1: Figure how how many kWh of electricity you have used. If you aren't sure, use the formula (.34 x miles) over a given period. Some calculators just ask you for your electric bill and figure the power automatically. This won't work if you only want to offset a part of your bill.
2: Use a solar calculator like this one : http://www.nrel.gov/rredc/pvwatts/
" onclick="window.open(this.href);return false; from the national renewable energy lab.
3: Contact a local installer to get your real price for purchase or lease.
Note: you can lease a PV system for 0$ down and have monthly payments that are very close to the rate your utility charges. And likely, you will add value to your house!