Here is a Spreadsheet to estimate costs of a Solar Grid Tie Kit based on average consumption. The advantage with Grid Tie Kit is that when the solar panels are not producing enough power due to shade or other reasons the system compensates for it from the mains.

https://docs.google.com/spreadsheet/ccc?key=0AgzulydHCKZddHBiZUdtNS0xRkZNYlRJUlZrYzBzMXc