Total Storage amount in datacenter

I'm trying to get a script that will give me the total amount of space used in an entire data center. I will like to run it against the data center from the virtual centers. There are a total of about 1900 virtual machines and the VM's are spread across multiple LUNS. I need to get the overall figure for a migration.

PS:  I will like to get the figures in GB instead of Megabytes and I also want it to spit out a report in .csv to my local machine (c:\scripts\xxxx.csv)

Thanks guys.