I came across this not too long ago. I'm currently rigging up a system in my room for a couple 12 volt items (car radio, laptop computer, car charger for cellphone/iPod, MR-16 halogen) plus some normal 120 volt items (TV, VCR/DVD, two fluorescent lights, and a fan) through a 300 watt inverter.
http://www.advancepower.net/advcalc.htm
You can enter your system voltage into the first box followed by a description of your load and what it draws. Example:
System Voltage = 12VDC (Default is 24 volts)
Load Name Quantity Energy Usage (In Watts) Hours(of useage)
Car Radio 1 6 4
It will give you the watt hours when you click "Calculate"
The idea is the same for the 120 and 240 volt loads
You then get to the battery bank section where you enter the make/model, voltage, and capacity of your batteries (Default battery is a Concord L-16 with 350amphours) followed by any other information (system voltage, temperature derating, wire gauge, and inverter efficiency if you have any AC loads connected).
My figures for the things in my room and how long I use them resulted in me needing about 160 watts worth of solar panels to keep up with my usage. I could get away with smaller panels though if I'm more conservative; try changing the length of time you use some of your loads (say, watch less TV or reducing the amount of time you have the lights on)
