After you've established what your load is, here is where to look for size of array needed to power it.
http://www.nrel.gov/rredc/pvwatts/
PVWatts can take your location, panel aim & tilt, and tell you how much, on average, to expect from it.
for grid tie, the numbers are pretty close. For battery/off grid, you may need almost 2x the PV array to account for losses in battery charging.