Do you know any reliable firms in production modeling for grid tied systems?

One engineering firm that does have a great deal of experience in this area is Aries Power & Industrial Co.  They have done a lot of work for renewable projects throughout the world modeling power production.  They have offices in both the US and in Spain.

Let me know if you find them useful.

-Will

For larger projects, the most well known Engineering firm for production modeling is BEW Engineering.  

For smaller residential projects you can use:

The State of California EPBB caluculator:  

or NREL's PV Watts