Knowing how much power you need is the first step to planning your array. Since solar panels are measured by how much energy they can absorb, this will tell you how many panels to buy, how efficient they need to be, and (perhaps most importantly) how much space you're going to require. Don't worry, this process doesn't require more than your utility bills and some basic math.
First, check out your utility bills to see how much energy you usually consume. Typically, this number falls around 900 kWh each month, but it varies wildly from household to household.
Next, find out the "peak sun hours" of your area. This number is a measurement of how sunny someplace is. On the West coast, this number is between six and seven hours; on the East coast, between four and five. The National Renewable Energy Laboratory has an excellent insolation map
. Follow the link for Photovoltaics under U.S. Solar Resource Map.
All that's left now is to do the math. At 900 kWh each month, you're burning 30 kWh each day. Divide this number by the daily peak sun hours. If I use 30 kWh in a day, and there's five hours of sunlight, then I need 6 kW worth of panels to match all of my usage.