He said 600 kWh per month - notice the "h" or hour. I typically use 700 to 1100 kWh per month at my all-electric 2000 sq ft home with a family of 4.
Without knowing details such as his location, obstructions, summer vs. winter, etc, I will assume 4 hours per day based on the link below:
600 kWh/30 days x 1 day/4 hours = 5 kW. That would be a rough estimate of the size of the solar panels needed assuming 4 hours per day of equivalent full capacity output.
http://www.solardirect.com/pv/syste...hours.html