Just how did you figure that and using what solar technology?
The most efficient panels on the market are Sanyo HIT series at 17%. To get 800 watts using them requires a minimum of 54/ft2. Using a panel with less efficiency over the same area will yield less.
I do a lot of off-grid battery systems and it doesn't look like you factored in charge efficient, charge controller efficiency, wiring losses, tilt angles, negative temperature coefficient, and time of year.
If you mount the panels directly to the surface, you are going to loose a lot of power from heat.
Anyway in SoCal with 0 degree tilt angle the worse case Sun Hours in December is 2.6, and 6.6 in July. So taking just charge controller and charge efficiency into consideration a 800 watt panel can generate up to 800 watts x 2.6 hours x .8 = 1664 wh in December, and 800 watts x 6.6 hours x .8 = 4224 wh.