / 190W / 5h = 20 panels (Evergreen)

19,000 kWh / 200W / 5h = 19 panels (Sanyo)

The area needed for 20 panels (not including the spaces between them):
Evergreen: 5.150 x 3.125 x 20 = 322 square feet (or 30 square meters)

Sanyo: 4.325 x 2.93 x 19 = 241 square feet

Of course, the energy would have to be stored in batteries or the system would have to be connected to power grid.
I recently (October 2007) also read an account of a purchaser that the cost of installation per Watt in California was about $6.91 after state and federal rebates. That would give:
20 panels x 190W x $6.91 = $26,258
If the electric monthly bill was $120, the very simplified return on the investment would be:
$120 x 12 months / $26,258 = 5.5%
The investment will break even, not including inflation and price increases, after 18.2 years. As you can see, it pays to invest in energy reducing techniques before investing in solar electricity. For example, to get a new Energy Star fridge may very well bring your energy usage to 18 kWh, if your fridge is 5 years or older. As well, compact fluorescents and/or LED lighting will bring your lighting down another kWh a day or so.

Grants play a big factor in some states. The magazine Home Power has studied some cases where grants would make the payback 6 years.




EDIT: As of 2010 the cost per watt is at $5~ the average payback in southern California is about 7 to 10 years depending on electric company pay system and type of panels/inverters you are using.


EDIT: Solar Panel efficiencies vary greatly. ranging from about 60w to 245w. Obviously the higher the efficiency the lower the number of panels needed.